Bump steer question

[nicbowman]

Hi, Apolgies if you have read this in other forums, just looking for maximum advice!

My TVR Griff feels like it has lots of bump steer, very unstable on bumpy roads lots of kick back via the steering. Also Griffs tend to wear the steering racks out, both point to bump steer. I have measured 1mm additional toe out on rebound (per wheel) at 2 inches from centre. Doesn't sound a lot? But, is that a lot. How much is reasonable?

Any advice apprecaited.

Nic

[steve_d]

Bump steer is a result of the steering rack being at the wrong height relative to suspension travel. Unless you or a previous owner has change the suspension travel, wheel size, springs etc. then I doubt TVR got the design wrong in the first place or you would not be the first to squeal about it.

Far more likely is wear in one or more of the suspension or steering joint or even the rack itself. Time to get it up in the air and take a tyre lever to the joints and look for play.

Steve

[leorest]

steve_d said:

Far more likely is wear in one or more of the suspension or steering joint or even the rack itself. Time to get it up in the air and take a tyre lever to the joints and look for play.

Take it for an MOT. Quickest and cheapest way to get the "joints" inspected.
Leo

[Avocet]

1mm at full rebound isn't brilliant but it's not a disaster either. When the wheel is at full rebound, there's no real load on it so it doesn't matter that much. The important measurement to try and get is nearer full bump. It can really only be done accurately on a surface table with the spring removed and the rack pinion clamped. The chassis should also be clamped to the surface table. We used to use a granite surface table and two dial gauges clamped to the wheel studs and set the same distance apart as the wheel rim opposite sides. You then jack the suspension up with a trolley jack taking readings every 20mm or so until the bump stop is (say) half compressed. It's very difficult to do accurately enough because you normally need to reposition the surface table a few times during the process because the dial gauges run out of travel. I can't be certain but I'd have expected something like a Griffith to have had maybe 0.25mm to 0.5mm of bump steer at full bump. From what I remember, it's pretty much impossible to shim the rack up and down because it's mounted at an angle. Also, the cars did have something of a reputation for "tramlining" which might be a very similar feeling. Has someone fitted wider-than-standard front tyres? Also, is there any play in the rack itself (there's a stack of shims under the plate opposite the pinion) and / or the universal joints in the steering lower column or the bearing where it pases through the toeboard?