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. (Ford Motor Company press photo of 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.

According to author Karl Ludvigsen, 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 c. 1945. Copyright 2014 General Motors LLC. Used with permission, GM Media Archive.

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 circa 1946. Copyright 2014 General Motors LLC. Used with permission, GM Media Archive.

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, so overall length appears to be roughly 150 inches (3,800 mm). 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 under a CC BY 2.0 Generic license

Contrary to some assumptions, the 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. The Holden development team knew little about the Cadet and any resemblance between the two cars was coincidental. The 48/215 was longer, had a significantly shorter wheelbase and a substantially wider track, and, save for its “Aerobilt” unitized construction, shared none of the Cadet’s novel engineering features. (Photo © 2011 Sicnag; modified 2014 by Aaron Severson and used under a Creative Commons Attribution 2.0 Generic 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 been promoted to vice president of engineering two years earlier. Crawford and MacPherson had never seen eye to eye and their disagreements over the Cadet that summer 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 likely minor. (The Vedette did have independent front suspension, but contrary to many reports, 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 (mounted atop the upper wishbone) that are sometimes incorrectly described as struts. Ford briefly contemplated using MacPherson struts for the front suspension of the 1958 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, he had little patience for anything he viewed as frivolous, but his engineering talents were considerable.

1959 Ford Thunderbird hardtop © 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.

30 Comments

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  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!

  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. 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.

  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. 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.

  11. 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.

  12. 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.

  13. 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 ).

  14. 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.

  15. 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.

  16. The Holden 48-215 was derived from an earlier GM compact-car prototype, the 195-Y-15, which preceded the Cadet. So, the 48-215 was not a development of the Cadet itself, but sort of a cousin or fraternal twin — a different offshoot of the same conceptual starting point.

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