Bumpsteer.....
 

Hi All....

Really starting to get to grips with the cars setup now. Sorted out my creaky suspension and set Camber & toe better now (approx 0.5 - 1 deg of neg camber works well on each wheel, rear wheels set slightly toe in (1mm combined) and front wheels ever so slightly toe out (0 - 0.5mm). Also ride height now set at approx 145 - 150mm below chassis rails and the damper rates set right down (6/7 clicks in only)...tyres at 17lbs.

Set like this gives a good ride (not too harsh) but lets you have enough fun as well when you push it!

....but I do seem to suffer a little from bumpsteer...particularlyon uneven road surfaces......

Any ideas on the height the track rod ends needs to be set at to minimise this?

Rich

Hi Rich.

We (me and Merlinman (Ian)) was looking at this some months ago. While Ian's was up in the air, Ian mentioned to me about bumpsteer. One thing I remember Ian saying about it was rosejoints should be used as well as some other things. I'll get in touch with Ian and add some comments.

Also found this article. Not sure if it's any use.

From here:
http://www.thedirtforum.com/bumpsteer.htm

In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm (See diagram below).

Good article John...I'll get my tape measure out & see where I get to.

Rich

John

i believe we had a conversation like that with Ian maybe at Stonleigh last year, my concerns though, was he was considering the rose joints for the rear.

Steve

Hi Rich. Did you have much success with your bump steer elimination? I too am at the stage when I'm now getting happier with the handling but am not free of bump steer. Going to play with the setup again tomorrow I think and make sure my ride height is sorted, and then turn my mind to the bump steer issue.

Ric H (just to avoid any confusion) - your car seemed to lean quite a bit more on track than any of the other cars. Mark did mention to me in passing a few months ago that he's now running higher rate, shorter springs, and the road cars respond very well to this. Maybe time to play with these too?

Have you ordered the new rear uprights and top suspension arm?

Yeah, I noticed that myself. Particularly obvious in a couple of the photos. IIRC I've got 250s on the front, and 300s on the rear - already pretty short but could probably afford to go up some. on rate. That said, I'd hope I can trim out some bump steer as is without having to resort completely to reducing movement!

Haven't ordered the new uprights yet - sorting some other bits and pieces and giving everything a good run-in first. Then new uprights and calipers to follow sharpish I reckon. Then where to stop? New master cylinders with balance bar maybe too...

John

Intersting you mentioned about Mark useing shorter springs (8 inch 350LBS was mentioned to me), he used these when he had the issue with the wishbones rubbing, which smacked of a cheap and simple way of overcomoing the problem. I did explore this myself, to the point of speaking to the technical rep for Gaz shocks, he considered the idea of useing 8 inch springs, with the shock we have from Marlin to be inapropiate. The reason was, given the make up of the shock, the shorter spring would be likey to coil bind, and the front would have little or no suspension.

Interstingly Mark reccomended to me I used 10inch 225 LBS springs when I was building the car, as Dylon had some sucess with them I believe, I dont seem to get to much leaning, but I do find the tyre rubs the wings sometimes under very heavy braking on bumpy roads, although the ride height may still need taking up.

Steve

Steve,

Your comment about coil binding, confirms what I suspected when Mark mentioned the shorter springs to me. I'm not sure what springs I've got, but I can see me following your lead with the stiffer springs, if I have the same set-up as Rich.

All academic to me at the moment, so I ought to just shut up, and get building!

John.

The only way you will loose some of the bump steer is running a firmer set up and tweking the ride height to get the suspension arms more level.
The only way you will get rid of it totally is spend a bit of money and have the car setup by the pros, and have the pick-up points moved and new top arms made like Ken Robson has done.

This is not the cheap option though but is the best.