The Ultimate Guide to Suspension and Handling – Bump Steer/Toe Steer

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Other suspensions are designed to toe in under roll, the versions of the semi trailing arms found in the Porsche 928 and FC RX-7 have links and selectably compliant bushings to make the suspension passively toe in under roll.  This will help the car transition predictably to understeer as the limits of grip are reached.

Interestingly in the 80's, both Porsche with the 928 and Mazda with the FC attempted to correct a semi trailing arm suspension's tendency to toe out with an additional link.  This worked by having the extra link go through a tight arc which forced the control arm to toe in through bushing compliance.  Sorta cool until a bushing goes bad or someone locks up the rear suspension by trying to put stiff bushings in place.  This was one of the first times suspension geometry correction by dynamic bushing flex was attempted. 

Excessive lowering of a car can make toe steer worse by placing the suspension links in a static position far from where the factory designers intended them to sit.  A beam axle suspension with trailing arms found in many small FWD cars is a good example of this.  In stock condition the trailing arms are usually parallel to the ground.  When the car rolls the outboard arm goes down and the inboard arm goes up. The axle's toe change is zero because although the arms are swinging in different directions, their arcs are the same so the net effect on toe is the same.

Poor 911, bad toe control and a high rear polar moment of inertia made for interesting handling.  They still managed to win a lot of races.

Now if the same car gets dumped to the weeds using a Ricky Ricer lowering spring kit, the front of the trailing arms will point down towards the ground.  Under roll the inside link will now push the axle outward while the inner link will suck the axle inward causing a toe in condition and worsening under steer.

One way to change the toe curve is to move suspension pick up points.  A no fabrication solution that some companies offer is to use bushings with eccentric inserts that can move the suspension pick up points quite a bit.  This is a Whiteline part for instance.  By moving a toe link or traction rod pick up point in a multi link suspension you can really make a big difference in the toe curve even with a relatively small move.

Here is an offset bushing in action, as you can see the pivot point can be moved quite a bit.

The ways to reduce the negative effects of toe steer are many so just a few can be discussed here. For semi trailing arm suspension, to keep the car from toeing out under roll, the suspension should be made super stiff limiting its movement.  Then the suspension should be set up with a lot of static toe-in.  Although this solution seems like caveman engineering, remember that both the 510 and 3 Series BMW enjoyed a long and successful heritage in racing.