The steel coil springs that most cars use for suspension these days do a great job of smoothing out the ride and keeping the car’s tyres on the road. But they’re not very good at staying in one place, so the typical suspension system also consists of a series of arms and links to locate the spring.
These arms and links hold the spring and attach it to the car at various points. The arms pivot off these points and allow the springs to compress and do their job. Sometimes the pivot point is a simple hinge, other times it’s an attachment that allows for movement in multiple planes.
Suspension arms can also work as fulcrums, maximising the effect of the spring without making the spring itself too bulky or large. Any movement in the suspension arms beyond their design will cause a car to feel ‘loose’ and rattly. In some cases, this excess movement can lead to the car actually steering itself randomly… when the driver would really rather go in a straight line.
Now read about how ball joints work.