Rear Suspension Tech 101
Engine torque is transmitted to the rear axles through the ring and pinion gears. Due to the nature of the action of the pinion on the ring there is a torque load (lifting force) transmitted to the rear of the chassis. This load can be controlled with the suspension components.
The rear differential housing rotates around an "instant center". A proper rear suspension can allow for adjustment of this instant center front, back, up, or down.
The springs support the weight of the vehicle. They also store and release energy.
The shock absorbers (really suspension dampeners) slow down movement of the chassis on the springs in both compression and extension based on their valving. Ideally, each shock should be mounted on the same arc traveled by the wheel when it moves up and down.
Sway bars behave as infinitely adjustable springs which act only when acted upon to resist vertical movements of one wheel relative to the other on the same axle.
4-link suspensions means there are 4 adjustable bars (2 per side) mounted between the rear axle housing and an adjustable forward mounting bracket attached to the frame. With the 4-link the instant center is adjustable front to back, up and down. It is possible to have the front end lift on acceleration (good for rear wheel drive vehicles to increase downforce and therefore traction on the rear tires), remain neutral, or by exerting a lifting force between the rear axle and chassis, transfer weight to the front end while simultaneously adding downforce to the rear (which is ideal for AWD vehicles). The adjustments on the 4-link are quick and easy, so changes can be readily made and evaluated. The characteristics of the 4-bar or ladder bar could easily be duplicated if desired.
Ladder bars are like 4-link bars with the fronts of the bars joined together. The instant center is the front mounting point. There can be a small amount of adjustment of the instant center up or down, but not front or back. There is good potential for improving rear traction but it is not nearly as adaptable as the 4-link.
The 4-bar is a parallel 4-link suspension. There is no instant center and no torque is transferred. No additional downforce is created. Therefore there is no traction benefit.
Lateral location of the vehicle body relative to the wheels is provided by linkages such as a Panhard bar, Watts linkage or angled control arms. The Panhard bar is the simplest in that it usually comprises a link with rod ends attached at one end to the body and at the other end to an unsprung portion of the suspension. Since body roll can cause the Panhard bar to steer the attached suspension, the Panhard rod is happiest when it is as long as feasible and mounted level. Generally the Panhard bar determines the roll center at the associated end of the vehicle, though stiff leaf springs can compromise this.
Leaf springs can under certain conditions transfer a torque load and create downforce. Unfortunately the are heavy and prone to deformation and twisting under load. If the leaves are clamped together at the forward end and permitted to slide at the rear end of the spring, the net effect is similar to a ladder bar.
