Bump steer

I’m trying to get a better understanding of bump steer.
Adjustments pretty much just move the curve along y-axis.
Makes sense since I’m not altering any suspension component length, but rather just moving the steering rack attachment point vertically.
The idea (may be terribly wrong) was to minimise toe out in bump and allow toe out in droop. See curve below.
Or does it make more sense to set it symmetrical?

There are some measuring errors in there I know

There is an excellent article on the causes, effects and remedies of bump steer here:



In my case, from the article:
Symptom 4. Always toes out on compression and rebound.
Cure 4. Shorten tie rod as it is too long.

It’s the cure I don’t get. How can that be done at the front on an Exige?
If I shorten it I will change my toe setting, no?

Just to add.
The “cure” to “toe out both in bump and rebound” at the front a on a Lotus is the opposite to the article.
It needs to be longer, not shorter.

Shorten or lengthen depends on rack position - in front or behind the axle line.

Sorry I can’t add much tech info, but I do find this interesting. What’s the scale up the Y axis?

Whatever adjustment you make on the track rod, you need to do the opposite at the rack, hub or split between both to maintain the correct toe setting.

Essentially what you are doing is altering the distance between the pivot centres of the track rod geometry so both inner and outer coincide with the lines through the inner and outer suspension arm axes.

double post

Y-axis I s wheel travel where 0 is wheel position at your set ride height. Positive values is Bump, negative is Rebound.
X-axis is toe

Clear, do we have any other way of adjusting rack height than the rack riser plates?
I now have both 8mm riser plates and Spitfire VRSAS.

Also, is my bump steer curve bad compared to std?
If I just add a shim at the track rod I move the curve down and bump and rebound will be close to mirrors.

Got it, was looking on a phone last night and it was a little confusing to look at. Makes sense!

You might want to read this article from exiges.com, especially the remarks from Lotus about why a certain amount of bump steer was designed into the geometry:

After a bit of fiddling around I settled for 1mm shim on the VRSAS steering arms and the rack raised using 8mm riser plates.

@40mm bump I get 0.7mm toe out
This is @115mm front ride height on a S2 Exige.

I could move the curve upwards by going for 0 shim.
That would get me close to 0 toe out @40mm bump, but… it would mean I would also get up to 0,5mm toe in from 0 to 40 in bump.

Shoot me down in flames because my experience of looking at bump steer is in Elans, but you don’t really want toe out when breaking, ie you are going into mild bump.

From linked thread above:

Dave Minter explains what’s going on. ‘The Elise had
too much grip at the front originally,’ he says of the
prototypes, 'and in the wet it could spin suddenly.We
had to fit an anti-roll bar and make the front wheels
toe-out over bumps, to add understeer and have some
chance of the rear gripping. That’s why the steering
writhes. It’s very “un-engineering” but we had to do it."

Well I agree, it certainly is unstable to have increasing toe out under heavy braking. Surely that makes it undesirable. Perhaps the quirkes of the Elise chassis mean that other positives outway the draw backs…