DeTomaso Mailing List: March 2000, Message #249

Tony wrote:

>I have seen rack mount spacer kits advertised to reduce bump steer.  They
typically say "especially non-lowered suspensions."  Is that "lowered" like
removing the factory spring spacer, or lower?  If one removes the factory
spring spacer, does the rack spacer kit still yield a substantial
improvement?  Moreover, does the kit do what it claims either way?

>>>I have one of these bump-steer kits and elected not to install it.  The 
fundamental geometry problem is caused by the fact that the body of the car 
was raised by spacers relative to the spindle height; the steering rack was 
thus raised with it.  On cars with the stock spacers, the tie rods droop down 
to the top of the spindles.

Thus, when you hit a bump and the suspension compresses, the tie rod end 
moves through an arc.  Not only does it move up, but the tie rod end also 
moves OUTWARD slightly.  Thus if you hit a bump with both wheels together, 
the toe suddenly goes from inward (toe-in) to outward (toe-out.)  This is 
normally imperceptible to the driver.

But if ONE wheel hits a bump, then that wheel turn outward while the other 
wheel remains in the same relative position.  The result is that the car 
suddenly veers to one side, the side that hit the bump.

By raising the tie rod ends so they are more or less parallel with the ground 
(by any means), this effect can be greatly reduced.  If you lower the car 
(remove the spacers) this can go a long way; if you lower the rack in the car 
(which is what the factory Group 4 cars did; check the factory parts book for 
the spacers they used) you can have the same effect.  The bump-steer kit is a 
sort of band-aid which raises the tie rod end relative to the top of the 
spindle.  I suppose it works too.

I happen to have ride-height adjustable shocks/springs on my car, and in 
their current position, the car is lowered enough so the tie rods are just 
about parallel with the ground.  Good enough for me!

Note:  If your car is lowered and you fit the bump-steer kit, you can create 
the opposite problem; if the tie rod has to reach up to get to the spindle 
arm, then when you hit a bump, the tie rod end will move inwards as well as 
upwards, causing the affected wheel to steer towards the center of the car, 
and possible give you bump-steer in the opposite-from-normal direction.

If you install a bump-steer kit, make sure you do it EXACTLY as the 
directions call for.  Remove the spindle arm and mill it out, or at least use 
a drill press and secure the spindle arm properly; don't just hack at it with 
a hand-drill.  Getting that hole centered is critical for strength (and thus 
safety)

Mike