The MacPherson Strut

It’s one of the most common suspension designs used on modern cars, found on everything from the lowliest Proton Savvy to the formidable Porsche 911 Turbo. It’s also frequently misunderstood and often misspelled. This week, we will try to set the record straight about the origins and workings of the MacPherson strut suspension system.

THE CHEVROLET CADET

Ordinarily, we wouldn’t consider the early life of Earle MacPherson to be terribly relevant to this article, but the amount of misinformation in even reputable, published sources is so immense that it’s worth laying out the basic facts.

First, his full name was Earle (not Earl, as it is often misspelled) Steele MacPherson. He was not born in England, as some source assert; he was born in Highland Park, Illinois, a suburb of Chicago, on July 6, 1891. After graduating 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 1, working as an engine mechanic for the Aviation Section of the U.S. Army Signal Corps (the ancestor of the U.S. Army Air Corps) — he was not a fighter pilot, as has sometimes been reported. 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 1923, MacPherson left for Hupp Motors, where he remained for more than a decade. In 1934, he joined General Motors as the assistant to the corporate vice president of engineering. The following year, he was promoted to chief design engineer of Chevrolet division, reporting to Chevrolet chief engineer James Crawford.

In the spring of 1945, Chevrolet general manager M.E. Coyle made MacPherson the chief engineer of Chevrolet’s new “Light Car” project, later known as the Chevy Cadet. The Light Car project emerged from Coyle’s fears of a postwar recession, like the one that had paralyzed the auto industry at the end of World War 1. GM senior management was not as pessimistic about the postwar market as Coyle was, but they nonetheless authorized development of the Cadet as a potential companion for the full-size Chevrolet.

The Cadet was by no means a one-man operation, but MacPherson was its guiding force, directing every aspect of its development. Although it had a target price of just under $1,000 — about 10% less than the cheapest full-size Chevy — the Cadet was a very sophisticated design, with novel features like unibody construction and an overhead-valve six-cylinder engine. It had a top speed of about 70 mph (113 kph) and offered 50% better fuel economy than a big Chevy. Furthermore, despite its modest, 2,200-lb (1,000-kg) curb weight, the Cadet had significantly better ride and handling than its larger brother, thanks to its novel independent suspension.

At that time, independent suspension (that is, suspension designs allowing independent movement of each wheel) 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 until 1949. Independent rear suspension was even less known, outside of a handful of exotic European cars. GM and Chevrolet management protested its inclusion on the Cadet, claiming that it would be too expensive, but MacPherson insisted vehemently that its benefits outweighed its costs.

Recognizing the severe cost pressures of the Cadet project, MacPherson set about designing the most cost-effective independent suspension he could concoct: the ancestor of what we now call the “MacPherson strut” suspension. He filed for a patent on his design in March 1947; it was granted in 1953. As with many automotive inventions, the concept was not a wholly new idea — Guido Fornaca, former managing director of Italy’s FIAT, had applied for a patent on several conceptually similar suspension designs back in 1927, of which MacPherson was presumably aware.

The Cadet would have been a groundbreaking design for a U.S. manufacturer, but by early 1946, its projected costs were becoming a crippling problem. To make money on the Cadet, Chevrolet would have to sell 300,000 units a year for at least three years. The Chevrolet sales organization, which had not been involved in the development of the concept, balked at that prospect. Since the resumption of civilian automobile production in late 1945, the market had been booming. There was no postwar recession; buyers had money to spend and nearly four years of pent-up demand. With buyers lining up to pay full list price (if not more) for every new car they could get, the sales force insisted that the inexpensive Cadet would only hurt Chevrolet’s profit margins.

In September 1946, GM announced that production plans for the Cadet would be suspended. The announcement did not mention the Cadet’s sales prospects, only concerns about the availability of raw materials, a big problem for all automakers in the immediate postwar years. Development work continued for a time, but in May 1947, the project was canceled entirely.

THE MACPHERSON STRUT

By then, MacPherson had been soured by his repeated clashes with his former boss, James Crawford, who had been promoted to corporate VP of engineering in 1945. Around the same time, Ford Motor Company executive vice president Ernest R. Breech — formerly the head of GM’s Bendix division — was scouting for current and former GM executives to help revitalize Ford. At Breech’s suggestion, Ford chief engineer Harold Youngren called MacPherson and offered him a job. MacPherson left Chevrolet for Ford in September 1947. (Contrary to some accounts, MacPherson did not move to Europe; he remained in the Detroit area.)

Although the “MacPherson strut” suspension design was not well suited to Ford’s contemporary body-on-frame domestic cars, it subsequently found its way onto some of Ford’s small European cars, including the Consul and Zephyr, from Ford of Britain; the Taunus, from Ford’s German subsidiary; and the second-generation Ford Vedette, from Ford SAF, the company’s French subsidiary.

Other manufacturers were relatively slow to adopt the design, presumably due in large part to a reluctance to pay royalties on the use of MacPherson’s patents. Once those expired, the floodgates opened. Porsche adopted strut-type suspension for the 911 in 1963 (albeit with torsion bars, rather than coil springs), while Volkswagen began using MacPherson struts in the late 1960s. After the original patents expired, the MacPherson strut quickly proliferated throughout the industry, particularly for compact, front-wheel-drive cars like the Volkswagen Golf.

MacPherson had originally intended his strut suspension design to be used on all four wheels, although for cost reasons, many production models had MacPherson struts only on the front wheels, retaining a live axle in back. However, in 1957, Lotus’s Colin Chapman developed a very similar strut-type rear suspension for the Lotus 12 Formula Two racers, as well as the subsequent Type 14 Lotus Elite production cars. The Chapman strut, as this design was known, was essentially a rear MacPherson strut located by a trailing link and the halfshaft, which does double duty as a control arm.

Curiously, the MacPherson strut wasn’t used on any domestic Ford products until the 1970s. Another of MacPherson’s achievements, however — front suspension ball joints, replacing the traditional kingpins — was adopted in the mid-fifties, and became universal on American cars by the end of the following decade. (MacPherson didn’t invent the ball joint suspension, which was Ford’s version was largely the work of Ford supplier Thompson Products — and had been common in Europe for some time — but he was responsible for applying it to full-size American cars, something many contemporary engineers had thought infeasible.)

Thanks to these successes, MacPherson was promoted to corporate vice president of Ford engineering in May 1952, succeeding Harold Youngren. He remained in the VP slot for six years, finally retiring in May 1958, at the age of 66. He died in 1960.

BASICS OF INDEPENDENT SUSPENSION DESIGN

To understand how the MacPherson strut works, we must first consider why you would want independent suspension in the first place. Beam axles are simple, cheap, and sturdy, which is why they serve perfectly well for horse-drawn carriages and heavy-duty vehicles. However, an independent suspension has several major advantages over a beam axle. First (and most obviously), it allows each wheel to move separately, so that a bump that affects one wheel doesn’t necessarily affect the other. Second, it avoids the uncontrolled oscillations created by a beam axle, which would otherwise cause wheel tramp and shimmy, hurting both handling and directional stability. Third, independent suspension reduces the vehicle’s unsprung weight.

(A car’s sprung weight is the mass supported by the suspension: the body, engine, passengers, and cargo. The car’s unsprung weight is the mass of the suspension components, wheels, tires, and anything that moves with them, such as the brakes, if they are mounted in the wheels, or the beam connecting the wheels of a beam-axle suspension. Every time the car hits a bump, its suspension transmits the force to the body; the greater the unsprung weight, the more severe the shock. High unsprung weight also increases the inertia of the suspension components, making it harder to change their direction, or to quell their motion once they’ve started moving.)

As a result, a car with independent suspension — particularly for the front wheels — tends to have better handling and ride quality than one with a beam axle, which is why IFS became standard on most cars by the late forties. (Of course, as with all things, the theory and the practice are often different things. Ford’s first independent front suspension, introduced in 1949, had very poor geometry, and its early IFS cars handled notably worse than their beam-axle predecessors.)

Outside of a brief flirtation with Dubonnet cylinders, the most common form of independent front suspension on American cars of the forties (and for about thirty years thereafter) was the unequal-length control-arm or double-wishbone layout.

1971 Chevrolet Chevelle suspension
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: wishbone-shaped upper and lower control arms supporting the wheel spindle, with a coil spring surrounding the shock absorber, mounted on the lower arm. Not evident in this photo is a front anti-roll bar. On Chevy 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.

As you can see from the above photo, a double-wishbone suspension connects the wheel spindle to the frame with two transverse control arms, each shaped like a wishbone or a capital A. The upper control arm is typically shorter than the lower arm by 20-50%. A tubular shock absorber is mounted between the arms. The actual suspension is usually by coil springs, which are sometimes mounted over the shock absorbers (“coil-over”) to save space; some cars mount the spring on the inboard side of the lower arm or on top of the upper arm, while others substitute torsion bars or semi-elliptical leaf springs. On many cars, there is also an anti-roll bar, a torsion bar spring that connects the lower control arm to its counterpart on the other side of the car.

A properly designed double-wishbone suspension offers a number of benefits, compared to either a beam axle or other types of independent suspension, like swing arms or trailing arms:

  • Low unsprung weight: The control arms themselves are relatively light, and only a portion of their mass is actually part of the unsprung weight, which keeps the total unsprung mass quite low.
  • Strength: The triangular wishbone shape of the control arms makes them stiffer without making them heavier, helping them resist bending and distortion — important to avoid shimmy and wheel tramp.
  • Long swing-arm length: Swing-arm length is the radius of the arc through which the wheel moves as it goes up and down on the springs. If this radius is relatively short, as on a swing-arm or swing-axle suspension, the wheel traces a long arc between the extremes of its travel. This often results in major changes in wheel alignment, which produces erratic handling. Making the effective swing-arm length as long as possible keeps the geometry of the wheel closer to constant, giving more predictable handling.
  • Camber gain: Tires have the best traction when they are vertical — that is, when their camber angle is zero. With a beam axle or trailing arms, as the body leans toward the outside of the turn, the top of the outside wheel tilts outward, reducing its grip on the pavement; this is called camber loss, and it reduces the car’s maximum cornering power. With a double-wishbone suspension, the longer lower control arm causes the lower half of the wheel to tilt faster than the upper half, which keeps the wheel’s camber closer to zero, even as the body leans; this is called camber gain, and it greatly improves handling ability.

A double-wishbone suspension has three major drawbacks:

  • Cost: An unequal-length control arm suspension is fairly complicated (particularly compared to a beam axle), which costs more to build and install.
  • Width: The control arms need to be relatively long to provide good suspension geometry, which takes up more space in the body.
  • Weight: Along with their bulk, the wishbones are relatively heavy, which adds to the unsprung mass. This can be alleviated by using lightweight materials, such as aluminum or magnesium, but that in turn adds to the cost.

THE MACPHERSON STRUT

Earle MacPherson confronted both of these limitations when designing the Chevrolet Cadet in the mid-forties. The Cadet’s track width was only about 48 inches (122 cm) — fully a foot (30 cm) narrower than the track width of a contemporary full-size Chevrolet — which didn’t leave a lot of space for suspension components. Furthermore, the ambitious price target meant that the cost had to be reduced as much as possible. Beam axles would have been easier, but they would not have provided acceptable ride or handling, particularly considering the Cadet’s low sprung mass.

MacPherson’s strategy was essentially to simplify the unequal-length control arm layout. The Cadet’s suspension retained the lower control arm, which was actually formed by a relatively narrow transverse arm and a skinny, diagonal radius rod. Instead of an upper control arm, however, the wheel spindle was mounted on a vertical strut, mounted rigidly to the body. The strut incorporated a tubular shock absorber, and it served both as the upper control arm and as the axis around which the front wheels were steered. The coil spring was mounted over the upper part of the strut, near where it attached to the body; this saved space, and allowed the lower control arm to be thinner, since it didn’t have to handle the loads generated by the springs.

The refined version of this design, first used in the Ford Consul and found on many modern cars, dispenses with the radius rods. Instead, it uses a torsion bar spring, connected to the outer end of each lower control arm. The torsion bar acts as an anti-roll bar and also triangulates the control arms, acting as the front half of each lower “wishbone.”

MacPherson strut suspension of a 1998 Saturn SL1
A badly damaged late-nineties Saturn SL1 shows off its MacPherson strut front suspension. The upper control arm of the SLA suspension is omitted entirely, its locating function provided by the shock absorber/strut, which is mounted directly to the wheel spindle. The lower control arm is narrower and lighter, but it gains strength through its triangulation with 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.

By eliminating several components and making others do double duty, the MacPherson strut design is both cheaper and lighter than a double-wishbone suspension. It’s also narrower, which is helpful in smaller cars with transverse engines. Although MacPherson didn’t have front-wheel drive in mind when he designed this suspension, it has an additional advantage for FWD cars in that there are no suspension components to interfere with the halfshaft, which is not the case for many double-wishbone designs.

Most MacPherson strut suspensions use coil springs mounted high on the struts, like MacPherson’s original designs, but that isn’t universally true. Porsche has frequently used MacPherson strut suspensions with torsion bars, rather than coil springs, while both Ford’s Fox platform and GM’s 1982-1992 Camaro and Firebird used “modified MacPherson struts” with the coil springs mounted on the lower control arms, rather than on the shock towers. By the same token, there are many suspensions with coil-over shocks that are not MacPherson struts. What defines a strut suspension is not the location or integration of the spring, but the use of the shock tower as the upper control arm.

MacPherson struts offer many of the benefits of a double-wishbone suspension, including strength, long swing-arm length, and low unsprung weight, but without the cost and space penalties. However, they also have several significant drawbacks, including:

  • Unsuitability for body-on-frame vehicles: The vertical strut must be firmly attached to the body, which in turn must be strong enough to absorb the loads created by the suspension. That generally requires a unitized or semi-unitized body; with a body-on-frame car, the body is generally not rigid enough to handle those loads. (This is why MacPherson struts were not common on domestic cars until the 1980s.)
  • Excessive height: Although a MacPherson strut suspension isn’t very wide, it is taller than a double-wishbone layout, which means it doesn’t fit well in cars with a low hood line. Some MacPherson-strut cars (like Mitsubishi’s 1990-2001 GTO/3000GT and the related Dodge Stealth) have noticeable bulges in the hood or front fenders to provide clearance for the shock towers.
  • High replacement cost: Because the vertical struts are also the shock absorbers — and sometimes incorporate the springs, as well — replacing the shocks on a car with MacPherson strut suspension is usually more expensive than changing the shocks on a comparable vehicle with double-wishbone suspension.
  • Limited camber gain: Because the top of the vertical strut is mounted rigidly to the body structure, MacPherson struts offer little provision for camber gain. You can get some from the lower arm or wishbone, but basically, the wheels lose camber as the body leans. You can compensate to some degree by designing the suspension with a few degrees of static negative camber (that is, aligning the wheels so that the upper halves are tilted slightly inward when the car is level), but too much negative camber causes uneven tire wear. The only alternative is to use stiffer springs and/or anti-roll bars to reduce body lean, which results in a stiffer ride. It’s possible to make a MacPherson strut car handle very well, as Porsche, Volkswagen, and BMW have repeatedly demonstrated, but it compromises ride quality more than would be the case with a double-wishbone suspension.

Despite these drawbacks, the MacPherson strut suspension remains very popular for both economy cars, and for any vehicle where space is at a premium. Even automakers like Honda, which has traditionally preferred double-wishbone suspensions, have gone to MacPherson struts for their smaller cars. That’s not a bad legacy for an engineer — even if nobody can spell his name right.

# # #

NOTES ON SOURCES

Our sources for the life of Earle MacPherson included Craig Fitzgerald, “Earl S. MacPherson,” Hemmings Sports & Exotic Car November 2005 (although he too misspells MacPherson’s name!); Karl Ludvigsen, “The Truth About Chevy’s Cashiered Cadet,” Special Interest Autos #20 (January-February 1974), pp. 16-19; and the Auto Editors of Consumer Guide, Cars That Never Were: The Prototypes (Skokie, IL: Publications International, 1981). The spelling of MacPherson’s first name is wildly varied in published sources; we went with the spelling on his patent applications on the assumption that he would certainly have spelled his own name correctly in such a context!

For the workings of the suspension itself, we consulted Earle MacPherson’s patents: “Vehicle Wheel Suspension System,” U.S. Patent No. 2,624,592, filed March 21, 1947, issued January 6, 1953, and “Wheel Suspension for Motor Vehicles,” U.S. Patent No. 2,660,449, filed January 27, 1949, issued November 24, 1953. We also looked up Guido Fornaca’s patent, Guido Fornaca, “Wheel-Suspension Means for Motor Vehicles,” U.S. Patent No. 1,711,881, filed 13 July 1927, issued 7 May 1929. To clarify our understanding of some basics of suspension design, we also referred to Herb Adams, Chassis Engineering (HP1055) (New York: HPBooks, 1993).


21 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.

  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. 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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