Now that you know what instant center is, let's talk about roll center. Every suspension design sets a point at which the vehicle "rolls" from side to side. Most of the time the point moves around depending on where the suspension is in its travel, but it is still important. On a straight-axle-type rear suspension the roll center is "set" by whatever is used to eliminate lateral movement (e.g., Panhard bar or even a triangulated-type link system). What happens is that the center of gravity acts upon the vehicle through its roll center. The higher the roll center, the lower the vehicle feels in cornering. Don't get greedy, though. Too high of a roll center and the car will act odd. The chances of having a high roll center at the front suspension is not good, and the vehicle will act unbalanced. Any decent rear-suspension design will have a roll center around the center of the rearend, which is fine.

Anti-squat is really not a major concern, except that it should be at least taken into consideration while building a link system that is limited to short bars. Anti-squat also affects roll steer and vise-versa. (Roll center can also affect roll steer but isn't a concern with a properly designed system.) What anti-squat does is fight the effect of weight transfer trying to "squat" the rearend while under acceleration. To create or eliminate anti-squat, you would design the link system to have the instant center a certain height related to the center of gravity. A higher instant center has more anti-squat, while a lower instant center has less. At some point, too high of an instant center will actually lift the rear up as the vehicle accelerates. And too low will push the rear down (though a little tougher to achieve).

Roll steer is a little more important and could cause some really poor dynamic effects. Roll steer also comes in flavors: roll understeer and roll oversteer. Every straight-axle rear-suspension design that actually affects performance has roll-steer in some amount. Roll steer is always measured when the car is leaning toward the outside of a corner. For example, if you were to turn left, the vehicle would lean to the right, so with the right rear wheel being compressed and the left wheel in droop, and geometry being what it is, the rearend would try to steer the vehicle into or out of the corner. The issue is caused when the rearend steers out of the corner, causing roll oversteer. What happens at the controls of the vehicle is, once the corner is entered and the vehicle takes its "set" into the corner, the rearend suddenly steers the car further than initially anticipated. Now, under normal driving conditions this is at best annoying, but really driving into a corner and having the rearend oversteer can put the car into a spin. In really severe conditions the vehicle would be extremely difficult to drive over 30 mph. A small bump in the road would really upset the chassis and almost can't be controlled. However, the confines of the typical mini-truck don't really leave much room for severe oversteer, and severe understeer is hard to achieve.

Pinion angle is probably the one thing that most builders are completely aware of but many are not sure how to set up properly. The pinion angle should be between one and three degrees pointed down. Most of us have already heard that, but the thing is that the major detail left out is that it should be one to three degrees pointed down from parallel to the transmission (or the front driveshaft on a two-piece driveline). What that means is the pinion could, under certain circumstances, point up. We don't have nearly enough space here for me to explain how to design the suspension that you may desire, but give us some time; we'll get there.