MANY drivers are aware that their vehicles have suspension, but just to the extent they know that such parts serve only to absorb bumps and give a comfortable ride. Most are unaware of what type of suspension their car has. Luckily, there are only a few variants of car suspensions and since Citroen has dumped their gas-charged spheres suspension system, most manufacturers use these common designs.
Last week, we saw a few examples of springing and damping and how they function as part of a vehicle’s chassis. The other part that works in conjunction with the springs and dampers to complete the picture is the suspension system.
The four wheels of a car are connected in twos, the front wheels are one unit while the rear are the same. This means that the front and rear suspension are different as they have different functions. The front suspensions need to allow the wheels to steer while the rears need to keep the wheels on the ground and also to drive the car forwards. Oftentimes, the front wheels also steer and provide drive in the case of a front wheel drive car.
Older or vintage cars kept it simple. The front wheels are connected by an axle as are the rear wheels. This form of arrangement means the pairs are non-independent whereby the movement of one wheel affects the other. What this usually means that if a bump affect the front right wheel, the left front will also move in unison. Not ideal if the bump is only on one side of the vehicle and the steering is being applied. Thus, newer cars were developed with fully independent front suspensions to allow each wheel to deal with obstacles without affecting the other. The only time you might find these beam front axles nowadays is on heavy trucks and lorries.
Independent Front
Suspension
The most common independent front suspension setup are MacPherson struts, developed by Earle S. MacPherson of General Motors in 1947. These combine a shock absorber and a coil spring into a single unit or strut. They provide a more compact and lighter suspension system that can be used for front-wheel drive vehicles. This is the reason why it is the most common suspension setup in the world. The MacPherson strut is ideally suited to the front-wheel drive car.
The other type of suspension is the double-wishbone suspension, also known as the A-arm suspension, and is another common design. The more complex design and better performing A-arm is usually found on more expensive and/or higher performance cars.
There are different possible configurations of this design, but typically it uses two wishbone-shaped arms to locate the wheel at the top and bottom. Each wishbone has two mounting positions to the frame and one at the wheel connected by an upright. A shock absorber and coil spring is attached to the lower arm. The beauty of the double-wishbone suspension is that it allows for more control over the camber angle of the wheel, which is the angle in which the wheels tilt in and out. They also minimise roll or sway and provide for a more consistent steering feel. Because of these characteristics, this suspension design is common on racing cars whereby the camber can be adjusted easily through the use of adjustable rose joints.
On the rear, the aforementioned solid axle is still a viable form of suspension. If you peek underneath a 4-wheel drive vehicle, you will usually see this type of beam axle. The axle is usually suspended on leaf springs and has shock absorbers and an anti-roll bar attached. This type of suspension is ideal for its simplicity and ability to bear heavy loads. The ends of the leaf springs attach directly to the frame, and the shock absorber is attached at the clamp that holds the spring to the axle. The same basic design can be improved with coil springs replacing the leaf springs. In this case, the spring and shock absorber can be mounted as a single unit or as separate components. American car manufacturers prefer this design, but the non-independent nature of the design meant limitations in handling ability.
The best form of suspension is if all four wheels move independently of each other. Naturally, four-wheel independent suspension mean that any suspension design can be used on the front of the car or on the rear. As a matter of course, the rear of the car is devoid of the steering rack, which turns the wheels. This just means that the rear independent suspensions can be simplified versions of front ones, although the basic principles remain the same. The most common form of independent rear suspension is the trailing arm, which pivots on the chassis before the rear wheel and has the spring and shock absorber mounted to it. The complex double-wishbone is only fitted to higher performance vehicles.