Rear Steer

Rear steer happens when the rear axle is not perpendicular to the center line of the chassis.  If the right side of the axle is forward of square or the left side is further back of square, the chassis will be steered to the right making the car feel as though it has understeer or “tight” as we dirt track racers call it. If the rear axle is skewed the other direction with the right side back and the left side forward, the chassis will feel like it has more oversteer or “loose”.

The rear axle can be “steered” one way or the other statically (without movement, sitting still) by simply lengthening or shortening the rear locating mechanism. Most racers typically set their car up square or ¼” one way or the other depending on the track they race on or driving preference depending if they like a tighter or looser feeling car.
Dynamically (in movement) however, the rear axle will skew when:
  • It moves vertically while absorbing bumps
  • When it moves for spring deflection due to cornering and forward acceleration g-forces
  • The rolling of the chassis from the wing forces.
Figuring out which way your rear axle steers the car dynamically can be a little tricky.
There are two standard designs of rear axle geometry used by the 600cc chassis manufacturers. The z-link, which locates the rear axle longitudinally (forward and back) by use of one radius rod on top of the bearing carrier and the torsion arm on the bottom. This is also the standard on full size sprints and midgets. The other type is the wishbone which uses a weldment that has two rod ends bolted to the bearing carrier and one bolted to the frame.

Figuring out how a wishbone steers the car is simple. As the rear axle moves vertically, it moves in an arc centered at the front pivot point where the wishbone it bolted to the frame. If the front mounting point is level with the center of rear axle it will skew front if it moves in rebound or droop (shock is extending) or bump (shock is compressing). Some cars have extra mounting holes for the front of the wishbone which will change the rear steer. If the mounting hole is above center, it will skew the axle back as it bumps and front as it droops. In this case, as the car rolls right and the rear axle bumps, it will steer the car loose. This can be an advantage when hitting a cushion to keep the car from getting tight. Of course, there are fairly big changes to anti-squat by making this change as well. We will have to save that for another article.
 
On a Z-link design, how much the rear axle steers is a function of rack height, torsion arm length, top radius rod length, top radius rod mounting height, and where the torsion arm and top link mount to the bearing carrier.
 
If the top rod and torsion arm are the same length, mounted the same distance from the center of the axle on the bearing carrier and are parallel at any time during the movement, then the rear axle will not steer at all as it moves vertically. That is the true Watt’s link geometry, I call it Z-link to avoid confusion with it’s shortened name “W-Link” which dirt track racers think (incorrectly) is a Jacob’s Ladder. I digress.
 
Any change in this true Watts Link (Z-link) geometry will cause the rear axle to steer. It will steer in an S shape pattern generally moving front in bump and back in drop. How much it steers is a function of how much the geometry has deviated from the true Watts design. There are many different designs on the market both in bearing carriers and frame designs. A few general things to consider are, If only moving the mounting points on the bearing carrier, the shorter the distance is between the axle center and the rod's mounting point, the less steer there will be. More rear torsion arm angle means more rear steer. A longer top radius rod means more rear steer.

I recommend that you make a full-size cardboard cutout of your exact geometry recreating your mounting points. You can use thumb tacks to hold the mounting points in place. Put a pencil at the axle center, move the axle up and down and draw the path the rear axle moves in. I think you will find it very interesting and educational. If you know CAD you can also easily draw your rear geometry, set up driven dimensions and get exact results.
 
Figure out how the right side and left side moves when the car rolls right, and wings left (if you race winged) and figure out when and how much it is steering the car loose or tight.
 
Knowledge brings freedom.