coil springs support the car's weight (this is the real "suspend" in suspension), lifting the vehicle's body and frame above the wheels, keeping the two separated. When doing this, they also maintain the vehicle's height, or "stance," on the road, keeping the proper distance between the upper and lower halves. They also hold the other suspension components -- tires, shocks, ball joints, control arms -- in proper position, allowing the other parts to function properly. If a spring should go out of tolerance it will affect the performance of -- and eventually, the wear and tear on -- all the other components.
Coil springs can be cylindrical or barrel-shaped. Their advantages include low weight, moderate cost, and limited space needs. Another big plus: they require no maintenance. On the downside, they have limited load-bearing capabilities and require additional components (sway bars, control arms, bushings, shims, ball joints) to control wheel travel.
Leaf springs are generally found in the rear of large cars and trucks. The most common leaf-spring setup consists of a series of flat steel leaves bolted together to form a single unit. This design is called a semi-elliptical leaf spring. A U-bolt attaches the springs to the rear axle, and the two ends of the leaves are bolted to the bottom of the frame using spring shackles. The shackles allow the springs to "travel" in response to the car's motion
Leaf springs have excellent load-carrying traits, making them ideal for trucks and other heavy-duty vehicles. They also are better than coil springs at transferring forces from the road to the frame. One disadvantage: leaf springs sometimes require maintenance, as interleaf friction can cause noise. Plastic inserts will generally fix this.