Discussion
Hi motor-racing fellows. I have switched forum temporarily to see if anyone knows about bump steer. I have a TVR Griff, which has 300 bhp but won't steer in a straight line! Seems a common complaint but no-one fixes it (so it seems?). Question is, I can measure 1mm toe additional toe out (per wheel) on rebound (not done compression yet) at 2 inch travel from centre. Is that a lot?
How much bump steer would be acceptable? (I know zero would be good!).
I believe I need top lower the rack to fix, this is not possible so I can only increase camber to effectively raise the steering arm at teh wheel end, same effect. (rack at teh front, bolted down onto the chassis, so can only space up).
Any advice appreciated.
On a slightly different subject, is tramlining the same as bump steer??
Thanks a bundle.
Nic
How much bump steer would be acceptable? (I know zero would be good!).
I believe I need top lower the rack to fix, this is not possible so I can only increase camber to effectively raise the steering arm at teh wheel end, same effect. (rack at teh front, bolted down onto the chassis, so can only space up).
Any advice appreciated.
On a slightly different subject, is tramlining the same as bump steer??
Thanks a bundle.
Nic
touringfan said:
Sorry.....pressed the wrong key before I finished.
Bump steer is, as it sounds - the wheels tend to overwhelm the steering when on compression or rebound (hitting a bump in the road).
Your issue sounds like it is a case of the camber/caster setting.
Surely caster affects bump steer.....
It doesn't sound too bad
Try to get a reading of how the toe changes when in bump. When in bump does it go to 0 toe or even to toe in? Or is it always at toe out? Just one more question, is the steering rack in front or behind the centre line of the wheel(think its behind)?
>> Edited by nightdriver on Monday 16th January 11:55
Surely caster affects bump steer.....
Yes it does also, which is why I suggested he checks his caster (as well as the offset wheels issue).
However his description of the problem doesn't really relate to bump-steer. Bump-steer is a factor if the car is reacting on "bump", not merely if the car does not track in a straight line. Its sounds more like a tracking/geometry issue.
Yes it does also, which is why I suggested he checks his caster (as well as the offset wheels issue).
However his description of the problem doesn't really relate to bump-steer. Bump-steer is a factor if the car is reacting on "bump", not merely if the car does not track in a straight line. Its sounds more like a tracking/geometry issue.
Hi, rack is in front. The car rides in a straight line on a flat road, but on a bumpy road the steering kicks quite aggressively, and the car veers to either side. It may be that more camber will aid, and this should also effectively raise the outer end of the rack, making some change to the bump steer (either way).
All in all, although the geometry will affect the bump steer, i.e it can be wrong in any setting, but aren't they seperate issues? (i.e. you can set the castor at what you like and still avoid bump steer?)
Ta
Nic
All in all, although the geometry will affect the bump steer, i.e it can be wrong in any setting, but aren't they seperate issues? (i.e. you can set the castor at what you like and still avoid bump steer?)
Ta
Nic
touringfan said:
Surely caster affects bump steer.....
Yes it does also, which is why I suggested he checks his caster (as well as the offset wheels issue).
However his description of the problem doesn't really relate to bump-steer. Bump-steer is a factor if the car is reacting on "bump", not merely if the car does not track in a straight line. Its sounds more like a tracking/geometry issue.
No it doesn't if taken in isolation, but changing castor on the griff does in this case as you can't separate the steering arm from the hub. Castor is a separate thing in it own right. bump steer is purely hte change in toe as the suspension goes through it's bump/rebound range, nothing more and nothing less.
If you have incorrect offset and this is taking the wheels one way then that's down to incorrect offset putting a leverage about the kingpin inclination, that's not bump steer.
If you go over a bump and the car is thrown off the bump one way then that's just the car being thrown off the bump, it's not atually bump steer.
You can modify almost all parameters of suspension independantly without affecting anything else if you understand what's going on.
On the griff you can tilt the hub and raise the steering arm to eliminate bump steer a significant amount, but you increase the trail/castor at the same time. If you wanted to keep that the same as std then you'd need to lower the rack as nick says (difficult) or raise the steering arm independant of the hub (difficult on tapered track rod ends but possible with rose joints and spacers).
Changing the camber on the griff will have a small but tiny effect on bump steer as you're effectively altering the length of the top wishbone pivot points, but it's effect is insignificant in the grand scheme of things.
By far the easiest way is to tilt the hub to raise the steering arm as first suggested ..
joo.
>> Edited by trackcar on Monday 16th January 14:46
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