Bump steer

[SUSIT]

I realise in an ideal world we would want zero bump steer on our cars but some of us dont live in an ideal world (we own Allora's) So can I ask those of you with more knowledge (or books) than me what would you consider an Acceptable amount, having spent close to a day on my car we have managed to reduce it from 1 degree 10 mins on full droop ( dampers & springs removed) to 30mins. Had to move the rack down 1 inch to get this. Any further movement of the rack made things worse again.
Any comments welcome.

[chris.richard]

I think the amount on bump is more important than droop isn't it? i.e. when landing/braking with a lot of weight on the front.

[SUSIT]

Good point Chris, on full bump it is the same ie 30 mins toe in. My original post was'nt that clear. As I said we had the springs & dampers removed and used the ramp to lift & lower the car with fancy plates under the tyres to allow them to slide. we measured through the full range of suspension travel.

[SUSIT]

Still a long way to go

[strat6v]

Steven,

Just as Chris said, reduction in bump rather than droop is what you want.

John.

[SUSIT]

John,
Thanks we have reduced it from 1 degree 10min in bump to 30 min but without major modification cant get it lower, just trying to work out if its worth a lot of hassle time effort and money to try and improve on the figure of 30 min that we now have.

[chris.richard]

Can you get it to 0 on bump, but more on droop?

[rutthenut]

Have you made changes to the outboard side of the suspension (steering arm/tie-bar position), or just the inboard side at the rack mount?
J.R.

[SUSIT]

Have you made changes to the outboard side of the suspension (steering arm/tie-bar position), or just the inboard side at the rack mount?
J.R.


Only moved the inboard side John

[rutthenut]

Probably harder to do, but you may get better results if you can also change the height of the outboard end of the steering arm/track rod.

If this is converted to a rose-joint mounting, that would give you more options to adjust height than working with a nromal track rod end.

For some back-of-fag-paper calculations, you could try to work out the relative position of the inner end of the track rod (end of steering rack) as a fraction of the height between upper and lower suspension arm pivots. Then aim to get the outer end (track rod joint) to be at a similar relative position between the outer points of the suspension arms. For instance, inner point may be about 2/3 of the way between lower and upper pivot points, so aim for outer joint to be a similar 2/3 height between pivot points on the upright.

That's just a rough guess though, from which you would continue with yet more of your fine-tuning and aim to get less bump steer in positions where suspension would be under load (during bump or braking weight transfer).

The lateral position of the different mounting points would of course affect the results, as well as the vertical position, but that will be much harder to modify without changing steering rack or uprights.

Apologies if that is all stating the bleeding obvious.

Sounds like a lot of time and effort in any case. Hence your question as to what may be an acceptable figure - and which nobody could really answer. Other than stating that the ideal would be zero under bump!

J.R.

[chris.richard]

Spotted elsewhere re TVR :
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.