Bump Steer Mod

[apearce]

I have an early 90's chassis which I believe is the 'bump steer' one, so I thought while the engine is out this winter, I'll put in the latest bracket and rack, BUT:

Looking at it yesterday, it doesnt appear that the rack could go any further forward or up because the chassis rails and track rods are in the way. It could go down or backwards but then would be less well aligned with the travel of the wheel and would increase bump steer. In other words, it appears to be pretty optimal already....

My rack is definately mounted differently to the current ones as it is bolted upward onto the bracket (rather than onto the rear of it), but appears to be in very much the same overall position.

So to get to the point, does any one know what the change is with the later chassis - is it just a few mil different ?

cheers
Andy

[mikey]

I guess its the Dax you are on about. You dont need to change the rack position, just use the later suspension suppotrt plates etc - the difference is amazing.
Mike

[apearce]

you most certainly have my attention - i love a cheap mod! could you be more specific about the bits to change >?

(also, did you notice a problem beforehand or just an improvement afterwards since (as others have confirmed) not all of us can actually detect the legendary bump steer...

cheers
A

[dave.n]

I'm interested in this as I've been told that SR's suffer from terrible bump stear. As yet I have never experience it and I've had the car a couple of years now.
All kinds of roads in all kinds of weather. When and where do you get it.

[apearce]

Dave - the theory is this: you'd only really notice it going in a straight line as the wheels individually move up and down with undulations in the road- this would manifest itself as movements in the steering wheel (even though you are going straihght - hence its a flaw). going over a drain cover might seem more severe, like kisk-bask through the steering.

It sounds pathetic for me to say that i havent really experienced it but i guess what i mean is that the overall drive is a bit 'kit-car' ish and i cant specifically isolate bump steer from this overall mess.

cheers
Andy

[dinosoar]

Bump steer arises from a poorly positioned steering rack.
What happens is the uprights are steered by the up and down movement of the suspension. Ideally the ball joint for the steering drag link should fall on a line drawn between the top and bottom inner wishbone pivots.
Viewed from behind, these form a sort of V shape. Therefore adjusting the rack position up and down with shims will help to get it to the idea position.
The feeling manifests itself by making the steering wheel try to dance about on a rough road and can also result in the front wheels toeing out under braking. This will make the car dart about.

I have an early Dax chassis from the Eighties that I am in the process of updating to MarkIV front. The steering rack on these is from a triumph dolomite and runs over the top of the main chassis rails. They are renowned for bumpsteer problems and heavy diving under braking. I have jigged the dimensions from a markIV chassis and have removed the old setup and replaced with new. I just need to order a new rack, rollbar and column extension to finish. In all about a weeks work.

I think Mikey's suggestion is to replace the "springpan". This will have no effect bumpsteer but will improve the damping action due to a revised location of the bottom mount of the coilover.

I will put some photo's of the work in my gallery shortly.

Andy

[apearce]

thanks for that, so if my rack is BELOW the chassis rail then it isnt the one with the bump steer problem after all (even though the bracket is different to the current one) ?

A

[dinosoar]

I'm not that familiar with the characteristics of your type of chassis.
I think there were some issues with the rack type or location but you will have to get some feedback from others to confirm.

Andy

[clive a brown]

I would have thought that DJ's are the ideal people to ask?

send Gary or Peter a Email

Regards Clive

[mikey]

Originally Posted by dinosoar

Bump steer arises from a poorly positioned steering rack.
What happens is the uprights are steered by the up and down movement of the suspension. Ideally the ball joint for the steering drag link should fall on a line drawn between the top and bottom inner wishbone pivots.
Viewed from behind, these form a sort of V shape. Therefore adjusting the rack position up and down with shims will help to get it to the idea position.
The feeling manifests itself by making the steering wheel try to dance about on a rough road and can also result in the front wheels toeing out under braking. This will make the car dart about.

I have an early Dax chassis from the Eighties that I am in the process of updating to MarkIV front. The steering rack on these is from a triumph dolomite and runs over the top of the main chassis rails. They are renowned for bumpsteer problems and heavy diving under braking. I have jigged the dimensions from a markIV chassis and have removed the old setup and replaced with new. I just need to order a new rack, rollbar and column extension to finish. In all about a weeks work.

I think Mikey's suggestion is to replace the "springpan". This will have no effect bumpsteer but will improve the damping action due to a revised location of the bottom mount of the coilover.

I will put some photo's of the work in my gallery shortly.

Andy

Spot on. When i spoke to Pete at Dax I mentioned about raising the rack. He suggested to try the new spring pans first. The new suspension location moves the rack to near perfect position with the older dax rack location.

I did this but I didn't think it would sort it out fully. However exept for a very slight feeling of bump steer which only ever happened on long undulating roads it totally cured it. Drove one of the later Daxes with the steering raised and it felt no different - to be honest I had got mine so well sorted it really did handle confidently. 90% of that was with the new spring position anhd the rest sorting out caster and camber with a gauge set I borrowed. You do need to set your ride heights correctly also.

Mike