On rear-wheel-drive vehicles with independent suspension, the final drive unit is fixed to the vehicle frame. Drive is transmitted to each wheel by external drive shafts.
Suspension is normally by coil springs, and each wheel unit is located by a combination of lateral, and longitudinal control arms, or by semi-trailing arms to the frame.
Front suspension systems are normally independent, but in addition to their suspension function, they have to allow for swiveling of the front wheels during steering. This is catered for by ball joints, or bearings, that also allow for suspension movement. The ball joints locate the wheel assemblies on lateral wishbones, or control arms.
Depending on the design, and the location of the springs, the lower arm may be a wishbone, with two pivot points, or a control arm with one pivot point.
Since the front wheels perform most of the braking, the wishbone pivots are widely spaced, to sustain the braking torque.
With a single arm, an angled bracing strut is needed. It locates the arm at its outer end and during braking; it prevents forward movement, and rearward movement. Similarly, during acceleration.
In front-wheel-drive vehicles, the driving thrust from the wheels pulls the vehicle along the road.
This additional force is generally accommodated by more substantial bracing, or by setting the pivot points on wishbones, further apart.
The rear suspension on front-wheel-drive vehicles must maintain alignment of the rear wheels with the front, and also with the frame.
On vehicles with 4-wheel steering, the rear suspension must also allow for swiveling of the rear wheels.
External forces, such as kerb impact or a collision, can damage control arms or linkages and move the wheel units from their correct position.
This can make a vehicle pull to one side, cause abnormal wear in the tires, and make the vehicle difficult to drive.