Bump steer...

[mark030358]

Gents,
Does anyone have a really good write up on how to set up an elan to eliminate bump steer?

Even though all my suspension settings are within tolerance I still get an uneasy feeling when at speed. Talking to a chap called Ian at Swettenham today he suggested bump steer....so here goes.

cheers
Mark

[twincamman]

try a 1/8- total toe in --a 16th a side ---bump steer is when the chassis describes a arc up and down and the wheels go to toe out ---ed

[sk178ta]

I know little or nothing about suspension adjustability, in fact I don`t think there is any apart from toe in/out. But I do know that when you buy a new chassis there are shims pre-fitted to ensure the height of the steering rack is millimetrically perfect, otherwise you get bump steer. These are calculated at the factory on a special jig and beyond the realms of the home mechanic. I should contact one of the specialists for advice, maybe Spyder.
I was also at Sweetenham, though we didn`t speak. Is your Elan Lagoon Blue over White?
Jim

[mark030358]

Jim,
Thanks for the reply. Yes mine was the lagoon blue car. Maybe next month we can have a chat!
Ed,
Sent you a pm on the subject re your problems is september. Toe in is as you prescride.

cheers
Mark

[marcfuller]

Take a search in the archives. Or a read of Fred Puhn's book "How to Make Your Car Handle"

Adjusting bump steer on an Elan is within the realm of anyone that claims to have mechanical abilities, even a home mechanic. There are at least four good discussions regarding methods of measuring and setting bump steer on an Elan in the archives. Search on the following names and bump steer.

Paul Adamson has a simple lowtech method that uses just a white board, marker and tape. And beer.

Keith Franck (type26owner) has a method that uses a home built Autocollimator using (costing about $25 total) laser pointer and a mirror. He walks you through the materials and assembly and use.

And there are others in the Forum archives. Rob_LaMoreaux, Stan Arhus and Bill Tebbutt also have some good contributions on bump steer in an Elan.

[mark030358]

Gents,
Thanks for the information, just a couple of things I need to know:-

1) Is it an abosolute requirement to lift both the front and rear of the car, or will just the front do?
2) Over what range do you measure the bump steer?
3) What is the tolerance to look for?
4) Is it best to always toe in or out on bump and droop if you cannot get it exact?
5) Fully understand (I think) about raising the rack, but not sure what moving fore and aft does. Can anyone explain?

cheers
Mark

[paros]

To correct bump steer you need lots - and I mean lots of home made shims to go beneath the stering rack.
Checking involves removing spring and dampers.
Set car under X member at ride height and wheels off car BUT stub axle at correct height from ground.
Place a flat piece of steel or formica onto hub and bolt it on with the stud nuts of KO.
Then move the stub axle and wheel up and down from the static position.
The plate will toe in or out.
To check I use a gallon can full of water at each end of the car with apiece of string between each can running along the length of the car. Usually just touching the rear wheels to give a datum so the rest of the string comes forward past the plate.
Then measure the gap as you move the plate up and down. Now strat on the shims as you find it goes toe in on bump and out on droop. Basically the track rod ar, is describing an arc as you move the plate and this pulls the wheels in or out - bump steer!
Takes a day and much beer
Richard

[M100]

Don't waste your time trying to eliminate all bumpsteer as you will end up with a car that is a pain in the ass to drive near the limit. It's not a case of an incorrectly positioned rack, Lotus designed in the bumpsteer (as they continue to do today on the Elise for instance) to actually ensure stability so the main thing you must ensure is that the bumpsteer is *always* be in the right direction.

As you turn in to a corner the car will roll, so one front wheel effectively goes into bump the other into rebound. To prevent the car from turning too rapidly (going unstable) you need the suspension geometry that in roll will turn the wheels slightly against the direction you are turning, otherwise the car will turn in very quickly as positive steer angle is induced by the geometry.

If this is not done, then depending on the roll stiffness this extra steering movement can make the car turn in so hard that oversteer is induced and the car becomes very hard to drive smoothly. Introduce the correct amount of bump steer (i.e. get the slope right) and make sure it is in the appropriate direction and the car will be *much* easier to drive.

[mark030358]

Martin,
Thanks for the reply.
So if I read you correctley then the thing to do is get it to toe out and not in? Question is by how much? ie if I measure with a DTI across the width of the tyre what movement am I looking for?
I am doing this as this is the only thing left that I can think of that might try and cure my straight line stability issues. I think I said earlier that a laser alignment showed nothing amiss with any of the geometry front or rear, but yet the car is not stable.
I have tried all kinds of tyre pressure combinations and nothing seems satisfactory.
This situatin is really depressing me.

thanks
Mark

[john.p.clegg]

Not stable?
Have you checked the steering rack,arms top,balljoints etc for wear...
The reason for this being after my plus2 passed its MOT it felt a little vague and on investigation,found plenty of play in one top balljoint....

John

[reb53]

I see no mention of tyres.
You haven't just got some new ones, different from usual? I ask because I once had a similar problem with really squirmy, snap over-steer type handling which turned out to be my new rear tyres.

Ralph
71 Sprint.

[M100]

Look at situation when going round a right hand bend. The car will roll to the left with the left wheel going into bump and the right into rebound. To counteract the steering angle input (from the driver) to the right, the wheel experiencing bump needs to turn slightly away from the direction of travel rather than towards it, so as you rightly identified the requirements are:

toe out on bump, toe in on rebound

A lot depends on what toe setting you are using and the roll stiffness but look to getting a max increase of around 1/2 of the static toe measurement at full bump - say around 1mm toe out increase from normal ride height to full bump at each side.

Have you looked directly under the steering rack mounting rubbers to see if there are any markings engraved on the chassis?

As John said I'd double check the balljoints, track rod end, steering rack balljoint and steering rack end bush for wear. Also check the right hand end of the rack for wear (there is no bush) and the pinion endfloat.