The MacPherson Strut

Although frequently misunderstood and often misspelled, MacPherson struts are one of the most common suspension systems used on modern cars, found on everything from the Proton Savvy to the most formidable Porsche 911 Turbo. In this newly revised and updated installment of Ate Up With Motor, we’ll take a look at the origins and workings of the MacPherson strut, including modern variations like the Toyota Super Strut, GM HiPer Strut, and Ford RevoKnuckle.

Author’s note: This article has been extensively rewritten to clarify some points and correct certain factual errors. If you’re already familiar with the origins of the MacPherson strut (or really don’t care), skip ahead to page 2 for the technical nitty-gritty.

EARLE S. MACPHERSON

Earle Steel MacPherson (not Earl McPherson, as it is often misspelled in even reputable sources) was born in Highland Park, Illinois, a suburb of Chicago, on July 6, 1891. After earning a bachelor’s degree in mechanical engineering from the University of Illinois in 1915, he moved to the Detroit area and went to work for the Chalmers Motor Company.

MacPherson served in Europe during World War I, initially as a civilian engine mechanic for the Aviation Section of the U.S. Army Signal Corps (not a fighter pilot, as has sometimes been reported) and then as a captain in the American Expeditionary Forces’ aviation technical division. When the war ended, he returned to Detroit and took a job with the Liberty Motor Car Company. After Liberty was bought out by Columbia Motors in 1922, MacPherson left for Hupp, where he remained for about a decade, eventually becoming assistant chief engineer.

Earle MacPherson press photo c. 1950s, copyright © Ford Motor Company

Earle MacPherson in the 1950s. (Photo copyright © Ford Motor Company; used with permission)

In 1934, with Hupmobile ailing badly, MacPherson and several other Hupp engineers (including future Hudson body engineer Carl Cenzer and future Nash engineer Ted Ulrich) departed for General Motors, where MacPherson became assistant to the vice president of engineering. One of their early projects was developing a prototype for a future small Chevrolet using Budd-patent unitized construction. Since the prototype was undertaken by the central engineering staff and not the division, we assume this was primarily a research project, but it became the basis of the 1935 Opel Olympia and the 1938 Vauxhall 10-4, GM’s first unit-body production cars.

Cenzer and Ulrich subsequently left for The Budd Company, where they continued working on unit body engineering, but MacPherson remained with GM. In May 1935, he was transferred to Chevrolet Division, reporting to then chief engineer James M. Crawford. MacPherson subsequently became Chevrolet’s chief engineer for passenger car and truck design.

THE CHEVROLET CADET

In the spring of 1945, Chevrolet general manager Marvin E. Coyle persuaded GM president Charlie Wilson to authorize the creation of a new Light Car Division and made MacPherson its chief engineer. The Light Car Division’s goal was to develop a cheaper, more economical compact car that Chevrolet dealers could sell alongside the standard Chevrolet.

Chevrolet’s Light Car project was prompted by Coyle’s fear that the imminent end of the war would bring another severe recession like the one that had paralyzed the auto industry shortly after the end of World War I (and nearly undone H.M. Leland’s fledgling Lincoln Motor Company, leading to its acquisition by Ford). However, Coyle was undoubtedly also aware that Ford was developing its own postwar Light Car, something that had been leaked to the press the previous summer and confirmed by Ford in July 1944. Since the small Ford was expected to undercut the price of a standard Ford (or Chevrolet) by a substantial and worrisome margin, it only made sense for Chevrolet to start working on a response.

Chevrolet Cadet B&W artist's rendering 163970 - Copyright 2014 General Motors LLC. Used with permission, GM Media Archive. (GMMA 19720)

An artist’s rendering of the Chevrolet Cadet. The Cadet was styled by Ned Nickles (best known today for designing the 1963 Buick Riviera and giving Buick its famous portholes), assisted by Chester Angeloni. The enclosed front wheels were a styling theme later adopted by Nash-Kelvinator for their 1949 Airflyte models and the 1950 Nash Rambler. (Image copyright 2014 General Motors LLC. Used with permission, GM Media Archive.)

The Light Car — subsequently christened Chevrolet Cadet — gave MacPherson a unique opportunity to develop a truly new design embodying his most advanced thinking. Some of the Cadet’s ideas were quite radical by contemporary American standards, including not only monocoque construction, but also hydraulic clutch actuation and an unusual centrally located manual transmission, connected to the clutch via a CV joint and a tubular driveshaft encased in a rigid steel tube. The engine, also all-new, was a lightweight OHV six with oversquare dimensions and dual flywheels, yielding 65 gross horsepower (48 kW) and 108 lb-ft (146 N-m) from 133 cu. in. (2,173 cc).

The Cadet was to be offered only as a four-door sedan, compact in exterior dimensions but boasting approximately the same interior room as a big Chevy of the mid-thirties. Target weight was only 2,200 lb (1,000 kg), about half a ton lighter than Chevrolet’s contemporary full-size cars, which contributed to excellent fuel economy. Despite its very modest curb weight, the Cadet also had decent handling and a surprisingly comfortable ride, thanks in large part to the Light Car’s most remarkable and controversial feature: fully independent suspension.

CADET SUSPENSION

In the mid-forties, independent suspension was still a relatively new development in the United States. Independent front suspension had only become standard on big Chevrolets in 1941 and Ford wouldn’t offer it at all until the 1949 model year. Independent rear suspension was even less known outside of a handful of exotic European cars. Including it on a car intended to sell for less than $1,000 (about 10% less than a full-size Chevrolet) was a bold move and naturally made Chevrolet management very nervous.

The Cadet’s suspension, described in detail in MacPherson’s 1947 patent application, was the ancestor of his later strut design, although both layouts had other antecedents, including a 1929 patent filed by former FIAT managing director Guido Fornaca and William Stout’s 1935 Stout Scarab prototype. (Interestingly, the Fornaca patent, which as far as we know was never applied to a production car, is not cited in MacPherson’s 1947 application, but is among the references listed in his 1949 patent.)

Each of the Cadet’s wheels was suspended on a vertical strut that incorporated the wheel spindle and a coil spring wound around a tubular shock absorber (itself a novel feature at the time — contemporary GM cars still used lever-action dampers). Each front strut was located by a radius rod and two lateral links while each rear strut was located by a single trailing arm and a curious diagonal “swinging link” that connected the base of the strut to a point on the opposite side of the body, behind the rear axle line. The halfshafts, which had universal joints at both ends, did not contribute to wheel location.

B&W Chevrolet Cadet prototype side view 163969 - Copyright 2014 General Motors LLC. Used with permission, GM Media Archive. (GMMA 19720)

A photo of the Chevrolet Cadet prototype, probably taken sometime in 1946. We don’t have complete dimensions for this prototype, but the wheelbase was 108 inches (2,743 mm) and 12-inch wheels were specified to reduce unsprung weight. The four-door sedan was to be the only Cadet body style. (Photo copyright 2014 General Motors LLC. Used with permission, GM Media Archive.)

MacPherson was a thoroughly methodical engineer and he was firmly convinced that this layout offered the best compromise between cost, packaging efficiency, handling, and ride. By most accounts, Cadet prototypes with this suspension worked very well, but the cost was problematic and the idea of GM’s cheapest U.S. model being more sophisticated than the priciest Cadillac probably sat ill in some quarters. MacPherson was obliged to develop a number of cheaper rigid-axle alternatives, if only to demonstrate the superiority of his fully independent setup. (One of these alternatives, incidentally, was a modified Hotchkiss drive layout with mono-leaf springs like those used on the later X-body Chevy II/Nova and first-generation Chevrolet Camaro/Pontiac Firebird.)

Had the Cadet been built as MacPherson wished, it would have been a landmark automobile, but by 1946, Chevrolet’s enthusiasm was fading rapidly. One reason was the departure of Marvin Coyle, whose promotion to group vice president in June 1946 left the project without a clear champion other than MacPherson himself. Another factor was the raw materials shortage that plagued all automakers in the immediate postwar years, a problem that forced a postponement of Cadet production plans that September and made the $1,000 target price — probably never very realistic to begin with — even more unlikely.

Moreover, the postwar recession Coyle feared had not materialized. Since civilian auto production resumed in late 1945, business had been booming. The real problem was not a lack of buyers, but a shortage of cars due to strikes and a scarcity of materials. The Chevrolet sales organization, which hadn’t had much voice in the Light Car project, saw no particular need for a smaller, cheaper car and balked at the idea of selling 300,000 of them a year, the minimum volume the corporation calculated Chevrolet would need to make any money on the Cadet.

GM senior management finally pulled the plug on the Light Car Division in May 1947, although MacPherson and a few of his team were transferred to the corporate Engineering staff to continue working on the Cadet as an advanced research project.

1948 Holden 48/215 sedan front 3q © 2011 Sicnag, modified 2014 by Aaron Severson and used under a CC BY 2.0 Generic license

Contrary to some assumptions, the Chevrolet Cadet was not the basis of the Holden 48/215. The 48/215, which was developed during approximately the same period as the Cadet and debuted in Australia in November 1948, was actually based on an earlier small car prototype, the 195-Y-15, which GM Engineering had developed shortly before the war; any resemblance between the 195-Y-15 and the Cadet was coincidental. The 48/215 was longer, had a significantly shorter wheelbase and a substantially wider track, and shared none of the Cadet’s novel engineering features save for its “Aerobilt” unitized construction. (Photo: “1948 Holden FX Sedan” © 2011 Sicnag; resized and modified 2014 by Aaron Severson and used under a Creative Commons Attribution 2.0 Generic license with modifications offered under the same license)

MACPHERSON AT FORD

The return to corporate Engineering was not a happy one for MacPherson, in large part because it meant once again working with his former boss, James Crawford, who had become corporate vice president of engineering two years earlier. Crawford and MacPherson had never seen eye to eye and their disagreements over the Cadet were particularly tense.

That situation soon came to the attention of Harold T. Youngren, who had been the chief engineer of Oldsmobile from 1933 to 1944 and had recently been appointed vice president of engineering at Ford Motor Company. At Youngren’s invitation, MacPherson left GM to become Ford’s executive engineer for design and development in September 1947. Without him, the Cadet project expired for good a year later.

When MacPherson arrived at Ford, the company’s own Light Car Division had already been canceled, but the car itself had caught the interest of Maurice Dollfus, head of Ford’s French subsidiary, who decided to buy the design, convert it to metric dimensions, and put it into production as the French Ford Vedette. We don’t know if MacPherson had any involvement in the engineering of the Vedette, which debuted about a year after his arrival at Ford, but if so, it was probably minor. (The Vedette did have independent front suspension, but contrary to many reports (and our own earlier error), it did not use struts.)

MacPherson would have the opportunity to apply some of his small car ideas to other products for Ford’s English and German subsidiaries, which in that era were still heavily dependent on the corporate headquarters in Dearborn for both engineering and styling. In January 1949, he applied for a patent (assigned to Ford) on what we would now recognize as the “classic” MacPherson strut suspension, described in further detail on the next page. This was in many respects a further refinement of the Cadet suspension, intended to minimize weight and production costs.

B&W front 3q of a Ford Consul photographed in Windsor circa 1951 (neg 83K75). Copyright © Ford Motor Company.

The first production cars with MacPherson struts were the 1951 English Ford Consul and the six-cylinder Ford Zephyr. This is a Mk1 Consul, which was 164.8 inches (4,185 mm) long on a 100-inch (2,540mm) wheelbase and was powered by a 1,508 cc (92 cu. in.) inline four. The Zephyr was 4 inches (101 mm) longer in wheelbase and 7 inches (178 mm) longer overall to make room for its 2,262 cc (138 cu. in.) inline six. Both cars, incidentally, were a little smaller than the abortive Chevrolet Cadet, although the English cars had a wider front track. (Photo copyright © Ford Motor Company; used with permission)

Later that year, the new suspension was incorporated into prototypes of the English Ford Consul, which in late 1950 would become the first production application. Unlike the Cadet, the Consul (and its six-cylinder sibling, the Ford Zephyr) did not have independent rear suspension, retaining cheaper Hotchkiss drive instead. Although MacPherson’s patent application noted that the strut design could easily be adapted for use at the rear wheels, Ford would not use rear struts on any production model until the arrival of the Mk3 Ford Escort in 1980.

MacPherson strut front suspension was subsequently applied to all of Ford’s English models and some iterations of the German Taunus. Curiously, Ford did not use struts on any U.S.-built models until the first Fox-platform Fairmont in 1978. Even early unitized Ford products like the 1958–1960 Lincoln and the original Ford Falcon retained double wishbones, although some of those cars used high-mounted springs (carried atop each upper wishbone) that are sometimes incorrectly described as struts. Ford briefly contemplated using MacPherson struts for the front suspension of the 1958 Ford Thunderbird, but eventually opted not to because the potential cost savings were outweighed by the lack of commonality with other Ford models.

Other manufacturers were slow to adopt MacPherson struts, presumably due to the preexisting patents, but in 1957, Lotus Engineering’s Colin Chapman essayed a novel variation on MacPherson’s theme for the Lotus Type 12 race car. The so-called “Chapman strut,” used only at the rear wheels, employed the double-jointed halfshafts as control arms, supplemented by a trailing link on each side. Lotus also used Chapman struts on the Type 14 Elite from 1959 to 1962, but abandoned them on the later Elan for a more conventional rear strut layout.

The MacPherson strut was certainly Earle MacPherson’s most recognized contribution at Ford, but far from the only one. Others included working with supplier Thompson Products to develop front suspension ball joints suitable for full-size American cars (first adopted by Lincoln in 1952 and Ford and Mercury in 1954) and pushing for the adoption of monocoque construction for the 1958 Thunderbird and Lincoln. MacPherson could be sharp-tempered and, like many determinedly rational people, had little patience for anything he viewed as frivolous, but his engineering talents were considerable.

1959 Ford Thunderbird hardtop front 3q © 2010 Aaron Severson

The 1958–1960 Ford Thunderbird did not use MacPherson struts (although Ford had seriously contemplated doing so), but was one of Ford’s first U.S.-market unitized cars, largely at MacPherson’s insistence. (Before anyone asks, the earlier Lincoln Zephyr and Continental‘s bridge-and-truss structure was not a monocoque in the modern sense.) (author photo)

MacPherson was promoted from executive engineer to chief engineer in 1949. In May 1952, he succeeded Harold Youngren as Ford’s vice president of engineering. Health problems and approaching retirement age prompted MacPherson to step down from that role in April 1957, succeeded by Andrew Kucher, but he remained with Ford for another year as vice president and engineering policy adviser. MacPherson died in 1960 at the age of 69.

A few years later, as MacPherson’s original patents expired, MacPherson strut suspensions began a rapid proliferation in the U.K., Europe, and Japan. Struts took longer to catch on among other Detroit automakers, particularly for their U.S.-built offerings, but today, there are very few automakers anywhere that don’t use MacPherson struts for at least some models — even companies like Honda, which had long eschewed struts in favor of double wishbones.

MACPHERSON STRUTS VS. DOUBLE WISHBONES

The MacPherson strut can be envisioned as a simplified version of the double wishbone layout that was virtually the default front suspension for American cars between about 1940 and 1980. That characterization doesn’t quite convey how advanced MacPherson’s ideas were for the mid-forties, but it does provide a useful starting point for understanding the basic principles.

A double wishbone front suspension locates each front wheel with two A-shaped control arms, usually of unequal length. The inner pivots of each A-arm are mounted on the frame rail or, on monocoque vehicles, a reinforced section of the body shell or a crossmember or subframe. The outer end of each A-arm is connected to the steering knuckle by a kingpin (or, later, ball joints) to allow the knuckle to turn with the steering wheel.

Diagram of a double wishbone suspension © 2014 Aaron Severson

A double wishbone front suspension with unequal-length A-arms, coil springs, and telescopic shock absorbers. Note that this is NOT to scale! (author diagram)

Double wishbone suspensions typically use coil springs mounted on the lower arm, acting against the frame rail or crossmember/subframe, although some cars instead use high-mounted coils acting on a reinforced section of the inner fender (which is generally feasible only with monocoque construction). Others trade the coil springs for torsion bars, generally mounted longitudinally and using the lower wishbones as lever arms. (There are also numerous other variations that are beyond our scope here.) The springs are sometimes but not always supplemented by an anti-roll bar connecting the left and right lower A-arms, compressing (by twisting) whenever one wheel rises or falls relative to the other.

When double wishbone suspensions were first introduced, they commonly used lever-action hydraulic shock absorbers with the upper wishbone acting as the lever. By the late forties, lever-action dampers were on their way out, at least in the U.S. industry; they would linger elsewhere into the seventies, notably on the MGB. Lever shocks were replaced by telescopic shock absorbers, usually mounted adjacent to or inside the springs.

Double wishbone suspension in a 1971 Chevrolet Chevelle hot rod © 2009 Aaron Severson

The double wishbone front suspension of a much-modified 1971 Chevrolet Chevelle. This appears to be an aftermarket installation, but it illustrates the basic components typical to most American cars from about 1950 to 1980: upper and lower A-arms supporting the wheel spindle and a coil spring and tubular shock absorber mounted on the lower arm. Not evident in this photo is a front anti-roll bar. On Chevrolet front suspensions of this vintage, the anti-roll bar usually connects to the lower control arm approximately where the empty hole is located, outboard of the spring. (author photo)

While it’s common today to think of double wishbones as the hot ticket for good handling, handling in the modern sense was really not in Detroit automakers’ vocabulary when independent front suspension (IFS) was first adopted in the mid-thirties. Instead, the principal goals were to improve ride quality, reduce steering effort, and eliminate the wheel shimmy that was endemic to using a beam axle with steered wheels.

Double wishbone suspensions have the following advantages:

  • Independent wheel action: The most obvious advantage of any independent suspension is that a one-wheel bump doesn’t necessarily affect both wheels. (In practice, this advantage is compromised by the presence of an anti-roll bar, which tries to force the wheels to remain on the same level and can cause the vehicle to rock or “waddle” back and forth over one-wheel disturbances.)
  • Low unsprung weight: One important factor in ride quality is unsprung weight, the portion of the vehicle’s mass not supported by its springs. In general, the lower the ratio of unsprung weight to total mass, the better the ride. While a double wishbone suspension’s A-arms may be relatively heavy, only a portion of that mass is actually part of the unsprung weight. Even in the early days of IFS, the unsprung weight of a double wishbone suspension was substantially less than that of a tubular beam axle. For example, Cadillac’s early double wishbone suspension (which still used kingpins and lever-action shocks) had more than 20% less unsprung weight than the previous solid axle layout.
  • Strength: The triangular shape of each wishbone makes it more rigid, allowing it to better resist bending and distortion and maintain proper alignment.
  • Flexible geometry: Double wishbone suspensions give chassis engineers considerable latitude in setting the various aspects of suspension geometry that influence how a vehicle rides and handles, allowing engineers to fine-tune the chassis balance by adjusting the length, mounting points, and relative angles of the A-arms. (This is one of the reasons double wishbones are still preferred for race cars.) Notably, double wishbones permit:
    • Anti-dive: Mounting the A-arms’ front and rear inner pivots at different heights (in essence tilting the wishbone upward) can produce an effect called anti-dive, which partially counters the forward weight transfer that causes the nose to dip when the brakes are applied.
    • Camber gain: Tires have the most traction when they are perpendicular to the road surface — that is, when their camber is zero. A beam axle forces the wheels to maintain a constant camber, which keeps them upright going over bumps, but forces the wheels to lose camber as the body leans, reducing the tires’ cornering power. With double wishbones, camber loss can be partially mitigated by using unequal-length, non-parallel wishbones. If the lower wishbone is longer than the upper, the lower ball joint will move outward more quickly than the upper ball joint as the body leans. This allows the wheel to remain more nearly upright, an effect called camber gain. (It should be noted that not all double wishbone suspension are actually set up to provide meaningful camber gain.)
    • Long swing-arm length: Camber gain can be a double-edged sword because it is inversely proportional to effective swing-arm length, the radius of the arc the wheel traverses as it jounces or rebounds on its spring. (This length is not a constant because it decreases as the spring compresses.) A short swing-arm length, as on a swing-axle suspension, provides ample camber gain, but can also introduce new problems, including undesirable camber changes caused by road disturbances and a tendency toward jacking, where the suspension arm acts as a lever, pushing the body upward (a problem described in greater detail in our Corvair article). With a double wishbone suspension, the swing-arm length is a function of the lengths and relative angles of the A-arms (and can be several times greater than the width of the car), which allows chassis engineers to select a length that will provide a useful degree of camber gain without making the ride and handling erratic.

Double wishbones also have several significant drawbacks:

  • Cost: Double wishbone suspensions have a lot of components (particularly compared to a beam axle) and cost more to manufacture and assemble than simpler alternatives.
  • Weight: While double wishbones have less unsprung weight than a beam axle, their total mass can actually be greater, which is one of the reasons many modern B-segment cars still use beam axles in back. That mass can be reduced by using lightweight aluminum or magnesium components, but that drives up costs even further.
  • Width: Unless the spring is mounted atop the upper arm (as on the Falcon or the Rambler), double wishbone suspensions are not very tall, but they are wide. That’s not a major concern for full-size American cars, but is a problem for smaller cars, particularly ones with transverse engines, potentially forcing unhappy compromises in packaging or suspension geometry.

The goal of the MacPherson strut was to mitigate these drawbacks by reducing the number of components. In the “classic” MacPherson strut front suspension, as defined by MacPherson’s 1949 patent application, the steering knuckle is rigidly connected to the base of a tubular shock absorber to form a more or less vertical strut with a coil spring wound around it. The strut’s upper mount includes a ball joint that allows the entire strut to turn with the front wheels.

MacPherson strut front suspension diagram © 2014 Aaron Severson

Head-on view of a MacPherson strut front suspension. The spindle (red) is rigidly connected to the lower portion of the strut (dark blue). As the front wheels are steered, the strut pivots on its upper and lower ball joints (green). A lower control arm (orange) is triangulated by the anti-roll bar (black). Please note that this diagram is not even remotely to scale! (author diagram)

The strut assembly completely replaces the double wishbone suspension’s upper A-arm, performing the upper wishbone’s locating duties as well as providing steering, springing, and damping. The lower wishbone, meanwhile, is replaced by a simpler transverse control arm (sometimes called a track control arm or TCA), which is connected to the base of the knuckle via a ball joint. An anti-roll bar connects the right and left control arms, which serves to triangulate each track control arm (allowing it to act like a wishbone) as well as performing the anti-roll bar’s normal functions.

MacPherson strut suspension of a late-90s Saturn SL1 © 2009 Aaron Severson

A badly damaged late-nineties Saturn SL1 shows off its MacPherson strut front suspension. As you can see, on a strut suspension, the double wishbone suspension’s upper arm is omitted entirely and its locating function is provided by the shock absorber/strut, which is affixed directly to the wheel spindle. The simple transverse control arm is triangulated by the anti-roll bar — note how the anti-roll bar and control arm form a wishbone shape. Also note the location of the engine driveshaft (identifiable by the rubber CV boots on each end). With a MacPherson strut, there are no suspension components to block the halfshaft, one of several reasons struts are very common on front-wheel-drive cars. (author photo)

The savings this arrangement provides in weight and cost are fairly obvious: fewer components, fewer parts to buy or manufacture, and fewer operations required to install the suspension in a car. However, MacPherson struts involve a number of tradeoffs:

  • Height: MacPherson struts take up less space horizontally than double wishbones, which is useful for narrow compact cars with transverse engines and front-wheel drive — not a consideration in the forties, but definitely significant now. However, struts are generally taller than are double wishbones, which may require a higher hood line. This again was not a major concern when the layout was first developed (and is becoming less of an issue today thanks to European pedestrian safety standards), but has forced some low-slung cars with struts (e.g., the 1991–2001 Mitsubishi GTO/3000GT and Dodge Stealth) to resort to fender blisters to cover the tops of the shock towers.
  • Unsuitability for body-on-frame vehicles: One of the reasons double wishbone front suspension is common on body-on-frame vehicles is that the control arms can be mounted on a frame member, allowing the frame to bear all suspension and spring loads. By contrast, a MacPherson strut transmits its spring loads directly into the body, which must be strong enough and rigid enough to handle those stresses without twisting or distorting. That usually requires a unitized body with reinforced shock towers or fender aprons. (While MacPherson’s 1949 patent application suggests that struts can be applied to vehicles with a separate frame, we can’t think of any body-on-frame vehicle that uses struts with high-mounted coil springs.)
  • Limited camber gain: Because a MacPherson strut’s upper ball joint is at the top of the strut, above the spring, the effective upper control arm length is quite long and the spindle height (the vertical distance between the upper and lower ball joints) is very large. Both of these factors serve to lengthen the effective swing-arm length, which minimizes camber changes as the wheel moves through its travel (good), but also sharply limits any potential camber gain (not so good). That means significantly limiting camber loss due to body lean means (a) lowering the center of gravity and/or widening the track (not always feasible); (b) increasing roll stiffness (which can have negative effects on both ride quality and handling balance); or (c) altering the wheel alignment to include a few degrees of static negative camber (which can result in uneven tire wear in normal driving). This doesn’t mean cars with MacPherson struts can’t ride and handle well, but it is an intrinsic limitation.
  • Large scrub radius: Scrub radius (also known as kingpin offset) is the distance between the horizontal center of the tire’s contact patch and the point where the kingpin axis (the imaginary line connecting the upper and lower ball joints) intersects the ground. The shorter this distance, the less effect road disturbances or cornering forces will have on the steering. Because a MacPherson strut puts the upper ball joint atop the strut, minimizing the scrub radius typically requires either using narrow tires or increasing the kingpin inclination (i.e., tilting the strut toward the car’s center line), which reduces the effectiveness of the shock absorber and causes caster loss as the wheels are turned off center or the springs compress.
  • Higher replacement costs: MacPherson struts may cost less to manufacture and install than double wishbones, but that doesn’t necessarily make struts any less expensive to service or replace. In fact, replacing a worn-out strut often costs more than replacing a conventional shock absorber, particularly if the vehicle’s struts don’t allow the damper (which usually wears out well before the spring) to be replaced without replacing the entire strut.
double wishbone suspension scrub radius diagram © 2014 Aaron Severson

Scrub radius is the horizontal distance between the center of the tire’s contact patch and the point where the kingpin axis (represented here by the blue line) intersects the ground plane. The illustration shows a double wishbone suspension (not to scale) with a small positive scrub radius. If the kingpin axis had intersected the ground on the opposite side of the contact patch center, the scrub radius would be negative. (author diagram)

REAR STRUTS

We typically think of MacPherson struts being used only at the front, but as Earle MacPherson’s 1949 patent application noted, they can also be used at the rear. Four-wheel struts were very common on FWD sedans of the eighties and nineties, but in recent years have been largely supplanted by beam axles for cheaper cars and multilink rear suspensions for more expensive models.

A rear strut is basically similar to a front strut, but can dispense with ball joints (unless the vehicle has four-wheel steering) and typically uses trailing links to triangulate the lower arms and transmit braking forces to the body. (Cars that have rear struts can and often do use rear anti-roll bars, but the bar generally does not contribute to wheel location.)

As noted on the previous page, the Chapman strut, devised by Lotus in 1957, is a type of rear strut suspension in which the axle halfshafts do double duty as lower control arms, supplemented by a single trailing link or trailing arm on each side. The term “Chapman strut” is sometimes incorrectly applied to any rear strut suspension (a mistake we also made in an earlier version of this article), but more properly applies only to struts that uses the halfshafts as locating arms.

MACPHERSON STRUT VARIATIONS

Over the years, there have been innumerable variations on MacPherson’s original design. Some common modifications include:

  • Omitting the anti-roll bar: While MacPherson’s dual-function anti-roll bar is clever, cheap, and elegant, a front anti-roll bar is not always desirable, particularly for lightweight, front-heavy cars that already have fairly stiff front springs. However, if the front anti-roll bar is omitted, some other means must be provided for triangulating the lower control arms. Some automakers resolve this dilemma by replacing the lower arm with a lower wishbone. Others, including early British Mk1 Ford Escorts and the Mk1 and Mk2 Ford Fiesta, use radius rods (leading or trailing links) to locate the lower control arm. One advantage of using radius rods in this way is that they can be designed to allow some fore-aft compliance for better ride quality.
  • Single-function anti-roll bar: Having the anti-roll bar do double duty as a radius arm may force designers to accept a spring rate for the bar that is either higher or lower than ideal for optimum ride and handling. Therefore, it’s sometimes desirable to locate the lower control arms with radius rods or use lower wishbones even if the vehicle has a front anti-roll bar. This costs and weighs more, but allows better anti-roll bar geometry and more freedom in selecting the bar’s spring rate. An additional advantage is that with either a lower wishbone or a “wishbone” formed by a control arm and a radius rod, it is possible to provide a measure of anti-dive geometry by setting the front mounting point at a different height than the lower ball joint.
  • Relocated springs: Mounting the coil spring around the upper part of the strut is simple and tidy, but, as noted above, requires the fender structure to be reinforced to withstand spring loads, resulting in tall, bulky strut towers. An alternative is to relocate the spring to the lower control arm, as in a typical double wishbone suspension. Some manufacturers have used coil springs in this manner, but a few (notably Porsche) have substituted longitudinal torsion bars, usually using the lower wishbone or control arm as a lever. Either way, the primary advantages are better packaging and a lower fender line; struts without high-mounted coil springs are also compatible with body-on-frame construction, which conventional struts are not. Struts with offset springs (or torsion bars) are sometimes called “modified MacPherson struts,” although technically any of these variations could be so described.
  • Double-pivot struts: Patented by BMW in the late seventies and applied to the E23 7-Series and many subsequent BMW cars, a “Doppelgelenk” front suspension locates the strut with a conventional lower control arm triangulated by a short diagonal leading link. The lower control arm is attached to the spindle via a ball joint in the conventional manner. The diagonal link connects to the strut via a second ball joint mounted above and slightly ahead of the first. Together, the link and control arm form a wishbone angled upward at the front to provide anti-dive. More significantly, the additional lower ball joint serves to alter the kingpin inclination and therefore the scrub radius. With two lower ball joints, the kingpin inclination is determined by the line between the upper ball joint and the point where the axes of the lower control arm and leading link intersect (the virtual steer center). The virtual steer center moves as the front wheel turns, so the scrub radius is no longer a constant, increasing as the wheel is steered away from center. The idea is to reduce bump steer in straight-ahead cruising while increasing self-centering action in turns.
Diagram of a modified MacPherson strut with low-mounted coil springs © 2014 Aaron Severson

A so-called “modified” MacPherson strut with low-mounted coil springs. Suspensions like this were used on the Ford Fox platform (used by the Ford Fairmont and the 1979–1993 Ford Mustang) and the third-generation F-body Chevrolet Camaro and Pontiac Firebird. (author diagram)

SUPER STRUT

All of the above variations are fairly straightforward and by now quite common. A significantly more elaborate variation has emerged more recently, driven by the the emergence of powerful FWD sporty cars. The challenge for automakers is to fortify those models to cope with a big infusion of horsepower without sacrificing their commonality with the mundane family sedans and hatchbacks on which they’re based, many of which use MacPherson struts for cost and packaging reasons.

In the early nineties, Toyota unveiled an optional front suspension package called Super Strut for certain sporty FWD and AWD models, including some versions of the AE101 and AE111 Corolla and Sprinter, the Celica and Curren coupes, and the Carina and Corona. (Super Strut was included on some export versions of these cars, but was never offered on any U.S. Toyota.) Put simply, Super Strut was an attempt to approximate the geometric advantages of a double wishbone suspension in a package that could interchange with Toyota’s standard MacPherson strut front suspension.

Super Strut suspension from a Toyota Celica GT-Four © 2005 Kris Carter (used with permission)

The (mostly) complete Super Strut assembly. The connector plate joining the two lower control arms (lower foreground) attaches to the bottom of the knuckle (not shown). The eyelet on the base of the strut is for the drop link that connects the strut to the front anti-roll bar (also absent here). Super Strut is theoretically interchangeable with standard MacPherson struts, although doing so requires replacing the entire front suspension, including spindles and brakes! (Photo © 2005 Kris Carter; used with permission)

On Super Strut cars, the base of each front strut (above the steering knuckle) has a curved extension shaped a bit like an inverted letter “C.” One end of the extension forms the mounting point for the upper ball joint, which is relocated to a point just below and outboard of the base of the strut. The lower end of the strut extension, meanwhile, is connected via a ball joint to a short lever arm (the assist link or “figure-eight” link), which is in turn connects (via another ball joint) to a point at approximately the center of the rear lower control arm (which Toyota calls the camber control arm).

Super Strut lower control arms from a Toyota Celica GT-Four © 2004 Kris Carter (used with permission)

The front and rear control arms of the Toyota Super Strut suspension, illustrating the way the assist link (“figure-eight” link) connects to the center of the rear (camber control) arm. The outer ends of the control arms connect to the front subframe, the rear arm (camber control arm) via a spherical joint. (Photo © 2004 Kris Carter; used with permission)

There is also an additional front lower control arm, a longer, curved arm that Toyota fans have dubbed the “banana bar.” The outer ends of both lower arms are connected via ball joints to a small connector plate on the steering knuckle, allowing the arms to pivot relative to one another as the knuckle turns. The front anti-roll bar, which does not contribute to wheel location, is connected to the strut itself via a drop link with ball joints at each end.

The geometry of this system is quite complicated, but there are five principal effects:

  1. Separating the kingpin axis from the strut. Since the upper ball joint is mounted outboard of the strut itself, Super Strut’s steering axis is similar to that of a double wishbone suspension, greatly reducing the scrub radius. An interesting side effect is that the strut no longer turns with the knuckle, although the strut does rock fore and aft as the wheel is steered.
  2. Creating a virtual steer center through the use of two lower ball joints. This appears to be analogous to the BMW Doppelgelenk system, although our information does not indicate to what extent Super Strut’s virtual steer center moves as the arms pivot.
  3. Reducing the spindle height by relocating the upper ball joint.
  4. Reducing caster changes as the springs compress or the wheels turn, which also serves to reduce camber changes in tight turns.
  5. Reducing the effective upper control arm length by pivoting the strut extension (via the “figure-eight” assist link) at the center of the rear lower control arm.

The end results are reduced torque steer — an important consideration when putting a lot of power through steered wheels — and significantly more camber gain than a conventional MacPherson strut would permit, improving front-end grip. To take advantage of the new geometry, Toyota specified wider, more aggressive tires and bigger front disc brakes (with two-piston calipers) for most Super Strut applications.

Super Strut suspension installed in a Toyota Celica GT-Four © 2004 Kris Carter (used with permission)

Super Strut as installed in a car, viewed from the rear. Note the way the curved strut extension connects to the assist link and the camber control arm. (Photo © 2004 Kris Carter; used with permission)

Super Strut was effective, at least as long as it was in good repair, but the system was both heavy and expensive. Its sheer complexity also made it less reliable than a conventional strut. There was a lot to wear out and the components were pricey to replace if they did fail. Toyota discontinued its last Super Strut model around 2006.

PERFOHUB, HIPER STRUT, AND REVOKNUCKLE

In 2004, Renault introduced a loosely comparable system for the Mégane RenaultSport (RS) hot hatch. Like Super Strut, the Renault “double-axis” system separated the steering axis from the strut using a relocated upper ball joint and a broad lower arm with a separate anti-rotation link that allowed the knuckle to turn without turning the strut. This arrangement, now dubbed PerfoHub, is still used on some current Renault Sport models.

RevoKnuckle diagram from a 2009 Ford Focus RS. Copyright © Ford Motor Company

A diagram of RevoKnuckle as used in the 2009 Ford Focus RS. (Ford Motor Company)

In 2009, both Ford and GM introduced their own systems. Ford’s, called RevoKnuckle, was used to allow the turbocharged Focus RS to cope with 305 PS (224 kW) without resorting to all-wheel drive. During the same period, GM introduced its similar HiPer Strut, offered initially on the Opel/Vauxhall Insignia OPC and later on the Astra OPC, Buick Regal GS, and Buick LaCross CXS. RevoKnuckle and HiPer Strut differ from the Renault layout in detail, but are functionally similar.

Diagram of HiPer Strut in the 2010 Buick LaCrosse CXS - X10AR_BU003- Copyright 2014 General Motors LLC. Used with permission, GM Media Archive. (GMMA 19720p2)

The complete HiPer Strut installation, including front subframe, of the 2010 Buick LaCrosse CXS. (Image copyright 2014 General Motors LLC. Used with permission, GM Media Archive.)

Where HiPer Strut, RevoKnuckle, and PerfoHub differ from the earlier Toyota system is that the newer layouts do not attempt to replicate the complex geometry of Super Strut’s figure-eight link and camber control arm. The GM, Ford, and Renault systems do provide some additional camber gain due to their reduced spindle heights, but the manufacturers’ own descriptions generally downplay that point, stressing instead that the primary goals are to minimize torque steer and reduce steering kickback over bumpy roads.

Exploded diagram of HiPer Strut from the 2010 Buick LaCrosse - X10AR_BU002 - Copyright 2014 General Motors LLC. Used with permission, GM Media Archive. (GMMA 19720p2)

An exploded view of HiPer Strut. In case it’s not obvious, “LCA” stands for “lower control arm.” (Image copyright 2014 General Motors LLC. Used with permission, GM Media Archive.)

The new systems are likely cheaper than the Toyota Super Strut layout, although there is still a significant cost and weight penalty that may limit the use of these suspensions to more expensive, more powerful models. It remains to be seen how broadly these layouts will be adopted.

CONCLUSION

The evolution of the MacPherson strut has been comparable to that of other automotive innovations like automatic transmission, which has also seen many changes and innumerable additions and modifications intended to minimize its weaknesses without sacrificing its basic selling points.

As with automatic, some of the MacPherson strut’s variations have caught on while others have not, but the layout’s virtues are still compelling enough to make it ubiquitous, if not quite universal. We have no doubt that Earle MacPherson’s strut suspension will remain in common use long after his name is forgotten. Many people have already forgotten how to spell it!

# # #

ACKNOWLEDGMENTS

The author would like to thank Jamie Myler of Ford Archives for providing archival images of Earle MacPherson, the Ford Consul, and RevoKnuckle (as well as a copy of Ford’s 1957 press bio of MacPherson); Kathy Adelson of GM Media Archives for her help in locating archival images of the Chevrolet Cadet and the HiPer Strut diagrams; and Kris Carter of the Celica GT-Four Drivers Club for the use of his Super Strut photos.


NOTES ON SOURCES

Our sources on the life of Earle MacPherson, the origins of the Chevrolet Cadet, and the origins of the MacPherson strut suspension included Herb Adams, Chassis Engineering (HP1055) (New York: HPBooks, 1993); the Auto Editors of Consumer Guide, Cars That Never Were: The Prototypes (Skokie, IL: Publications International, 1981); and “1951-1956 Ford Consul and Zephyr,” HowStuffWorks.com, 11 October 2007, auto.howstuffworks. com/ 1951-1956-ford-consul-zephyr.htm, last accessed 25 June 2014; Griffith Borgeson, “How Leland Lost Lincoln to Ford: The little-known battle of two Henry’s: dedication to an ideal vs. big business,” Motor Trend Vol. 19, No. 2 (February 1967): 58–62, 82–83; “British Fords Get U.S. Look,” Popular Science Vol. 158, No. 3 (March 1951): 150–151; David A. Crolla, ed., Automotive Engineering: Powertrain, Chassis System and Vehicle Body (Burlington, MA: Butterworth-Heinemann/Elsevier, 2009); “Editor’s Note: Earl S. MacPherson and His Invention” [the article whose errors prompted the original version of this article], VW Trends 23 April 2003, www.vwtrendsweb. com/features/ 0306vwt_macpherson_strut_suspension/, last accessed 27 June 2014; Craig Fitzgerald, “Earl S. MacPherson” [note that he too misspells MacPherson’s name!], Hemmings Sports & Exotic Car #3 (November 2005); “Ford,” Autodriver Vol. 57 (1957): 93; Ford Motor Company, Annual Report, 1957, and “MacPherson, Earle S. – Biography” [press release], 26 April 1957; Ford-Werke A.G. Köln, “Taunus 17M” [German brochure 9 P 115/2], 1957; Ken Gross, “Stovebolt Six with an Aussie Accent: 1948 Holden,” Special Interest Autos #49 (February 1979), pp. 26-33, 62; “Icons: Earle MacPherson,” Motor Trend Vol. 58, No. 3 (March 2006); Michael Lamm, “The Imagineer William B. Stout: Automobile and Airplane, His Goal Was to See Them Wedded,” Car Life Vol. 14, No. 7 (August 1967): 54–58; David L. Lewis, “Ford’s Postwar Light Car,” Special Interest Autos #13 (October-November 1972): 22–27, 57; and “Lincoln Cosmopolitan: The Gleam in Edsel Ford’s Eye,” Car Classics April 1973, reprinted in Lincoln Gold Portfolio 1949-1960, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1990): 5–17; Karl Ludvigsen, “The Truth About Chevy’s Cashiered Cadet,” Special Interest Autos #20 (January-February 1974), pp. 16–19; Mike McCarthy, “Honda’s Headliners,” Wheels August 1985: 38–43; “Necrology,” Automotive Industries Vol. 122 (1960): 53; “News,” Motor Truck News Vol. 47 (1958): 91; news, SAE Journal Vol. 34 (1934): 71; Jan Norbye, “Half-Hour History of Unit Bodies,” Special Interest Autos #18 (August-October 1973): 24–29, 54; “Personals,” Iron Age Vol. 160 (1947): 104; “Personals,” Iron Age Vol. 164 (1949): 43; personnel news, Electro-Technology Vol. 33, No. 3 (1944): 234; Don Sherman, “Volvo 242GL,” Car and Driver Vol. 20, No. 7 (January 1975); William K. Toboldt and Larry Johnson, Goodheart-Willcox Automotive Encyclopedia (South Holland, IL: The Goodheart-Willcox Company, Inc., 1975); the Suspensions section of Mark Wan’s excellent AutoZine Technical School (1997–2011, www.autozine. org/ technical_school/ suspension/ Index.html); Mary Wilkins and Franck Hill, American Business Abroad: Ford on Six Continents (Detroit: Wayne State University Press, 1964); and of course MacPherson’s patents: Earle S. MacPherson, assignor to General Motors, “Vehicle Wheel Suspension System,” U.S. Patent No. 2,624,592, filed 21 March 21 1947 and issued 6 January 1953; and Earle S. MacPherson, assignor to Ford Motor Company, “Wheel Suspension for Motor Vehicles,” U.S. Patent No. 2,660,449, filed 27 January 1949 and issued 24 November 1953.

Some information of the many variations on the theme included Sam Abuelsamid, “Quick Spin: 2010 Buick LaCrosse CXS gets HiPer,” Autoblog, 25 March 2010, www.autoblog. com, accessed 23 June 2015; Ian Adcock, “Renault Megane 265 Trophy,” Road & Track 20 January 2012, www.roadandtrack. com, accessed 12 July 2014; James M. Amenda, “GM’s HiPer Strut Rethinks MacPherson Suspension,” Wards Auto, 30 March 2010, wardsauto. com, accessed 17 July 2014; Ben Barry, “Ford Focus RS (2009) CAR test review,” CAR 22 February 2009, www.carmagazine. co.uk, accessed 12 July 2014; Patrick Bedard, “BMW 733i,” Car and Driver Vol. 23, No. 10 (April 1978), reprinted in BMW 7 Series Performance Portfolio 1977–1986, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1999): 29–33, 48; “BMW’s new big ones,” Autocar 28 May 1977, reprinted in BMW 7 Series Performance Portfolio 1977–1986: 5–9; “Brief Test: Peugeot 205 CTI,” Motor 2 August 1986: 46–49; Kris Carter, “Changing 205 Super Strut Suspension,” Icy Designs, March 2010, diy.icydesigns. com/ article/ 93/Toyota/ Celica/ 1994-99_(ST20x)/ Suspension/Brakes/ Steering/ Changing_205_Super_Strut_Suspension/, accessed 10 July 2014; Yung Chang Chen, Po Yi Tsai, and I An Lai, “Kinematic Analysis of Roll Motion for a Strut/SLA Suspension System,” World Academy of Science, Engineering and Technology Vol. 6, No. 5 (2012): 1172–1176; K.C. Colwell, “Ford RevoKnuckle and GM HiPer Strut Explained,” Car and Driver Vol. 56, No. 9 (March 2011), www.caranddriver. com, accessed 12 July 2014; Csaba Csere, “F-Tech,” Car and Driver Vol. 27, No. 7 (January 1982): 46–48; Richard Doig, “ST205 Superstrut Suspension,” New Zealand GT-Four Homepage, 2007, gtfour.supras. org.nz/ superstrut.htm, accessed 8 July 2014; Dan Edmunds, “Straight Line: 2010 Buick LaCrosse CXS: Hiper [sic] Strut Suspension Walkaround,” Inside Line, 2 April 2010, blogs.insideline. com/ straightline/, accessed 23 June 2015; ‘elansprint71,’ Roger French, and Peter Ross, “The ‘Chapman’ strut,” LotusElan.net, 16 November 2011 to 11 February 2012, www.lotuselan. net/ forums/ lotus-suspension-f42/ the-chapman-strut-t24055.html, accessed 24 June 2015; Ford Motor Company, “New Ford Focus RS: A Legend Returns” [press release], 4 July 2008; “Ford’s happy event,” Autocar 17 July 1976: 12–21; General Motors, “Buick LaCrosse’s Innovative HiPer Strut Suspension Delivers Improved Ride and Handling” [press release], 24 March 2010; Akira Higuchi, assignor to Toyota Jidosha Kabushiki Kaisha, “Suspension of Vehicle,” U.S. Patent No. 4,844,505, filed 14 December 1987 and issued 4 July 1989; “Kingpin Inclination, Caster, etc. for MacPherson Strut” [forum discussion], Eng-Tips, 20 January – 24 January 2005, www.eng-tips. com/ viewthread.cfm?qid=113181, accessed 27 July 2014; Randy Leffingwell, Mustang: America’s Classic Pony Car (Ann Arbor, MI: Lowe & B. Hould Pub., 1999); A.K. Legg, Peugeot 205 Owners Workshop Manual 1983 to 1985 (Sparkford Nr Yeovil, Somerset: Haynes Publishing Group, 1986); “Lotus Elite / Lotus Type 14,” Unique Cars and Parts, n.d., www.uniquecarsandparts. com.au, accessed 24 June 2014; Wolfgang Matschinsky, assignor to Bayerische Motoren Werke Aktiengesellschaft [BMW A.G.], “Independent Suspension of a Steered Wheel of a Motor Vehicle,” U.S. Patent No. 4,145,072, filed 30 March 1977 and issued 20 March 1979; John Miles, “More than Skin Deep,” Autocar 20 June 1981, reprinted in BMW 5 Series Gold Portfolio 1981–1987, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1993): 5–8; Chuck Nerpel, “Driving Impression: BMW 733i,” Motor Trend Vol. 30, No. 4 (April 1978), reprinted in BMW 7 Series Performance Portfolio 1977–1986: 26–29; new car price guide, 5 November 1993, Car and Driver (Japan Edition) 10 December 1993: 220–230; Yasushi Nishikiuma et al, assignors to Toyota Jidosha Kabushiki Kaisha, “Suspension for Steerable Driving Wheel in Vehicle,” U.S. Patent No. 5,048,859, applied 30 May 1990 and issued 17 September 1991; David Phipps, “The Lotus Elite,” Profile Publications #48 (1967); “Renault Mégane RenaultSport Review,” Autocar, n.d., www.autocar. co.uk, accessed 12 July 2014; “Renaultsport Megane 250 gallery,” Evo October 2009, www.evo. co.uk, accessed 16 July 2014; “Road Test: Peugeot 205 GTI,” Motor 26 May 1984: 20–23; Toshiyasu Santo, assignor to Toyota Jidosha Kabushiki Kaisha, “Strut Type Suspension,” U.S. Patent No. 4,995,633, filed 31 August 1989 and issued 26 February 1991; Don Sherman, “Ford Fairmont: Ford builds a Volvo, and it works,” Car and Driver Vol. 23, No. 3 (September 1977): 27–34; Showtime4U, “Toyota trueno & levin AE101 introduction year 1991,” YouTube, [disabled video], uploaded 24 February 2014, accessed 27 July 2014; “Super Strut Suspension,” Toyota Corolla FX GT, 2009, www.oocities. org/fxgt20/ superstrut.htm, accessed 26 July 2014; “Super Strut Suspension” and “205 Suspension,” Celica GT-Four Drivers Club, 2013, www.gtfours. co.uk/ what/ sstrut/ sstrut.htm and www.gtfours. co.uk/ pics/ suspension/ 205/sus.htm, accessed 10 July 2014; “Superstrut Suspension, ST202 – ST 205 suspention [sic],” 6G Celicas Forums, 18 August 2005 – 13 July 2014, www.6gc. net/ forums/ index.php?showtopic=28093, accessed 27 July 2014; Toyota Motor Corporation, 75 Years of Toyota: Vehicle Lineage, www.toyota-global. com, accessed last accessed 15 April 2014; Toyota Motor Sales, “Celica” [CD0016-9909], September 1999; “Corolla Levin” [Japanese brochure CE0047-9106], June 1991; and “Corolla Levin” [Japanese brochure CE0022-9505], May 1995; Ron Wakefield, “Ford Fiesta: Henry builds a minicar,” Road & Track Vol. 28, No. 2 (October 1976): 34–36; Jeremy Walton, Escort Mk 1, 2 & 3: The Development & Competition History (Sparkford, Somerset: Haynes Publishing Group, 1985); Mark Wan, “Focus RS,” AutoZine, 4 June 2009, www.autozine. org/ Archive/ Ford/old/ Focus_Mk2.html#RS, accessed 16 July 2014; “Megane RS (Renault Sport),” AutoZine, 23 November 2009, www.autozine. org/ Archive/ Renault/new/ Megane_III.html, last accessed 12 July 2014; “Opel Astra,” AutoZine, 18 June 2012, www.autozine. org/ Archive/ Opel/new/ Astra_2009.html, last accessed 12 July 2014; “Opel Insignia/Buick Regal,” AutoZine, 27 November 2011, www.autozine. org/ Archive/ Opel/new/ Insignia.html, last accessed 12 July 2014; and “Renault Megane,” AutoZine, 22 December 2006, www.autozine. org/ Archive/ Renault/old/ Megane_II.html, last accessed 12 July 2014; and xreknil, “Toyota superstrutsuspension [sic],” YouTube, https://youtu.be/c9tj43J8cJk, uploaded 12 June 2012, accessed 27 July 2014.

Additional background sources included the Auto Editors of Consumer Guide, Encyclopedia of American Cars: Over 65 Years of Automotive History (Lincolnwood, IL: Publications International, 1996); Bob Bolles, “Camber Crutches,” Circle Track 18 June 2004, www.circletrack. com, accessed 17 July 2014, and “Understanding and Learning Caster and Camber Settings,” Circle Track 2 April 2013, www.circletrack. com, accessed 17 July 2014; John R. Bond, “Porsche 901,” Road & Track Vol. 15, No. 4 (December 1963): 24–26; Gérard Bordenave, “Ford of Europe, 1967-2003,” Cahiers du GRES [Groupement de Recherches Economiques et Sociales, Université Monteqsquie-Bordeaux and Université des Sciences Sociales Tolouse], Cahier No. 2003-11 (September 2003); Carl Breer and Anthony J. Yanik, The Birth of Chrysler Corporation and Its Engineering Legacy, Second Edition (Warrendale, PA: Society of Automotive Engineers, Inc., 1960); Douglas Brinkley, Wheels for the World: Henry Ford, His Company, and a Century of Progress (New York: Viking Penguin, 2003); Arch Brown, “Chrysler’s Magnificent Mistake: 1934 Airflow ‘CU,'” Cars & Parts August 1992: 22–26; and “1935 Plymouth: ‘A New Picture of Car Value,'” Special Interest Autos #172 (July-August 1999), reprinted in The Hemmings Book of Plymouths: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2002): 14–21; Arch Brown, Richard Langworth, and the Auto Editors of Consumer Guide, Great Cars of the 20th Century (Lincolnwood, IL: Publications International, Ltd., 1998); Darren Burnhill, “How Caster Angle Affects Camber Angle,” RC Tek, 2009, www.rctek. com, accessed 17 July 2014; Bill Carroll, “Inside Pontiac’s Terrific Tempest!” Sports Cars Illustrated October 1960, reprinted in Car and Driver on Pontiac 1961–1975, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1985): 5–7, 16; Floyd Clymer, Floyd Clymer’s Independent Test Report and Investigation of 1949 Ford Cars (Los Angeles: Floyd Clymer, 1948); John A. Conde, “Rare Pair: Twin 1948 Nash Convertibles,” Special Interest Autos #37, (November-December 1976): 34–39, 64–65; Mike Covello, ed., Standard Catalog of Imported Cars 1946–2002, Second Edition (Iola, WI: Krause Publications, 2001); “Engineering the Camaro,” Car Craft January 1968, reprinted in Camaro Muscle Portfolio 1967–1973, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1992): 43–46; Edward Eves, “Designs Analyzed-8: Alfa Romeo Giulia GTV,” Autocar 21 December 1967, reprinted in Alfa Romeo Giulia Coupes Gold Portfolio 1963–1976, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1992): 68–71; Joe Fabrini, “Opel Olympia,” Better Parts Ltd., 7 July 2013, betterparts. org, accessed 28 July 2014; Jim and Cheryl Farrell, Ford Design Department: Concepts and Showcars 1932–1961 (Roseburg, OR: Jim and Cheryl Farrell, 1999); Guido Fornaca, “Wheel-Suspension Means for Motor Vehicles,” U.S. Patent No. 1,711,881, filed 13 July 1927, issued 7 May 1929; Edwin Storm’s Free Car Brochures website at the Old Car Manual Project (storm.oldcarmanualproject. com); David Halberstam, The Reckoning (New York: William Morrow and Company, 1986); Maurice D. Hendry and Dave Holls, Cadillac: Standard of the World: The Complete History, Fourth Edition (Princeton, N.J.: Automobile Quarterly, 1990); Tim Howley, “Full Dress Falcon 1963 Sprint V-8,” Special Interest Autos #67 (January-February 1982), The Hemmings Motor News Book of Postwar Fords: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2000): 96–103; “1958 Lincoln Continental: Bigger and Better?” Special Interest Autos #157 (January-February 1997), reprinted in The Hemmings Book of Lincolns: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2002): 96–103; “1953 Ford: The Final Flathead,” Special Interest Autos #56 (March-April 1980), reprinted in The Hemmings Motor News Book of Postwar Fords: 40–47; and “There’s a Ford in Your Future: The Start of Ford’s Postwar Comeback,” Special Interest Autos #110 (April 1989): 36–39, 62–63; Mike Kojima, “The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub,” MotoIQ, 27 December 2011, www.motoiq. com, accessed 17 July 2014; David Knowles, MG: The Untold Story (Osceola, WI: Motorbooks International, 1997); Michael Lamm, “Bathub!” Special Interest Autos #9 (January-March 1972), reprinted in The Hemmings Book of Nashes: driveReports from Special Interest Autos magazine, ed. Richard Lentinello (Bennington, VT: Hemmings Motor News, 2002): 90–95; “Fishbowl: 1955 Ford Crown Victoria Skyliner,” Special Interest Autos #37 (November-December 1976), reprinted in The Hemmings Motor News Book of Postwar Fords: 48–55; “Magnificent Turkey,” Special Interest Autos #16 (April-May 1973) reprinted in The Hemmings Book of Pre-War Chryslers: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2002): 34–41; “1950 Mercury,” Special Interest Autos #12 (August-September 1972), reprinted in The Hemmings Book of Mercurys: driveReports from Special Interest Autos magazine, ed. Terry Ehrich (Bennington, VT: Hemmings Motor News, 2002): 48–63; “1950 Nash Rambler: America’s First Successful Post-war Compact,” Special Interest Autos #24 (September-October 1974), reprinted in The Hemmings Book of Nashes: 84–89; and “1934 Pontiac 8,” Special Interest Autos #19 (November-December 1973), reprinted in The Hemmings Motor News Book of Pontiacs: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2001): 12–16; Richard M. Langworth, The Thunderbird Story: Personal Luxury (Osceola, WI: Motorbooks International, 1980); C.A. Leslie, Jr., “Multi Cylinders and a New, Inadvertent Luxury Car: Engineering the Twin Six and the Twelve, the Ninth through Seventeenth Series 1932-1939,” Packard: A History of the Motor Car and the Company (Automobile Quarterly Magnificent Marque Books), Third Edition, ed. Beverly Rae Kimes (Princeton, NJ: Automobile Quarterly Publications/CBS Inc., 1978): 398–417; “London Show at Earls Court …” Automotive Industries Vol. 77, No. 19 (6 November 1937): 676–681, 686; Karl Ludvigsen, “SCI Analyzed Ed Cole’s Corvair,” Sports Cars Illustrated November 1959, reprinted in Corvair Performance Portfolio 1959–1969 (Cobham, England: Brooklands Books Ltd., ca. 1998): 5–13, 17; Steve Lyman, “Suspension 101,” Collegiate Design Series (Detroit: DaimlerChrysler AG, 2002); Alex Meredith, “SIA comparisonReport: 1948 and 1951 Chevrolet First vs. Second-Generation Fastbacks,” Special Interest Autos #82 (July-August 1984), reprinted in The Hemmings Book of Postwar Chevrolets: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard A. Lentinello (Bennington, VT: Hemmings Motor News, 2001): 18–25; “1937, Vauxhall Goes Unibody,” Generations of GM History, GM Heritage Center, n.d., history.gmheritagecenter. com, accessed 28 July 2014; “1951 Chevrolet,” Special Interest Autos #17 (June-July 1973), reprinted in The Hemmings Book of Postwar Chevrolets: 12–17; N.V. Nederlandsche Ford Automobiel Fabriek, “Taunus 12M-56/57” [Dutch brochure H56-D13-A1-12000], 1956; “Opel History 1930–1939,” Opel Media – Europe, n.d., media.gm. com, accessed 28 July 2014; “Preliminary Road Test: Ford Falcon,” Sports Cars Illustrated November 1959, reprinted in Falcon Performance Portfolio 1960–1970, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1998): 4–7; Michael Sedgwick, Classic Cars of the 1950’s and 1960’s (Twickenham, England: Tiger Books International PLC, 1983); Don Sherman, “The Econoboxes: All the small cars that are fit to drive, Part One,” Car and Driver Vol. 24, No. 1 (July 1978): 33–56; Wayne Thoms, “Rambler American,” Motor Trend Vol. 15, No. 2 (February 1963), reprinted in AMC Rambler Limited Edition Extra 1956–1969, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 2004): 58–60; “Thunderbird: A Real Change… for the Better,” Motor Trend Vol. 13, No. 5 (May 1961), reprinted in Thunderbird 1958–1963, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1990): 60–63; and Jim Wright, “Mustang GT-350,” Car Life April 1965, reprinted in Shelby Mustang Ultimate Portfolio 1965–1970, ed. R.M. Clarke (Cobham, England: Brooklands Books, 2003): 12–14.


38 Comments

Add a Comment
  1. You can still get some camber change out of a strut.

    Looking at your diagram, when the suspension compresses, the lower control arm will move through an arc that will push the bottom of the knuckle outward.

    The bottom of the strut can’t rotate relative to the knuckle, so its upper mount will have to flex some to accommodate the change in angle.

    Nitpicking aside, it’s always cool to learn the story behind an eponym. When can we learn about Mr Cardan and his double-joint?

    1. MacPherson struts do have some camber change in compression, as do most independent suspensions other than pure trailing arms, but the amount is very small because the effective swing-arm length is quite long. What they don’t provide is camber [i]gain[/i] with body roll, the way a double-wishbone suspension or swing-arm suspension does. As the body leans and the outside spring compresses, the arc of the lower control arm does push the lower edge of the spindle outward (which would tend to create negative camber), but the magnitude of that force is much less than that exerted by the strut as body lean pushes it outward (which tends to create positive camber). The result is always a net camber loss.

      The fact that increasing positive camber is called camber loss and creating negative camber is called camber gain is the kind of thing that gives me a headache.

      Gerolamo Cardano didn’t actually create the DCJ, since he died in the 16th century — the universal joint was named after him because he invented the concept, but the double Cardan joint was a refinement developed many years after his death. He was an interesting guy, though…he and André Citroën would have gotten along well.

  2. Hmmm…true. Suspensions are funny because major differences in ride, handling and character are wrung from tiny geometry tweaks. Through much engineering education, differences in angle that we’re talking about would all be considered “vertical” to a reasonable approximation.

    The funny part about a double-cardan joint is that when there’s there’s one u-joint, it’s usually called a u-joint. When there are two (like the one I blew up on my Jeep recently), it’s called a double-cardan or CV, even though it’s not really a constant-velocity joint. No one ever talks about a single-cardan joint.

    The backwards/upside down one I love is gearing. “I’m going to lower my gears from 3.55:1 to 4.10:1.”
    huh?

    1. [quote]Suspensions are funny because major differences in ride, handling and character are wrung from tiny geometry tweaks.[/quote]

      This is true, which will tell you something about the horrors of swing-axle suspension — an early Corvair had 16.25° of camber change through its complete suspension travel (about six and a half inches). Holy Snap Oversteer, Batman!

    2. That’s the term because you go down the gears to a lower gear.

  3. Nowdays, MacPherson struts are being put on heavier cars, and on heavy 4×4 SUV’s, of which, most off the people who buy them will rarely take them off road, but some will, believing they are true off road vehicles, even when usually called, car based or crossovers.
    So my question or concern is this. Are stuts really strong enough for these uses, usually reserved for double wishbone or solid axles. Has the diameter of the piston rod increased, or is there an increase of the strength of the steel used for today’s struts, to keep up with the increasing demands placed on them.
    I would like to know.

  4. “Strength” is a matter of definition. Are modern MacPherson struts physically robust enough for their intended use, in terms of bending stiffness and so forth? Sure. Do they incorporate shocks beefy enough for heavy-duty off-road use? Rarely. Very few modern vehicles are really set up for that kind of abuse, since less than 5% of them are driven off road at all, even fewer in any serious off-roading; the shocks are usually designed for a civil on-road ride. If you take a Mazda CX-9, say, off road, you’ll probably kill the struts in short order, not because the struts lack bending stiffness, but because the dampers will be overloaded.

    The real limitation MacPherson struts present for severe duty is not the physical strength of the strut itself, but the way the strut transmits its loads into the body. In an SLA suspension with the spring between the control arms, the spring loads are taken by the control arms and transmitted either to the frame (on a body-on-frame vehicle) or, on most modern cars, a front subframe. With MacPherson struts, or with the suspension layout used by old Ramblers, Falcons, and Mustangs (with the coil mounted on the upper arm), the spring loads are transmitted into the structure of the upper fender. (That’s why MacPherson struts are almost always found on unit-body vehicles.) For on-road use, that’s rarely a problem, but for heavy-duty use, it can impose severe, uneven loads on the body structure. You could make a strut strong enough to deal with the loads on the strut itself, but the fender is another matter.

  5. I see that Ford has switched to SLA for the 2011 Explorer as opposed to the MacPherson strut set-up on all their other D platform cars.

    Why do you think they did that? Does SLA provide greater wheel travel or greater durability or some other advantage an Explorer would demand over say, the Flex?

    1. I’ve heard conflicting information about this. A couple of press reports say the new Explorer’s front suspension is an SLA set-up carried over (which I find unlikely) from the outgoing model, but Ford’s own official website says the new model has MacPherson struts, like the other D3 vehicles.

      MacPherson struts present a number of disadvantages for off-road vehicles (scrub radius with fat all-terrain tires, and the way they transmit load to the body structure), but Ford is not positioning the new Explorer as a hardcore off-roader. I would imagine that for Ford, the loss of commonality would probably outweigh any mechanical advantages, especially given that the market for the Explorer is a big question mark — sales of the existing model cratered long ago, and it’s unclear whether the new one will revive them.

      1. Yeah, I noticed the confusion as well but in the end i assumed the press release was right.

        1. First generation Mazda6, whose platform was shared by first-gen Ford Fusion and Ford Edge, had SLA front suspension. The D3 platform has MacPherson strut front suspension. It’s likely the SLA suspension will be phased out with the demise of the first generation Fusion. There was some confusion on this. Even the Mazda website was confused about whether the pre-Skyactive second-gen Mazda6 had SLA or strut front suspension.

          1. As a side note, I happened to learn the etymology of the term SLA suspension, which is not short-long-arm as you might expect. Cadillac applied that term to its first IFS prototype (around 1932), which had what became the early GM double wishbone suspension (with upper arms doing double duty as part of the lever-action shocks). Since that car also had a special aerodynamic body, it was dubbed SLA for Stream Line A, although there was apparently never a Stream Line B or C.

          2. BMW motorcycle’s “TeleLever” front suspension is a form of “modified” MacPherson strut. There are two sliders, one on each side of the wheel, these are sliders only. It uses a separate coil-over-shock that acts on the A-arm.

          3. Thanks for the information. I freely confess I know basically nothing about motorcycles and so I can’t speak intelligently about motorcycle engineering practice.

  6. I’ve sort of followed this progression with bemusement.

    When I was coming of age in the 60s, almost all American cars used unequal A arms in front (Chrysler used a torsion bar variant).

    By the early 70s, after some European and Japanese cars started appearing with struts, references to MacPherson struts appeared in ad copy constantly, it was a badge of sophistication.

    Then in the 80s, manufacturers (and ad copy writers) rediscovered A arms again, and that was the new mark of a sophisticated car.

    Now I’m waiting for solid axles to return (actually I own a Jeep Wrangler, so I am still on solid axles)

  7. Well, it’s not [i]just[/i] a matter of engineering fashion. Now that most cars are unibody, and very rigid, MacPherson struts are a lot more practical than they were 40 years ago. While struts have some downsides, they also have significant advantages. Most A-, B-, and C-segment cars have MacPherson struts for both cost and packaging reasons — it’s cheap, and it doesn’t take up a lot of internal space, important with small transverse-engine cars. Many D-segment cars use them, as well, because it’s an easy way to reduce production costs that most customers don’t perceive or necessarily care about.

    Because SLA or double wishbones are generally more expensive, heavier, and take up more space, they’re now sort of a luxury. Designers have to consider whether the advantages in ride and handling are worth the cost. For a high-end sports car, it might be; for an inexpensive B-segment hatchback, probably not.

    I doubt that [i]front[/i] solid axles will make a comeback — for anything other than off-roaders, the drawbacks are steep. Torsion-beam rear axles, though, are almost as common as MacPherson struts on smaller cars. Even Honda opted for a torsion beam for the Fit and the European Civic.

  8. Had to get my 2 cents in here regarding camber gain with struts. Camber gain can be achieved ,though very limited,if the inner pivot of the control arm is located higher than the ball joint in the static position. As body roll compresses the strut, the ball control arm pushes the wheel outward creating a bit of negative camber which will cancel some of the positive generated by the roll. The down side is a change in track during cornering as well as straight line travel. Raising the inner pivot will also raise the roll center which will affect the handling as well. A friend and I played with this mod on both front and rear drive autocross sedan years ago, and were able to gain some improvement in overall handling and cornering force. The big downer was heavy tire wear due to scrubbing as the track changed. Fun to experiment with though!

    1. That makes sense, and certainly, different manufacturers have found various tricks to mitigate some of the geometry limitations of the classical MacPherson strut (BMW and more recently Ford spring to mind). Of course, as you found, some changes are more suitable to the track than the street!

  9. An observation that may be worth adding is that on cars with MacPherson strut suspensions, the tire tread wears out on the edges much, much sooner than in the middle, even with camber correctly adjusted. On my Audi, tires show virtually none of this, and tires last a good deal longer.

    Another observation that may be worth adding is that in a conventional MacPherson strut using a single ball joint where the lower arm meets the wheel carrier, this ball joint resides on the steering pivot axis, and is thus aligned with the centerline through the strut. However, the strut is generally not perfectly vertical, and the steering pivot line typically insects the ground somewhere under the tire contact patch, as opposed to a point vertically under that ball joint, but also not at the center of the contact patch. Some cars, notably certain BMWs, replace the lower arm with two arms, each with an independent ball joint, the effect of which is to replace the single physical ball joint with a virtual steering pivot point located further outward. The strut has to be angled further from vertical to accommodate this, but not nearly so much as would be required to accomplished the same effect (causing the steering pivot line to intersect the ground plane at the middle of the contact patch) using a single ball joint located on the inner edge of the wheel carrier. The principal advantage is probably not with any handing improvement, but more likely with reduced tire wear and possibly greater life of the ball joint.

    As anyone who studied elementary geometry in high school may recall, a triangle is fully defined by any three parts. If at least one of those three known parts is a side, then the size of the triangle as well as its shape is determined, but even when the three known parts are angles, the shape is fully determined. You will have no difficulty figuring out that the lower arm corresponds to one side of a triangle, and that the chassis is effectively another side of a triangle. The upper mount point for the strut is basically a lazy suzan, and the distance between that point and the pivot joint for the lower arm is another fixed side of the triangle. That gives you two parts for the triangle, which tells you that no other part of the triangle can be fixed, least all the other parts of the triangle also must be fixed, in particular the angles at the ends of the lower arm. But the strut itself joins to the chassis at a fixed angle. As the suspension compresses and the wheel camber changes, the angle of strut body changes relative to the chassis. The upper rod remains at the same angle relative to the chassis, albeit rotating to accommodate steering rotation. It is thus apparent that it is necessary for the strut itself to flex, but none of the articles that I have thus far encountered have made any mention of this fact, which seems to me a defining characteristic of the MacPherson strut. I am interested in other people’s comments on this aspect of the MacPherson strut, and whether anyone has reliable knowledge of just how much flex is required.

    1. R.E. the location point of the top of the strut. The top of the strut is not a fixed point at all. While the strut may in fact flex, or bend to a certain extent, the amount is negligible unless the design loads are exceeded. For instance, fitting larger, sticker tires. Rather, it is located in a rubber mount, both for the purpose of isolating it to reduce NVH trasmission into the body structure, and to allow for the aforementioned camber gain/loss. These mounts also serve as a path for virtually all of the vertical load transmission as well as a portion of the lateral loads. As you might expect, they are highly stressed, and, depending on how well engineered they are, and the quality of the material used, in some vehicles they tend to wear and fail fairly rapidly. As a VW technician, I see this quite often, especially in the older A3 and A4 body Golf/Jetta.

  10. I’m a faithful reader of your blog, but there is a true mistake for this article : the first Ford Vedette was not the first production car with MacPherson struts.
    It’s a common mistake, but the first Vedette used the common SLA arrangement for front suspension.
    You can see a picture of the frame here :

    There is more explanation (in french !) in the message of Vega 770_0 posted 01-12-2009 (21:42:28 pm).
    In fact, the very first was the Ford Consul/Zephyr in 1950. Then, the second generation Ford Vedette was launched with MacPherson suspension in 1954, a few weeks before the takeover of the Ford SAF by Simca. The third one was the German Taunus 17M P2 three years later.
    Surprisingly, as far as I know, the first non-Ford MacPherson car was the Peugeot 404 in 1960…
    Moreover, I’ve seen in some Revue Automobile Suisse catalogs that many japanese brands started to use it in the sixties : Honda already had it on the tiny N360/N600.

    1. Thank you for the correction and the photo — I’ve amended the text accordingly.

  11. what were the earliest Ford models to use these struts in their suspensions (early 1970’s, I think)?

    1. Well, the first Ford product to have MacPherson struts was the Consul. The first Ford marketed in the U.S. with MacPherson struts? A number of European Fords were sold here in very limited numbers by dealers with English Ford franchises, so there were some Cortinas floating around in the mid-sixties. The first to be marketed here in any coherent way, maybe the Capri, followed by the Mk 1 Fiesta.

      The Fox platform used what Ford called modified MacPherson struts (which meant that the coil spring was mounted on the lower control arm, rather than on the strut). I think the first unmodified U.S.-specific car might have been the first-generation, U.S.-market Escort, although I’d have to check.

  12. Tom: I’ve wondered the same thing. In my mind, there needs to be some pivoting or flexing in the strut, otherwise it will bind. I asked some years ago on the AtlasF1 forum, and all I got were blank stares.

  13. Tom, the upper mounting of the strut is not a fixed point. While it is a bearing that allows the strut to rotate for steering purposed, the bearing itself is mounted in a very compliant rubber mount. As the strut compresses and extends and the angles change, the mount has enough range of motion to allow the angles to change without flexing the strut. Having said that, there is a certain amount of flex that does occur, as it must, but strut tubes and pistons are quite robust (in most cases, there have been some notable exceptions)and generally keep the tire located within the intended range of caster and camber.

    In regards to your mention of BMWs use of seperate lower links to create a lower arm, there is indeed a distinct handling benefit to it. The idea is to bring the center of the steering axis closer to the center of the contact patch, thereby reducing the scrub radius. Scrub radius is the arc the contact patch travels through as the wheel is steered left to right. A large scrub radius is undesireable, as it tends to create a “pull” felt at the steering wheel, especially when there is a difference in traction between the left and right wheel. As you noted, there is another problem associated with this is SAI, or steering axis inclination. That is the angle of the steering axis when viewed from the front of the vehicle as it is tilted toward the middle of the car. It has a very profound effect on wheel camber as the wheel is steered from straight ahead, and not in a good way. SAI tends to cancel out caster on the outside wheel in a turn, and multiplies it on the inside. Large SAI’s can even push the outside wheel into a positive camber angle. A good example of this is the Volvo 240 series. Just turn the wheel full lock in one direction and look at the wheels to see what I mean. Typically, you will find that the better handling cars (those that turn in well and remain relatively neutral, or even oversteer slightly) have low SAI’s. VW/Audi found an interesting solution to the problem with the B5 Passat/A6, by using 4 individual links to create 2 wishbones. The result was a nearly vertical SAI. The actual steering axis in this case was not even fixed. Rather, because of the movement of the joints, it moves also in an arc of its own.
    A facinating subject for sure. Hope this clears away some of the fog.
    Regards.

  14. Very informative piece, most of which I already knew, but with a few surprises. The only thing you missed is the inverted strut used on rally cars, where the piston in the strut carries the wheel spindle and balljoint, and the damper end fits into the wheel arch ( to reduce unsprung weight ).

  15. Aaron,

    Two quick notes.
    By some accounts, HiPer Strut is a Saab invention. For instance, Autocar (UK) writes that “This front suspension set-up was designed by Saab engineers for General Motors’ Global Epsilon project.” I’m sure the patent application lists the actual inventors, but I couldn’t find it.

    One of the key elements in scrub radius is wheel offset. It’s also something that amateur suspension gurus almost always get wrong when they “tune” cars by lowering them, fitting bigger wheels, and sometimes even adding wheel spacers. It’s very easy to add an inch or more of offset to a car, or even to take it for negative to positive offset (or vice versa), with dangerous results.

    1. Bernard,

      You make a good point about offset, although the impact of altering the design offset of a car’s wheels is sort of outside the scope of this article. There’s a lot of complexity there that’s really a separate topic.

      I hadn’t seen the item about Saab designing HiPer Strut, although it would make some sense. Saab really got a lot of criticism for the torque steer of some of the more powerful later 900 variations (particularly the Viggen, as I recall), so their engineers may have been giving the issue a lot of consideration.

      As far as patents go, there may not be any specific patents for HiPer Strut per se. There may be for certain specific elements of it, but as the article explains, there’s a lot of prior art in this area. Even with Toyota’s earlier Super Strut, there was no single patent that was recognizably the production layout (I wish there had been because it would have made it easier to figure out the mechanics!), although there was a whole series of patents covering certain elements. One included illustrations of something like a dozen possible variations.

  16. By the way, Colin Chapman didn’t abandon strut rear suspension on the Elan. He abandoned the trailing arm location, using a wide-based lower “wishbone” to control lateral and fore-and-aft location, and rubber doughnuts in the drive-shafts.Some modern aftermarket replacement Elan chassis do use double wishbones instead of the original struts,however.

    1. Thanks for the clarification. Splitting hairs, I would still not call the Elan’s rear suspension a Chapman strut layout — although some contemporary press articles did — because the halfshafts don’t provide lateral location in a way the halfshafts do on the contemporary Jaguar or Corvette Sting Ray independent rear suspensions. I’d just describe it as a MacPherson strut located by a wishbone.

  17. I thoroughly enjoyed reading your article about Earle, my great uncle. I never got to meet him, but I do know a few things about him. I didn’t know he had a short temper, but it doesn’t surprise me because he was such a genius and was a Scot. I love knowing stories about him. His father was an inventor, as well. I have some photos of Earle as a young man and in his WWI uniform. I wonder if you would like to see them!

    Thank you for the great article!!

    Liz

    1. Liz,

      I would love to see pictures of him earlier in his life. If you like, send me a message via the contact form.

      I don’t know that he had a <em>short</em> temper per se, but I got the impression that he was a person of strong opinions and didn’t suffer fools gladly. Admittedly, many of the anecdotes of people who had run-ins with him at Ford were in situations where someone was doing something they technically weren’t supposed to be doing or were just horsing around. For instance, there is an amusing story about some of the designers putting a giant plaster elephant in a wind tunnel where they were supposed to be doing aerodynamic testing, which I imagine was particularly infuriating to him because Ford was renting the wind tunnel by the hour!

  18. I cant help thinking that the passenger area of the Chevrolet Cadet prototype looks remarkably like that of the Vauxhall Wyvern, Velox and Cresta of the mid-fifties(Vauxhall is of course GM owned) .Could that Cadet six cylinder engine have also found its way into the Velox and Cresta, as the size and output seem remarkably similar?

    1. The stylistic resemblance is no great surprise, given the way postwar career advancement typically worked in GM Styling (and for that matter Ford). It wouldn’t have been terribly surprising to have at least a few of the same people have either worked on both or remembered seeing the earlier car, although it may have simply been a reflection of common styling trends.

      As for the engine, I think the resemblance is coincidental. My understanding is that the Velox/Cresta engine was essentially the 1952-vintage 1.5-liter four with two additional cylinders, and I don’t know that there was any relationship with the experimental Cadet engine beyond the broad similarities one saw between various GM OHV engines of similar vintage and displacement. (The Vauxhall six shares the 3 1/8-inch bore of the Victor four, versus the Cadet’s 3 1/16-inch, giving 2,262 vs. 2,173cc.) Vauxhall would have had to buy the Cadet design from Chevrolet — with a markup, in all likelihood — so I don’t think there would have been any particular advantage in using it versus one developed by Vauxhall itself. Vauxhall had been using OHV engines for 20 years and didn’t really need any technical help in that regard. The same logic is how GM ended up with no fewer than FOUR distinctly different 5.7-liter V-8s 15 years later!

      Vauxhall did, of course, offer its own Cadet for a while in the thirties, although there was no relation.

  19. This is easy for me to say 70 years after the fact, but if MacPherson had played up the superior handling of the fully independent suspension, the Cadet could have been the starting point for a sports sedan in the mold of the Rover P6/Triumph 2000/BMW Neue Klasse.

    Of course, it’s easy to see why this didn’t happen. The brief was to design an inexpensive compact, not a car for a market segment that wouldn’t exist for another 20 years. Sports cars were very much a niche market in the U.S. at the time. Earle MacPherson, much less GM management, couldn’t have been expected to think this far out of the box.

    1. I don’t really think the sport sedan in the modern sense was really on anybody’s radar at that time. I think for the most part, even when engineers considered the subject of handling, it was typically prefaced with “ease of,” which isn’t quite the same thing. The standards for maneuverability were somewhat higher in markets like the U.K. or Italy, where narrow, winding roads not originally intended for cars were commonplace, but in the U.S., “handling ease” was generally about stuff like steering effort and how wieldy a car was in tight spaces or for parking. The bigger preoccupation, and what I assume was MacPherson’s principal argument for independent suspension, was getting a smooth ride on what for an American car was a ridiculously short wheelbase.

      Mostly, I don’t think the cultural factors would have been present in the forties or fifties for a sporty sedan to catch on beyond folks like Tom McCahill and a small class of people Detroit generally dismissed as cranks well into the seventies. Some buyers would probably have been pleasantly surprised, I imagine, but even the later American enthusiast crowd would probably have judged it mostly on its suitability for having a big V-8 jammed under the hood.

Leave a Reply

Your email address will not be published. Required fields are marked *

Click here to read our comment policy. You must be at least 18 to comment. PLEASE DON'T POST COPYRIGHTED CONTENT YOU AREN'T AUTHORIZED TO USE!
Except as otherwise noted, all text and images are copyright © Aaron Severson dba Ate Up With Motor. (Terms of Use – Reprint/Reuse Policy) Trademarks referenced herein are the property of their respective owners and are used here for informational/nominative purposes.