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.
- 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.
- 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.
- 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.
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.