Understanding Rear Steer
To even begin to understand how the car will rear steer and to what extent, we first need to completely understand the movement of the chassis and what causes this movement. The chassis mounting points of the four-link and Z-link will move vertically as the car transitions into and out of the turns and even down the straightaway and the amount of movement dictates the degree of rear steer.

As a chassis rolls in the turns, three basic things can be happening overall: 1) the left side of the chassis may move up and the right side may move down, 2) the right side may move down and the left side may stay near that static location, and 3) the left side may move up and the right side may remain unchanged. With the same set of suspension link locations at each side, a car may well produce very different rear steer characteristics from each of the three scenarios.

A four-link can be made to produce varying amounts of fore and aft movement of each end of the rear axle in either direction, depending on the combined angles of the links. If we start at a neutral setting for the links, meaning that for a certain range of movement up or down, the axle won’t move fore and aft, let’s see how we can produce axle movement.

If, on a four-link, we move the chassis mount for the bottom link up, then as the chassis moves up, the rear axle will move more forward. On the Z-link, we see the same effect for the bottom link. The opposite is true if the chassis moves down. For both systems, the axle would move to the rear. That is exactly why we need to know which way the chassis is moving at each side of the car under all conditions.

Knowledge of the extent and direction of shock travels will come in handy as we plan out our rear geometry. We can translate shock movement to suspension movement. Using either shock travel indicators or data acquisition will tell us what is really happening. I don’t see widespread use of electronic data gathering on dirt cars, so some mechanical device must be used to help us understand our true movements.

A direct influence on chassis movement in the turns is the J-bar or Panhard bar. If the bar is mounted more parallel to the ground, then it will have little influence on the vertical location of the chassis in the turns and the chassis will move similar to Examples 1 or 2. If there is a lot of angle in the bar with the left end higher than the right end (chassis mount to the left side as is most popular), then as the car turns left, the bar angle will have a jacking effect causing the left side of the bar to want to ride up over the right side of the bar as in Example 3. This movement would raise the entire rear of the car.

Under those conditions, if the car rolls, and we know it does, and the whole chassis rises up, as we too can visually see, then the right side links may well remain in their static locations producing near zero rear steer at that side. On the other side of the chassis, there will be a combined vertical movement of those links to where the lift associated with the bar angle will be combined with the roll lift so that they can produce a large amount of forward movement of the left rear wheel. This movement pulls that end of the axle forward and the rearend will steer to the right.

Upon acceleration off the corners, the rearend will be driven forward and if the forward ends of the links are higher than the rearends, there’s a further movement of the chassis to a higher level.