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.)
- Low unsprung weight: An 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.
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 major 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.
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 use the halfshafts as locating arms.