Bump steering cure?

[aidanc]

Hi here in New Zealand the transport authorities have chosen to make it hard for anybody to convert thier LHD S1 to RHD by requiring the bump steer to be better than when new any help would be great. Is it possible to remove all bump steer?

[Gfhug]

How do they measure original bump steer for a car that is fifty years old? Do they have some sort of reference figure to compare it with? Wouldn't this be the same criteria whether you are converting LHD to RHD or just restoring a RHD car that's been in a barn for years?

Are you able to get any information from them that would help you know what you have to achieve? It would possibly be down to steering geometry which can be adjusted as per the manual. Would polybush mounts make a difference? I'll wait for the more technically knowledgeable to answer that.

Good luck in sorting out another piece of jobsworth

Geoff

[Heuer]

Polybush steering box mounts will make it worse so stick to stock Metalastic ones. Convert the car and take it to be tested claiming it is better than new. Can't see any way they are going to figure it out. You could mess with the toe-in settings temporarily but my view is a good well setup E-Type does not have any bump steer.

[jag68]

Bump steer or not is caused by the relationship of the suspension arms to the steering arms and tie rods. All rotate on different circumferences so there is pushing and pulling of the things they are attached to - the upper and lower ball joints and the steering arm respectively. When you hit a speed bump for example the suspension rises relative to the frame. The upper control arm is on a smaller circumference than the lower control arm (see front suspension diagram in the manual) which causes the upright to lean in at the top (goes into negative camber). The steering arm moves in and out with this, relative to the rack end, but if the distance of the steering arm to the attachment point on the rack through the tie rod doesn't change it has to push the steering arm out giving you more toe out. This is bump steer. It gets more complicated when you have chassis roll on a turn causing the suspension on each side to move in different directions, and the location of the rack/tie rod attachment point moving in and out as you turn the steering wheel.

Bump steer is built into the suspension design - it can't be altered by shims, alignment etc. Asking somebody to alter it on a safety check is insane. Typically a car is designed to minimize it by having the tie rod run more or less parallel to the upper or lower control arm - whichever is closest to the steering arm on the upright. To change bump steer you have to modify that relationship either by raising or lowing the rack, or the steering arm, or modify the movement of one or both the control arms.

An example of this was a modification Jaguar did to the E Type when they raced it. They lowered the inner pick ups for the upper control arm by about an inch. This arm then sat with the ball joint end higher than the inner bracket and fulcrum arm. This destroyed the relationship with the tie rod and led to unacceptable bump steer. The fix was to lower the rack until the tie rods were again mostly parallel to the control arm or alternatively redo the steering arm to raise the height where it connected to the tie rod end.

Measuring bump steer is easy/hard. You have to move the suspension through it's range of motion while measuring the changes to toe in and toe out. Racers do this all the time, but you generally have to disconnect the springs/torsion bar (the hard part) I can't believe anybody would ask you to do this ona safety check like procedure.
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[Gfhug]

It'd be nice to hear if the OP has made use of this information he's been given