The rear coil spring suspension is a variation of the rear leaf spring suspension often found in rear-wheel drive vehicles; the leaf springs have simply been replaced with coil springs. Since coil springs are lighter, they have less unsprung weight, creating a smoother ride. However, coil springs can't be used to hold the axle in line therefore other methods are necessary to do this. The typical rear coil spring suspension uses two lower control arms to control wheelbase, and one or more upper control arms to control side motion and axle rotation.