Bump steer survey - does everyone end up raising the rack 9mm ?

[Jason Plato]

🤔

 

Edited by - Dave Jackson on 26 Apr 2012 21:38:35

[Shad]

I haven't, but I'm wondering if I should. However, if I do then my rubber gators on the rack would foul on my sideskins

 

I've gotten used to the bump steer now to be honest.

[Paul Deslandes]

Yes Dave, so far it looks as though they do, odd though it may seem. Due to 'unforeseen circumstances' I've had two new long fronts on my car and they've both needed the rack to be raised by 9mm, one with the later front shock mount position. And that seems to be the experience of several others that have gone to the trouble of accurately measuring it.

 

Shad, you may have got used to the bump steer but you'll find it as revelation if you make an adjustment. But you're quite right, the gators do foul the side skin unless you cut the hole out a bit.

 

Paul

[EFA]

Ah well you should have got some of those steering joints on the bulk buy. They enabled you to lower the balljoint instead.

 

No more left now though.

[Damdy-Cash]

very interesting topic, that is a job I was also going to do on my 1992 DD chassis. So there is a fair chance that I didn't have to run the whole procedure because I will anyway endup with 9mm?

 

Cheers Volker

[Bricol]

Just be glad it's not an antique Dutton - had to move the rack an inch or two down to remove bumpsteer, and couple of inches back to induce ackerman effect, rather than the reverse ackerman it already had!

 

Bri

[stevemersey]

9mm so far....still a little fine tuning I think when I get some thinner shims.

[Eugene]

Classic chassis - no problem... 1984 live axle.

 

Interesting. I wonder what has changed, and why?

[Paul Deslandes]

I wonder whether the rack mounting plate height was set for the earlier live axle front suspension setup and has never been checked or changed for the DD suspension. Either way the error does seem to be remarkably consistent. I wouldn't worry too much about the last 1 to 0.5mm as I suspect you won't notice the difference and there is a small amount of variation with suspension rise and fall anyway.

[Mavic82]

Having had two of them I know of at least three different height rack platforms heights have been used on the DD chassis. As a rule of thumb the older the higher.

[ACJ]

I have spent a lot of time measuring bump steer and adjusting it out using the shim methon for race cars. I agree with the rule of thumb 9mm rise to get rid of most of the bump steer. (for the later chassis). Also the comment made earlier where the last 0.5 / 1 mm will not be noticed on the road is probably true as well.

 

A few years ago Catherham modified the rack mounting clamps so now they come with the lower half taller than the top half. They have counter bored the cap head bolts so you can still use the original bolts even though the overall clamp is taller.

 

For road use a easy fix is to check if your mounts are the same top and bottom, if not purchase the updated ones. Also don't forget to reset your tracking after moving the rack!

 

For race use I think the time consuming and more accurate measuring / shim method will always give the best results as everyone is looking for the last tenth!

[Jason Plato]

so is the general trend of 9mm only applicable to racks with equal sized mounts ?.

 

- those that have measured and corrected the height by 9mm - were your rack mounts all equal size ?

[Paul Deslandes]

The old style mount that I have raises the rack about 6mm or 1/4" above the bridge. I added an additional 9mm of spacers to raise the rack a total of 15mm above the bridge, i.e. 6mm for the mount plus 9mm of spacers.

[prs]

June 2006 Low Flying has got Paul's Bump Steer article

[Paul Deslandes]

or download it here. The online version has some additional bits on doing the measurements a bit more easily.

[Jason Plato]

I appreciate the articles, but my thought is that there is a common change here that corrects a significant % of bump steer that could be usefull for everyone to know if we were to bring collective knowledge together

 

could even pass it onto Arch for a correction in build spec if it is a common variance ?

[irrotational]

How do I know if mine was raised?

 

My car darts all over the road anyway so to be honest I suspect bump steer gets lost in the noise.

 

Whats the best way to test for it? fly over a bump and let go of the steering!?

[Paul Deslandes]

Look at the clamps holding the rack to the bridge. The top and bottom halves are probably the same and space the rack about 6mm off the bridge. That's standard. If there are some spacers between the bridge and the lower clamp then it has been raised, whether by the correct amount is anyone's guess.

 

If your car is darting all over the place and you don't have front toe out you could well have a bump steer issue. That was certainly the case with mine. A preliminary check can be to see if the track rods are about parallel with the bottom wishbone which they should be.

 

I entirely agree with Dave's approach and in the absence of any better info, 9mm looks about right.

[Myles]

I raised mineby about 4-5mm when I rebuilt the car. No science - a local had done all the hard work of measuring and correcting his, and I just reckoned on abit being better than none!

[7 wonders of the world]

 

 

Edited by - 7 wonders of the world on 28 Apr 2012 10:25:32

[mic]

I think if you look at the clamps introduced at least 2 years ago you will see they can be fitted with either the standard position or if reversed they have a 9mm lift.

[Jason Plato]

thanks Mic

 

let's not degenerate into a "why haven't they fixed it thread"

 

I guessed mic would know of the solution and has already done it 😬

 

I was originally curious if older chassis have a consistant variance - which they appear to have so that we can all gain from with a simple mod

[EFA]

My '93 HPC chassis had the rack platform 12mm too high.

 

The older chassis were set up for the steel rack. Also, the early TRE's had a different spacing to those available now.

 

I reigned it in by moving to an alloy rack and machining the brackets down.

 

Rack position has always been all over the place. Bump steer has to be terrible to be noticable on a road 7 - leading to massive instability at high speed on bumpy lanes.

 

On the track it reveals itself as a lack of grip in roll.

 

[7 wonders of the world]

Thanks for the update mic, I'm sure there are loads of little update tweaks that you have made over the years, that one is a little gem and I'd bet there's a fair few owners checking the orientaion of their mounts tihs weekend.

Thinking aload now - could there be a way of creating link perhaps on here for such Factory updates?, appariciate there will be a lot of variables on some changes but if it were possible to list the common ones to start with?

 

 

[Eugene]

On most cars, the steering rods from rack to steering arms should be close to horizontal for low bump steer...

 

Would be interesting to see piccys taken from the front of various cars to see if the majority of steering arms are indeed horizontal?

 

http://freespace.virgin.net/shaw.clan/images/Eugene3.jpg