The MacPherson Strut

MACPHERSON STRUTS VS. DOUBLE WISHBONES

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

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

Diagram of a double wishbone suspension © 2014 Aaron Severson

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

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

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

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

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

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

Double wishbone suspensions have the following advantages:

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

Double wishbones also have several significant drawbacks:

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

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

MacPherson strut front suspension diagram © 2014 Aaron Severson

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

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

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

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

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

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

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

REAR STRUTS

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

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

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

38 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!

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  15. Aaron,

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

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

    1. Bernard,

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

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

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

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

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

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

    Thank you for the great article!!

    Liz

    1. Liz,

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

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

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

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

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

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

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

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

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

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

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