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. Crawford had been promoted to corporate VP of engineering in 1945, and it was clear that he and MacPherson would not be able to coexist happily.
Around that 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 made him an attractive job offer. MacPherson subsequently 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. The first outside user — albeit only by a technicality — was Simca, which acquired Ford’s French subsidiary in 1954 and continued to build the Vedette under license until 1961. 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. For cost reasons, many production models have MacPherson struts only on the front wheels. 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 Lotus Elite production cars. As a result, rear strut suspensions are often (somewhat unfairly) called Chapman struts, rather than MacPherson struts, although the designs are basically identical.
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 largely the work of Ford supplier Thompson Products, but he was responsible for the design making it to production, which many contemporary engineers had thought impractical.)
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.
(For more information on the Chevrolet Cadet project, including MacPherson’s role in it, we highly recommend Karl Ludvigsen’s excellent article, “The Truth About Chevrolet’s Cashiered Cadet” from the January-February 1974 issue of Special Interest Autos.)
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.
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 Vedette 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.”
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.
- No camber gain: Because the top of the vertical strut is mounted rigidly to the body structure, MacPherson struts do not provide camber gain — 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 way to prevent camber loss 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, “Wheel-Suspension Means for Motor Vehicles,” Patent No. 1,693,111, filed July 13, 1927, issued November 27, 1928. To clarify our understanding of some basics of suspension design, we also referred to Herb Adams, Chassis Engineering (HP1055) (New York: HPBooks, 1993).