[ChrisL]

I hope that this question isn’t too dumb, but something just doesn’t quite gel in my mind and I want to understand it before I make the (not insignificant) effort to correct the problem.

I’ve performed some very simple (and approximate) calculations to determine what sort of movements are required to get a 2.5 mm toe out with the current geometry and this is what I got…

All measurements are approximate.

• Distance from chassis pivot point of A-arm, to outmost point of the wheel is 750mm
• Distance from center of wheel to leading edge of wheel is 300mm
• Length of lower trailing arm 800mm
• Length of upper trailing arm 1000mm

Triangulating the required rearward movement of the wheel to the chassis pivot point of the A-arm, to achieve a 2.5mm toe out, the wheel must be pushed back 6.25mm

Triangulating the movement of the lower trailing arm to achieve this 6.25mm, the trailing arm would need to move through about 7deg of a radius, or 100mm of upward travel. But at the same time the upper arm would extend by about 5mm so the effective extended length of the lower arm is only about 1.25mm.

So to achieve a difference of 6.25mm between the upper and lower trailing arm, I did some progressive calculations and concluded that the wheel would need to travel a wopping 220mm of bump (lower trailing arm would sweep 15.5deg) to achieve 2.5mm toe out. But in doing so, the wheel would also need to be pushed back a wopping 30mm.

220mm of bump and 30mm change in wheel base sounds excessive and unlikely to me. Furthermore, only a fraction of that overall movement would be a contributing factor in destabilization during hard cornering, since the tyre will provide maximum grip when the suspension is loaded and already compressed. It seems to me that although the toe-out theory is true in principle, in practice there would be very little in it… or have I missed something?