Bump Steer - Help!

[Sanj]

Hello from a new seven owner!

 

I've had my roadsport 160 just over a week now, having bought it delivery mileage from millwood. I have noticed on the tricky b roads in north yorkshire where i live the steering wheel seems to go a bit crazy over the bumps, i believe this is known as bump steer?

 

My car has 15 inch wheels with avon tyres (just normal road tyres not the sticky cr500s), and the superlight suspension upgrade.

 

Can things be improved by playing with tyre pressures? If not what can be done? I would prefer to take it somewhere to be sorted as its probably beyond my diy abilites and i would like an 'expert' to look at it! could anyone suggest somewhere a reasonable distance from York?

 

Sanj.

[Sue Dinnim]

In partial answer to two of your questions.

 

I have heard a few people say that the larger wheels tend to follow cambers quite a bit. Some of this can be changed by changing tyres although I can't say I know to what.

I have 14 inch rims on my 1.6SS and have found that the tyre pressures can make a big difference as to how the car reacts to bumps in the road.

Depending on tyre condition this can affect this too although I have no experience in a 7 I know I have had other cars suffer as the tyres get to the end of their useful life.

 

Someone will probably be along to correct me on all points in a minute but the above is my humble opinion.

 

Sorry I should have added welcome to 7 ownership. I haven't had mine long but I already wish I'd started years ago. You'll love it.

 

A slowly increasing number of photos here plus a shameless link to my company as I cannot do it on here

 

Edited by - DuncanP on 23 Aug 2004 11:17:28

[Shaun_E]

Try reading this thread here.

 

Yellow SL #32

[Sanj]

Right, so reading that thread i think ill check tyre pressures and tracking to start with.

 

Would still welcome any recommendations for a seven specialist/garage in the north.

 

Sanj.

[EFA]

If you reduce the ride height to accomodate the larger wheels at the recommended ground clearance you move the suspension position at the front to beyond the limits of ideal geometry, which increases the bump steer.

 

The solution is to get it measured and shim the steering rack higher in the chassis, although this will only imprve it rather than make it as good as it would be with 13" wheels.

 

Ex Fat Arnie

Fat by a different name.

[Sanj]

My 15 inch wheels are on 45 profile tyres, so i would assume the rolling radius would be the same as other sized wheels.

 

I think my plan of action would be to first see if the car is toeing out then look at steering rack as you suggest.

[Normans_Ghost]

Sanj, Welcome to the club (I assume you've joined, it's worth it). Why not have a nice blat down to Sheffield on Wednesday night and join the South Yorks nutters who will give you there collected wisdom. Watch Chit Chat for the venue as it's new. If you need any directions send me an email.

 

I don't think tyre pressures or toe-in will cure bump steer, if that is what you have. There was an article on this in Low Flying some time ago. I'll try and find time to find it for you but don't hold your breath as I'm catching up after being away for 2 weeks and am at Brize Norton over the weekend.

 

As Arnie (EFA) says it's mainly to do with the rack position relative to the suspension setting. One quick visual check is to see if the track rods are parrallel with the car on a flat floor. If they are angled up or down this could be the cause of the problem.

 

norman verona

1989 BDR 220bhp

Mem No 2166

the full story here

[dave1968]

Sanj.

 

I have a very similar issue when i first got my car. It had 16" wheels and 45 profile tyres and was frankly a nightmare handing-wise, esp on bumps.

 

I went down the 13" wheel route and it transformed the handling of the car. Im sure there are other things like pressures, tracking etc to look at but most of the feedback (Im no expert i hasten to add!) ive picked up is that smaller wheels work better on a 7. they certainly do on mine. I didnt have ride height issues either as 185/70/13 tyress are quite similar in diameter to 205/45/16 which is what my car had on it. You dont mention the width of your tyres so I cant tell if you will have issues with ride height etc

 

a useful link to wheel/tyre diameters is:

http://www.miata.net/garage/tirecalc.html

 

hope that helps

Dave

[Sanj]

Yes, i think the 13 inch wheels are the optimum size but mine came with 15s so will have to stick with them at least for a while!

 

my tyres are 195/45/15s. I think the tyres could be part of the issue as i drove an sv140 with Avon cr500's and the 10 spoke black alloys and it was fine over the bumps. The avons were very very soft compared to a normal road tyre so this may help things?

 

The cars' handling is fine apart from the bumpsteer - i cant say there are any other faults such as tramlining etc.

 

I upgraded to 17 inch wheels on my last impreza but soon put the standard 16s back on as the extra unsprung mass really knackered the damping up, so id have no qualms putting smaller wheels on if it were necessary apart from the expense of course!