Geometry, Setup, Bump Steer, Ride Height Questions?

[GF04RCE]

So i hand over my keys to a friend who has raced all sorts of different cars over the years, and wait for him to come back with a big smile on his face. No big smile and slight disappointment... His comments included questions of Bump Steer, twitchyness, and it not feeling setup right.

 

I have never thought about setup ever since Caterham did some work for me back in 2005.

 

15" cr500s on a 2004 roadsport, wide track.

 

I admit it is a bit twitchier than it used to be but i guess i just got used to it.

 

Looking into geometry and setup there seems to be no standard, depending on the type of driving you do. As for bump steer and questions about steering rack being parallell to lower wish bones, ride heights and some even raising their steering racks I question whether this is necessery or if there is a standard setup guide for sevens and a few simple rules to follow.

 

15cm ride height rake back to front

castor ?

camber front ?

camber back ?

Toe ?

Corner weighting % front to back

 

I could just get someone in to take a look but really interested in learning about setup myself.

 

I also still dont understand how things can go out of alignment over time.

 

So to reliable BC to point me in the right direction....

 

Thanks

 

David M

 

 

 

[Stationary M25 Traveller]

15mm rake may be safer !!

[Paul Deslandes]

Bump steer really is an issue with our cars and I wrote an articlefor Low Flying a few years ago that described how to measure it and calculate the required change in rack height.

 

A number of people have had a go since and it appears that in all cases that have been reported, the rack is about 9mm too low and should have spacers added to the underside of the bottom clamps to raise it by that amount. Doing this transformed the handling of mine and a number of other cars.

 

The rear camber, on a deDion at least, is set by the hub mounting ears which are machined to the correct angle. Front toe, if toe-out, can make the car twitchy but gives great turn-in. Parallel (zero) to 20minutes of toe-in is usually recommended for road and track use although a lot of folk quite like the extra responsiveness that a bit of 'out' gives.

 

Front camber of about 2 deg negative works well.

 

If you decide to have a go it be worthwhile reading the article, if only to see the order in which to make the adjustments.

 

Paul

[paul jacobs]

Which raises the question of why the factory let the cars out into the hands of the general public with the rack too low?

 

I don't suppose that happens anymore though!

[Z3MCJez]

New rack mounts available to cure this.

 

Jez

[GF04RCE]

Are the new rack mounts from Caterham? Or is it just the spacers £1.80?

 

 

[Richard Price]

Spacers and longer bolts will do the job

[Paul Deslandes]

The factory is still letting cars out like this. The good news is that the fix is simple, the only issue being that the steering rack gaiters can catch on the holes through the bodywork which may need to be relieved a little.

 

If you look at the article you'll see what a difference the adjustment makes to reducing the amount of 'steer' with bump and droop; it really is quite dramatic.

[Roadsport06]

Good evening Paul, my car was built in 2006, I remember reading your article in Low Flying. Will comparing the steering rack to the lower wishbones by eye be OK to check if my car suffers with bump steer? My car is the only seven that I've driven in anger so have nothing to compare my car to.

[Smithy7]

Quoting GF04RCE: 

Are the new rack mounts from Caterham? Or is it just the spacers £1.80?

 

 

I'd you purchase the car new from Caterham?

 

Dave

 

Edited by - Smithy7 on 6 Aug 2013 07:20:29

[Paul Deslandes]

Its not entirely clear whether the rack height has always been the same or whether there have been some variations over the years. This would particularly apply to the welded Caged chassis which may have been redesigned to take account of this problem. Talking to Bruce at Arch a couple of years ago, I got the impression that the height for theirs hasn't changed, certainly in recent (20+) years.

 

I'm not sure I could judge sufficiently accurately the geometry by eye and the only sure-fire way of knowing is to measure it, e.g. using the method in my article or, better still, using the amended method at the end of the link posted earlier. Increasing the rack height only changes the track rod angle by about 1.5deg so might be hard to see but, if your rack is on standard clamps with equal thickness top and bottom halves, has no packing spacers and has been built in the last 20 years or more, there's a good chance that it might benefit from raising the rack by 9mm.

 

The spacers on the CC parts website don't show the thickness so you'd need to check, or make your own from 2 or 3mm ali. or WHY.

 

Do bear in mind that altering the rack height WILL affect your toe setting quite significantly so if you don't check and adjust it after moving the rack, you could have a real handful. Whether it goes one way or the other will depend on the ride height setting and whether raising the rack has the effect of pulling in or pushing out the steering arms. YOU HAVE BEEN WARNED! Sorry for shouting but this is important.

[Roadsport06]

Much appreciated, mine is an Arch chassis. I'll have a look at the clamps on mine as to whether they are the same thickness top and bottom as a starting point.

A shame that Caterham let cars out of their factory with bump steer.

[DJ.]

My car 2010 metric S3 narrow track has a little bump steer, but I don't want to change it. The steering is full of feel and is very good, in fact it is close to being as nice as my old Elan Sprint

 

In his book Chris Rees says that after borrowing a Seven during the Elise development, Lotus returned it and made recommendations including increasing the bump steer.

 

On a bumpy road, the wheel does move around, but my car is impressively stable and seems to be self correcting. Also the faster I go the better the chassis seems to get

 

Duncan

[myothercarsa2cv]

I found the bump steer improved significantly after fitting Arnie's rod ends which effectively lowers the rack by lifting the tie rods.

 

Then the bridge bent and it's rubbish again. Really should take a hammer to that at some point...

 

Here is a good diagram showing (basically) what you need to achieve.

[Richard Price]

I found the bump steer improved significantly after fitting Arnie's rod ends which effectively lowers the rack by lifting the tie rods.

 

How do you know that 🤔

Have you measured it 🤔

 

 

[GF04RCE]

So are we saying that all sevens of my era circa 2004 would suffer bump steer, what gets me is that it used to be ok (or seem better) i am guessing that something else has changed but i find it odd that things can change when i have not actually adjusted anything....

 

Oh i want to remove tyre pressure from the equation, if someone can remind me what the standard psi setting is for 15" CR500's?

 

If anyone has any more diagrams or guides to recommend please do....

[Richard Price]

The bump steer characteristics are going to be slightly different depending on a few factors. desired ride height, camber, caster and toe settings, all affect bump steer slightly.

In comparison to some amateur modified cars, I believe the bump steer of any Caterham is not particularly bad, but it can be improved.

However, what is "right" for one car, may not ne "right" for another. I would be cautious about simply fitting rack spacers without actually measuring the effect.

 

His comments included questions of Bump Steer, twitchyness, and it not feeling setup right.

 

How is your car currently set?

What springs, dampers, and ARB's does it have?

I suspect it may have toe out, and too little castor angle, to make it twitchy.

 

 

[Garry7]

Standard pressure is 18psi for 15" CR500's for road use. HTH

[GF04RCE]

Richard, need to work all that out, components were whatever was standard in 2003/2004 for a widetrack roadsport on adjustable blistiens.... part of the game will be pulling together a complete profile and then changing measuring things one by one taking into consideration Pauls article regarding the order of things.

[Grim Reaper]

My 2002 chassised (?) superlight came with equal dimensioned rack mounts.here

I have recently fitted some 10mm spacers (made from some 10mm thick ally bar I bought at Stoneleigh) under each mount and after correcting the toe the difference is astounding.

Caterham sell two different spacers to effect the same mod, one would appear to be 3mm thick and they say you'll need 3 of these layered to effect the required raising of the mount, the other says you could need 2 or 4 of them so I have no idea what the thickness of that one could be here

here