The Basics of Rear Suspension Systems - Tail End Tales

From the February, 2012 issue of Street Rodder

By Ron Ceridono

When the time comes for most Street Rodders to consider the suspension that will go under their car, and the impact it will have on the way it steers, rides, and handles, consideration is given to the front of the chassis. However, when it comes to what goes under the rear of the car the main concern is often ride quality. However, the rear suspension is critical to the car’s handling as well.

Two of the most common terms used to describe an automobile’s handling are understeer and oversteer—in simple terms both determine how well the car goes around corners.

Understeer in a turn means the front tires are slipping and the car isn’t going the direction the wheels are turned—it feels like the car wants to keep going straight. Usually accompanied by the front tires howling in protest, understeer has been used by some factory engineers in an attempt to make cars “idiot proof.” Understeer scrubs off speed, which can serve to slow the car in a corner, at the expense of the tire’s treads and the driver’s dignity. Oversteer means what it implies, the rearend is coming around like a sprint car in a corner and the car is turning sharper than the front wheels are pointed. Many drivers find oversteer frightening (which is why most passenger cars are designed to understeer). In between understeer and oversteer is neutral steering, that is the car simply follows the front wheels and goes around corners with no muss or fuss.

When a car goes around a corner the body rolls about points at the front and rear, these are the suspension’s roll centers. (A related term that is often heard is roll center migration, which means roll centers move for a number of reasons, including suspension deflection.) Most race cars have roll centers that are close to the ground, which transfers less weight off the inside wheel in a corner, resulting in better traction and higher cornering speeds.

Another term to be familiar with is roll axis. If a line is drawn through the front and rear roll centers that’s the roll axis. In most cases the roll axis is at an angle and is lower in the front because in most cases the front suspension has a lower roll center than the rear. Picture a Model A chassis with stock transverse springs. The roll centers at each end are where the springs attach to the crossmembers, which in this case means the roll center in the rear is much higher than in the front, and the roll axis is at a relatively steep angle. As a result, going fast around a corner in a street rod with Model A springs would result in transferring a considerable amount of weight off the inside rear tire with a resulting loss of traction.

For our purposes we’re going to say that all street rods are rear-wheel drive, which means the suspension on the back end of the car has to cope with acceleration, braking, and cornering forces. That means the axle housing has to be located fore and aft (while resisting the rotational forces applied to it) as well as side to side. Those requirements bring up a few more terms that will be used when discussing rear suspensions—instant center, roll steer, and antisquat.