Most modern cars have MacPherson struts in the front, and rear suspension systems are usually torsion beams. One can better understand the reasons for such uniformity by looking at what has gone before.
FRONT SUSPENSION
Various ways of suspending the front wheels separately had been tried by a number of motoring pioneers even before 1900, but most of the designs were either not robust enough or interfered with steering and braking. The most famous was the Lancia sliding pillar system, which was used on various models until the late ’60s. By 1930, the major manufacturers had started to look seriously at independent front suspension, and focused on three main systems – torsion bars, as used by Adler and Citro’n; coil springs and transverse A-brackets, used by Mercedes-Benz and General Motors; and transverse leaf springs, found on Fiats and Peugeots.
One of the most successful front suspension types (if success can be measured by the ability to soak up major disturbances at speed) must be the trailing link and torsion bar arrangement found on the Volkswagen Beetle. It is extremely robust, is still used by many specially-built off-road racing vehicles, and has the additional advantage that the wheels remain parallel to the body when roll takes place.
In the US, a suspension linkage system known as SLA (short-long arm) became almost universal in the years after WW2. It was well suited to rear-wheel drive, and the arm mounting points and lengths could be adapted to cater for a wide range of wheel movement profiles. The arms are usually called A-arms in the US – wishbones elsewhere – and sometimes they’re simply called struts. Their qualities are determined by the geometry imparted to the wheels, so that some of these systems are a lot more successful than others. The most popular arangement is to have the top wishbone shorter than the bottom, so that when the wheel moves upwards the camber becomes negative, which improves wheel grip. However, equal length arms are sometimes used.
Enter the MacPherson strut. In the late ’40s, Earle Lee MacPherson left Chevrolet and joined the European branch of the Ford Motor Company after Chevrolet showed little interest in his suspension design. This was a time of great change in the way cars were designed, because bodies mounted on separate chassis were no longer good enough, being torsionally too flexible, prone to rattles, and too heavy. The answer was to adopt unit construction, which meant that the bodies were now designed as stress-bearing units. To achieve this, they not only had to be welded to the chassis but the complete structure had to be designed as a unit. This simplified production, but created a problem for suspension designers. They had to create special load-bearing attachment points for suspension arms where they would not intrude on the engine bay. Earle MacPherson realised the answer was to raise the attachment points for the top parts of the suspension to a point where the natural curvature of the body provided extra strength – and the famous strut was born.
The first car to be fitted with MacPherson struts was the 1949 French Ford Vedette, followed soon afterwards by the 1950 English Ford Consul and Zephyr. These cars created a sensation, because they were inexpensive but handled like cars costing a great deal more.
The MacPherson system uses a combined coil spring and damper as a strut. It swivels on a ball joint at the lower control arm and on a roller or other good quality bearing at the mounting point underneath the front mudguard or bodywork member. Today, the strut is chosen by many designers because it fits easily into the space available in a modern layout, and combines good wheel travel with reasonable handling. It is a relatively simple design, with a low unsprung mass, and allows the shock loads to be distributed widely over the body structure. However, it has the disadvantage of making the effects of an unbalanced wheel more noticeable. Most mechanics and do-it-yourself men hate the MacPherson strut because replacement usually means that a spring compressor is needed, and the suspension has to be stripped and re-assembled, so that wheel alignment has to be reset. Contrast this with a torsion bar or coil set-up, as used on modern LCVs, where you need one spanner, four bolts and a spare 10 minutes to replace two separate dampers.
REAR SUSPENSION
Non-independent rear suspensions on rear-wheel drive cars lingered on well past the introduction of independent set-ups for the front, no doubt because a good independent rear suspension system is expensive to make. A second-rate system merely introduces handling difficulties and excessive tyre wear. There are still a few cars in production, in the US and Australia, with beam rear axles, and this system is still common on LCVs and bigger trucks, because it is robust.
An independent system not only makes greater wheel movement possible, which improves comfort, but also reduces the unsprung mass because the differential is now carried on the frame, instead of being part of the axle. In the early ’30s, a number of manufacturers started to experiment with swing axles in an effort to improve passenger comfort, but the extreme camber changes, which resulted in sudden oversteer, counted against this set-up. Nevertheless, millions of Beetle enthusiasts throughout the world have learned to live with it, and Mercedes-Benz used it for many years before adopting a less vicious single-pivot swing arm.
Today, expensive rear-wheel drive cars such as Mercedes-Benz, BMW, Lexus and Jaguar have multi-link set-ups that control the wheel movement, while the drive is taken care of by open shafts with four universal joints.
Front-wheel drive cars often have either a dead axle or a torsion beam at the rear. A dead axle implies non-independence, and does not contain a driveshaft. When used to link the rear wheels on a front-wheel drive car, it can be suspended on leaf springs, coils or torsion bars. This layout is now losing ground to the torsion beam, which is a partially independent layout. It is effectively a much thicker version of an anti-roll bar, in that it links the non-driven wheels less rigidly than a dead axle would and so introduces a degree of flexibility, and even a degree or two of rear wheel steering, which tends to make the car more responsive.
Suspension systems can be divided into passive and active systems. Most cars have passive systems, ie, systems where the spring rate and damping characteristics are fixed and cannot be altered without changing components.
Semi-active suspensions have spring and damping elements that can be adjusted by an external control. These systems, sometimes called adaptive, use brake pressure, steering angle or suspension motions to trigger discrete changes in suspension damping or spring rate. This makes it possible to control pitch, bounce and roll to some extent, but the switch back to softer settings only occurs after a time delay, ie, the system cannot adjust continuously to a change in road conditions.
Full active suspensions utilise actuators to counter the forces fed into the suspension by the wheels. They control the stiffness of the suspension on a continuous basis, but these systems are still in their infancy, and are too complicated for most applications.
SUSPENSION TERMINOLOGY
Two components used to control wheel movement have special names, and require further explanation. A Panhard rod, named after one of the most famous early motor pioneers, René Panhard, is a rod that usually has one end attached to a beam axle and the other end to the frame-work. However, the rod must be parallel to the axle, ie, mounted transversely, to qualify as a Panhard rod, otherwise it is just a trailing or leading link, depending on whether it is mounted before or after the axle. A Panhard rod prevents any sideways movement of the axle, except for the very small amount due to the fact that any rod can only allow each end-point to rotate around the other end-point as a centre.
A Watt’s linkage, invented by the same James Watt whose name is used to measure units of power, was first used on steam engines. It is a set of two links, mounted on the same or opposite sides of an axle or hub unit in such a way that the unit can only move up or down in a straight line. This happens because each locating rod swivels about a centre, so that the other ends are forced to follow circular paths. If the axle is located by swivel joints at just the right position, it can move only in a straight, vertical line.
Anti-roll bars are essentially U-shaped torsion bars, mounted between wheels in such a way that when both wheels rise and fall together, the short ends of the bar move with them, but when one wheel rises or falls on its own, the ends move with the wheels, causing the long section of the bar to twist like a torsion bar. This stiffens the suspension during cornering, reducing body roll.
Discussion about various suspension systems often concerns the roll axis. This is the imaginary axis about which the sprung mass rolls when the car corners. It can be found by determining the instantaneous roll centre for the front and rear suspension systems, then connecting the two centres with a straight line. These centres can be determined by the geometry of the wheel movement, which, in turn, depends on the way the suspension links locate the wheel. They are called instantaneous centres because, in most suspension systems, their positions change as the wheels move up or down. If the front and rear roll centres are close to the centres of gravity of the front and rear sections of the car, then the body will roll very little during cornering. In practice, this is difficult to achieve, because the roll centres are often near the ground whereas the centres of gravity cannot be so low.