Bump Steer

[wobblyweb]

I hope I am OK asking this Lotus Seven question
I fitted new springs a shocks ages ago and the car has not handled properly since with bump steer.
I have taken off the shockers, with the car level if i lift the wheels they toe out ,drop them they toe in. But with the shockers fitted the car sits too high. (way above level)
What effect does adjusting the the springs have by moving the adjustable platform up, can I lower the car or does it just stiffen up the springs?? or do i need to slacken off to allow the car to drop.

[types26/36]

Rae,
Despite how high or low the car is it should not have bump steer, it will probably feel odd and handle strangely if it is too high but by adjusting the shocks it will alter the height and camber.
The wheels move up/down in an arc but the "toe in" should remain constant, if it is going from toe in to toe out (and vice versa) with the suspension travel then it sounds as if the steering rack height is incorrect and needs to be raised or lowered as necessary.
I had a similar problem with my Seven (Birkin), hit a bump and the car darted one way or the other, in a straight line under heavy braking (looking out the side) the wheels could be seen going from its normal slight toe in to a drastic amount of toe out and squealing the tyres.
I lowered the rack which sorted it out, when I repaired my Elan after a rally accident I made a jig from a friends +2 to get it in position but I spent hours with straight pieces of metal bolted to the vertical links (shocks fitted with no springs) raising and lowering the suspension and checking the toe stayed constant until I found the optimum position for the rack.
I believe there was a post on making a bump steer gauge either on the forum or the mailing list although it was quit a while back but may be a search will bring some details.

[twincamman]

too stiff a spring will transfer weight causing handling problems -------my 7 has 100 pound springs and the handling is controlled by rebound control and shock tuning --------stiffer is better in some things but not always in chassis tune ---the object of the game is to keep a contact patch on the ground so the tire works ----to do this compliance in the setup is some time required --as the 7 chassis design is prehistoric at the best ---rethink your set up back wards - the wheels must move and the a arms must go up and down -----too much of any one thing wont work ---ed

[john.p.clegg]

Rae
If you are happy with the shocks,springs and ride height all you can do is raise/lower the steering rack so that any deviation up/down from "normal" ride height produces the same effect ie. toe in or toe out (depending on wether the steering rack is placed infront or behind the axle)

If you manage this on both sides equally (remember altering one side will affect the other) then you will not be far out....

John
(complete amateur)

[twincamman]

an example ---the ?lan goes to 3 wheels on hi speed severe turns both left and right resulting in a dead feel 2 /3 ds of the way through the corner to the straight -but works well every where else - but to correct that problem would give bigger problems elsewhere -- stiffer pring rates -sway bar adjustment --chassis stress at the bar mounting areas -etc etc -now were chasing the car all weekend for one problem----some times its better to drive through a particular problem than compromise the rest of the handling ----ed

[types26/36]

all you can do is raise/lower the steering rack so that any deviation up/down from "normal" ride height produces the same effect ie. toe in or toe out (depending on wether the steering rack is placed infront or behind the axle)

Not following you there john but are you saying if the rack is either in front or behind the axle it will result in a different effect .....toe in as opposed to toe out? or am I misunderstanding your post?
The rack on a Seven is in front of the centre line but either way the suspension travel should not dramatically alter the toe (a Seven should have 0 to 3/16" toe in if I remember correctly)

[bill308]

wobblyweb,

Adjustment of the spring perch will have no affect on the spring rate. Adjustment of the spring purch lower will drop the ride height. Get this under control and then address the bump steer issue pver the normal running range. The rack height should be adjusted to minimize toe effects. I'm not familiar enough with your car to recommed which way the rack has to go.

Bill

[john.p.clegg]

Brian

Yes,if the rack is in front of the axle there will be toe-in,if behind toe-out.

As the wheel goes away from the normal ride height,up or down,if the rack height is set correctly,the wheel should toe-in in both cases by an equal amount ie. the left wheel will steer right and the right wheel will steer left...thereby negating each other....no bump steer....

John

[wobblyweb]

Thank you for the replies.
It is all becoming clearer now.
I have adjusted the spring perches the car sits lower and now the wishbones and rack are level and in line. The correct place to start from?
So, compressing the suspension should produce toe in or dropping the suspension should produce toe in by equal amounts ? Adjust the rack hight to suit ?

[steve71]

Hi wobblyweb,

Here is a thread about checking bump steer.

http://www.lotuselan.net/forums/viewtopic.php?t=2632

And a web site with a couple of pictures to help explain how to do it.

http://www.advancedracing.com/bump_steer_gauge.php

It isn't to hard to set up in the garage. You don't need a real proffesional looking setup. Just have to make sure the laser and paper you are plotting on do not move. Once set up it is really easy to see what the steering is doing as the suspension moves up and down. Have done this on an Elan and the difference after getting the rack hieght correct made the car a joy to drive.

Steve.

[john.p.clegg]

Rae
Yes,you've got it,just remember that the starting point is the axle at the right height relative to the chassis/body and any up/down movement should produce the same result...

The other thing to watch is that as you adjust the right hand side this will affect the left hand side also so you have to keep re-checking after adjustments and moving from side to side....


And as Steve says a laser pointer blue-tacked to the stub axle will give you a good guide as to the in/out movement...

John

[types26/36]

Here is a web site that explains bump steer better then us amateur's could ever hope too:
http://www.moddedmustangs.com/forums/bu ... vt875.html

[M100]

Despite how high or low the car is it should not have bump steer

I disagree with this!

Bump steer in the right direction is a very important aid to stability on turn in. A zero bump steer car feels dead and one that adds steering angle on the heavily loaded (and hence in bump) outer wheel is a nightmare to drive (very twitchy and prone to provoking oversteer)

Geometry that removes steering angle in bump makes a car much easier to drive.

[poiuyt]

I replaced the rear shocks and springs in my Elan and had terrible bump steer until I discovered that one of the shocks was bad. After replacing the shock the problem went away.

make sure yours are OK.

Steve B.

[types26/36]

Despite how high or low the car is it should not have bump steer[/quote

I disagree with this!

O.K....I stand corrected Would you agree if I were to change it to:
Despite how high or low the car is it should not have bump steer hitting a bump when going straight forward