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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.
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.
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.
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.
4 comments
I lowered my 991.2 Carrera GTS (2017) by approximately 30 mm in the front using Öhlins shocks. The lower control arms are hence directed somewhat upwards, however, the rod is also directed upwards. Maybe, they are nearly parallel. I am not sure if I should compensate for bump steer or this modern car compensates it automatically. I do not know if it developes a significant amount of bum steer as it is now.
As long as you didn’t correct the roll center you will be ok.
If I see it correctly, the roll center is lowered by lowering the car.
No