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.

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

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

THE CHEVROLET CADET

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

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

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.

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.

MACPHERSON AT FORD

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

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

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

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

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

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

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

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

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

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

MACPHERSON STRUTS VS. DOUBLE WISHBONES

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

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

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

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

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

Double wishbone suspensions have the following advantages:

Independent wheel action : The most obvious advantage of any independent suspension is that a one-wheel bump doesn’t necessarily affect both wheels. (In practice, this advantage is compromised by the presence of an anti-roll bar, which tries to force the wheels to remain on the same level and can cause the vehicle to rock or “waddle” back and forth over one-wheel disturbances.)

: The most obvious advantage of any independent suspension is that a one-wheel bump doesn’t necessarily affect both wheels. (In practice, this advantage is compromised by the presence of an anti-roll bar, which tries to force the wheels to remain on the same level and can cause the vehicle to rock or “waddle” back and forth over one-wheel disturbances.) Low unsprung weight : One important factor in ride quality is unsprung weight, the portion of the vehicle’s mass not supported by its springs. In general, the lower the ratio of unsprung weight to total mass, the better the ride. While a double wishbone suspension’s A-arms may be relatively heavy, only a portion of that mass is actually part of the unsprung weight. Even in the early days of IFS, the unsprung weight of a double wishbone suspension was substantially less than that of a tubular beam axle. For example, Cadillac’s early double wishbone suspension (which still used kingpins and lever-action shocks) had more than 20% less unsprung weight than the previous solid axle layout.

: One important factor in ride quality is unsprung weight, the portion of the vehicle’s mass not supported by its springs. In general, the lower the ratio of unsprung weight to total mass, the better the ride. While a double wishbone suspension’s A-arms may be relatively heavy, only a portion of that mass is actually part of the unsprung weight. Even in the early days of IFS, the unsprung weight of a double wishbone suspension was substantially less than that of a tubular beam axle. For example, Cadillac’s early double wishbone suspension (which still used kingpins and lever-action shocks) had more than 20% less unsprung weight than the previous solid axle layout. Strength : The triangular shape of each wishbone makes it more rigid, allowing it to better resist bending and distortion and maintain proper alignment.

: The triangular shape of each wishbone makes it more rigid, allowing it to better resist bending and distortion and maintain proper alignment. Flexible geometry : Double wishbone suspensions give chassis engineers considerable latitude in setting the various aspects of suspension geometry that influence how a vehicle rides and handles, allowing engineers to fine-tune the chassis balance by adjusting the length, mounting points, and relative angles of the A-arms. (This is one of the reasons double wishbones are still preferred for race cars.) Notably, double wishbones permit: Anti-dive : Mounting the A-arms’ front and rear inner pivots at different heights (in essence tilting the wishbone upward) can produce an effect called anti-dive, which partially counters the forward weight transfer that causes the nose to dip when the brakes are applied. Camber gain : Tires have the most traction when they are perpendicular to the road surface — that is, when their camber is zero. A beam axle forces the wheels to maintain a constant camber, which keeps them upright going over bumps, but forces the wheels to lose camber as the body leans, reducing the tires’ cornering power. With double wishbones, camber loss can be partially mitigated by using unequal-length, non-parallel wishbones. If the lower wishbone is longer than the upper, the lower ball joint will move outward more quickly than the upper ball joint as the body leans. This allows the wheel to remain more nearly upright, an effect called camber gain. (It should be noted that not all double wishbone suspension are actually set up to provide meaningful camber gain.) Long swing-arm length : Camber gain can be a double-edged sword because it is inversely proportional to effective swing-arm length, the radius of the arc the wheel traverses as it jounces or rebounds on its spring. (This length is not a constant because it decreases as the spring compresses.) A short swing-arm length, as on a swing-axle suspension, provides ample camber gain, but can also introduce new problems, including undesirable camber changes caused by road disturbances and a tendency toward jacking, where the suspension arm acts as a lever, pushing the body upward (a problem described in greater detail in our Corvair article). With a double wishbone suspension, the swing-arm length is a function of the lengths and relative angles of the A-arms (and can be several times greater than the width of the car), which allows chassis engineers to select a length that will provide a useful degree of camber gain without making the ride and handling erratic.

: Double wishbone suspensions give chassis engineers considerable latitude in setting the various aspects of suspension geometry that influence how a vehicle rides and handles, allowing engineers to fine-tune the chassis balance by adjusting the length, mounting points, and relative angles of the A-arms. (This is one of the reasons double wishbones are still preferred for race cars.) Notably, double wishbones permit:

Double wishbones also have several significant drawbacks:

Cost : Double wishbone suspensions have a lot of components (particularly compared to a beam axle) and cost more to manufacture and assemble than simpler alternatives.

: Double wishbone suspensions have a lot of components (particularly compared to a beam axle) and cost more to manufacture and assemble than simpler alternatives. Weight : While double wishbones have less unsprung weight than a beam axle, their total mass can actually be greater, which is one of the reasons many modern B-segment cars still use beam axles in back. That mass can be reduced by using lightweight aluminum or magnesium components, but that drives up costs even further.

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

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

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

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

Height : MacPherson struts take up less space horizontally than double wishbones, which is useful for narrow compact cars with transverse engines and front-wheel drive — not a consideration in the forties, but definitely significant now. However, struts are generally taller than are double wishbones, which may require a higher hood line. This again was not a major concern when the layout was first developed (and is becoming less of an issue today thanks to European pedestrian safety standards), but has forced some low-slung cars with struts (e.g., the 1991–2001 Mitsubishi GTO/3000GT and Dodge Stealth) to resort to fender blisters to cover the tops of the shock towers.

: MacPherson struts take up less space horizontally than double wishbones, which is useful for narrow compact cars with transverse engines and front-wheel drive — not a consideration in the forties, but definitely significant now. However, struts are generally taller than are double wishbones, which may require a higher hood line. This again was not a major concern when the layout was first developed (and is becoming less of an issue today thanks to European pedestrian safety standards), but has forced some low-slung cars with struts (e.g., the 1991–2001 Mitsubishi GTO/3000GT and Dodge Stealth) to resort to fender blisters to cover the tops of the shock towers. Unsuitability for body-on-frame vehicles : One of the reasons double wishbone front suspension is common on body-on-frame vehicles is that the control arms can be mounted on a frame member, allowing the frame to bear all suspension and spring loads. By contrast, a MacPherson strut transmits its spring loads directly into the body, which must be strong enough and rigid enough to handle those stresses without twisting or distorting. That usually requires a unitized body with reinforced shock towers or fender aprons. (While MacPherson’s 1949 patent application suggests that struts can be applied to vehicles with a separate frame, we can’t think of any body-on-frame vehicle that uses struts with high-mounted coil springs.)

: One of the reasons double wishbone front suspension is common on body-on-frame vehicles is that the control arms can be mounted on a frame member, allowing the frame to bear all suspension and spring loads. By contrast, a MacPherson strut transmits its spring loads directly into the body, which must be strong enough and rigid enough to handle those stresses without twisting or distorting. That usually requires a unitized body with reinforced shock towers or fender aprons. (While MacPherson’s 1949 patent application suggests that struts can be applied to vehicles with a separate frame, we can’t think of any body-on-frame vehicle that uses struts with high-mounted coil springs.) Limited camber gain : Because a MacPherson strut’s upper ball joint is at the top of the strut, above the spring, the effective upper control arm length is quite long and the spindle height (the vertical distance between the upper and lower ball joints) is very large. Both of these factors serve to lengthen the effective swing-arm length, which minimizes camber changes as the wheel moves through its travel (good), but also sharply limits any potential camber gain (not so good). That means significantly limiting camber loss due to body lean means (a) lowering the center of gravity and/or widening the track (not always feasible); (b) increasing roll stiffness (which can have negative effects on both ride quality and handling balance); or (c) altering the wheel alignment to include a few degrees of static negative camber (which can result in uneven tire wear in normal driving). This doesn’t mean cars with MacPherson struts can’t ride and handle well, but it is an intrinsic limitation.

: Because a MacPherson strut’s upper ball joint is at the top of the strut, above the spring, the effective upper control arm length is quite long and the spindle height (the vertical distance between the upper and lower ball joints) is very large. Both of these factors serve to lengthen the effective swing-arm length, which minimizes camber changes as the wheel moves through its travel (good), but also sharply limits any potential camber gain (not so good). That means significantly limiting camber loss due to body lean means (a) lowering the center of gravity and/or widening the track (not always feasible); (b) increasing roll stiffness (which can have negative effects on both ride quality and handling balance); or (c) altering the wheel alignment to include a few degrees of static negative camber (which can result in uneven tire wear in normal driving). This doesn’t mean cars with MacPherson struts can’t ride and handle well, but it is an intrinsic limitation. Large scrub radius : Scrub radius (also known as kingpin offset) is the distance between the horizontal center of the tire’s contact patch and the point where the kingpin axis (the imaginary line connecting the upper and lower ball joints) intersects the ground. The shorter this distance, the less effect road disturbances or cornering forces will have on the steering. Because a MacPherson strut puts the upper ball joint atop the strut, minimizing the scrub radius typically requires either using narrow tires or increasing the kingpin inclination (i.e., tilting the strut toward the car’s center line), which reduces the effectiveness of the shock absorber and causes caster loss as the wheels are turned off center or the springs compress.

: Scrub radius (also known as kingpin offset) is the distance between the horizontal center of the tire’s contact patch and the point where the kingpin axis (the imaginary line connecting the upper and lower ball joints) intersects the ground. The shorter this distance, the less effect road disturbances or cornering forces will have on the steering. Because a MacPherson strut puts the upper ball joint atop the strut, minimizing the scrub radius typically requires either using narrow tires or increasing the kingpin inclination (i.e., tilting the strut toward the car’s center line), which reduces the effectiveness of the shock absorber and causes caster loss as the wheels are turned off center or the springs compress. Higher replacement costs: MacPherson struts may cost less to manufacture and install than double wishbones, but that doesn’t necessarily make struts any less expensive to service or replace. In fact, replacing a worn-out strut often costs more than replacing a conventional shock absorber, particularly if the vehicle’s struts don’t allow the damper (which usually wears out well before the spring) to be replaced without replacing the entire strut.

REAR STRUTS

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

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

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

MACPHERSON STRUT VARIATIONS

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

Omitting the anti-roll bar : While MacPherson’s dual-function anti-roll bar is clever, cheap, and elegant, a front anti-roll bar is not always desirable, particularly for lightweight, front-heavy cars that already have fairly stiff front springs. However, if the front anti-roll bar is omitted, some other means must be provided for triangulating the lower control arms. Some automakers resolve this dilemma by replacing the lower arm with a lower wishbone. Others, including early British Mk1 Ford Escorts and the Mk1 and Mk2 Ford Fiesta, use radius rods (leading or trailing links) to locate the lower control arm. One advantage of using radius rods in this way is that they can be designed to allow some fore-aft compliance for better ride quality.

: While MacPherson’s dual-function anti-roll bar is clever, cheap, and elegant, a front anti-roll bar is not always desirable, particularly for lightweight, front-heavy cars that already have fairly stiff front springs. However, if the front anti-roll bar is omitted, some other means must be provided for triangulating the lower control arms. Some automakers resolve this dilemma by replacing the lower arm with a lower wishbone. Others, including early British Mk1 Ford Escorts and the Mk1 and Mk2 Ford Fiesta, use radius rods (leading or trailing links) to locate the lower control arm. One advantage of using radius rods in this way is that they can be designed to allow some fore-aft compliance for better ride quality. Single-function anti-roll bar : Having the anti-roll bar do double duty as a radius arm may force designers to accept a spring rate for the bar that is either higher or lower than ideal for optimum ride and handling. Therefore, it’s sometimes desirable to locate the lower control arms with radius rods or use lower wishbones even if the vehicle has a front anti-roll bar. This costs and weighs more, but allows better anti-roll bar geometry and more freedom in selecting the bar’s spring rate. An additional advantage is that with either a lower wishbone or a “wishbone” formed by a control arm and a radius rod, it is possible to provide a measure of anti-dive geometry by setting the front mounting point at a different height than the lower ball joint.

: Having the anti-roll bar do double duty as a radius arm may force designers to accept a spring rate for the bar that is either higher or lower than ideal for optimum ride and handling. Therefore, it’s sometimes desirable to locate the lower control arms with radius rods or use lower wishbones even if the vehicle has a front anti-roll bar. This costs and weighs more, but allows better anti-roll bar geometry and more freedom in selecting the bar’s spring rate. An additional advantage is that with either a lower wishbone or a “wishbone” formed by a control arm and a radius rod, it is possible to provide a measure of anti-dive geometry by setting the front mounting point at a different height than the lower ball joint. Relocated springs : Mounting the coil spring around the upper part of the strut is simple and tidy, but, as noted above, requires the fender structure to be reinforced to withstand spring loads, resulting in tall, bulky strut towers. An alternative is to relocate the spring to the lower control arm, as in a typical double wishbone suspension. Some manufacturers have used coil springs in this manner, but a few (notably Porsche) have substituted longitudinal torsion bars, usually using the lower wishbone or control arm as a lever. Either way, the primary advantages are better packaging and a lower fender line; struts without high-mounted coil springs are also compatible with body-on-frame construction, which conventional struts are not. Struts with offset springs (or torsion bars) are sometimes called “modified MacPherson struts,” although technically any of these variations could be so described.

: Mounting the coil spring around the upper part of the strut is simple and tidy, but, as noted above, requires the fender structure to be reinforced to withstand spring loads, resulting in tall, bulky strut towers. An alternative is to relocate the spring to the lower control arm, as in a typical double wishbone suspension. Some manufacturers have used coil springs in this manner, but a few (notably Porsche) have substituted longitudinal torsion bars, usually using the lower wishbone or control arm as a lever. Either way, the primary advantages are better packaging and a lower fender line; struts without high-mounted coil springs are also compatible with body-on-frame construction, which conventional struts are not. Struts with offset springs (or torsion bars) are sometimes called “modified MacPherson struts,” although technically any of these variations could be so described. Double-pivot struts: Patented by BMW in the late seventies and applied to the E23 7-Series and many subsequent BMW cars, a “Doppelgelenk” front suspension locates the strut with a conventional lower control arm triangulated by a short diagonal leading link. The lower control arm is attached to the spindle via a ball joint in the conventional manner. The diagonal link connects to the strut via a second ball joint mounted above and slightly ahead of the first. Together, the link and control arm form a wishbone angled upward at the front to provide anti-dive. More significantly, the additional lower ball joint serves to alter the kingpin inclination and therefore the scrub radius. With two lower ball joints, the kingpin inclination is determined by the line between the upper ball joint and the point where the axes of the lower control arm and leading link intersect (the virtual steer center). The virtual steer center moves as the front wheel turns, so the scrub radius is no longer a constant, increasing as the wheel is steered away from center. The idea is to reduce bump steer in straight-ahead cruising while increasing self-centering action in turns.

SUPER STRUT

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

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

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

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

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

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

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

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

PERFOHUB, HIPER STRUT, AND REVOKNUCKLE

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

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

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

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

CONCLUSION

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

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

FIN

ACKNOWLEDGMENTS

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

NOTES ON SOURCES

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

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