what does bump steer feel like?

[blueg33]

Just has suspension rebuilt on my G33. New wishbones, bushes etc

All aligned using 4 wheel laser stuff, but the car feels less planted and twitchy in a straight line. Accelerating can be scary as the car tries to wander, and it seems to wander very suddenly. (is this bump steer?)

Other changes are minor adjustment to roll centres

Some hints as to causes would be handy before I take it to my race car specialist (current set up done by none race car bloke, who says he has done a fast road set up)

Cheers

Nick

[Sam_68]

Could be. Bump steer typically exhibits itself as tendency to tramline/wander.

Why (and how) did you change the roll centres? This is a fundamental change to the suspension geometry and certainly not something you should enter into lightly!

[blueg33]

Sam_68 said:

Could be. Bump steer typically exhibits itself as tendency to tramline/wander.

Why (and how) did you change the roll centres? This is a fundamental change to the suspension geometry and certainly not something you should enter into lightly!

The roll centre issue is a known G33 problem that causes snap oversteer. The fix is a well recognised one used by G33 race cars. I am pretty sure changing the roll centres isnt the issue as the car is fine under cornering but wanders horribly on acceleration even on a smooth road.

[Sam_68]

At risk of stating the obvious, a race car is a very different animal to a road car; it's likely to be much stiffer, hence the suspension may not move enough to cause problems that would exhibit themselves on a road car, and it may not be such an issue that the car is a bit darty/tramliney, 'cos racetracks don't have quite so many potholes and lorry-ruts.

I ask again, though, how did you change the roll centres? Assuming you changed the front, this will introduce bump steer unless you re-set the rack height to suit.

Assuming the G33 is double wishbones at both ends, the only sensible way I can think of is to move the chassis suspension pickups, in conjunction with new wishbones? (eta: apparently the fix is to alter the upright geometry instead? Quite apart from anything else, the recommended solution will change your caster angles, which will affect steering feel and self-centring).

Are you sure that the toe settings are correct - the other obvious cause for your symptoms would be toe out (particularly at the rear, and particularly if the new bushes were deflecting enough to allow excessive toe changes under hard acceleration)?

Did you have the car corner weighted as part of the changes?

ETA: Ok... I made the mistake of googling 'Ginetta G33' suspension' to check that it was wishbones all round, and now I'm very afraid. I hope your insurance is up to date!

I'm hoping you didn't just give your chap 'with experience of fast road set up' the details from a certain website and ask him to follow the instructions...

Edited by Sam_68 on Monday 5th April 11:22

Edited by Sam_68 on Monday 5th April 11:51

[blueg33]

Sam_68 said:

At risk of stating the obvious, a race car is a very different animal to a road car; it's likely to be much stiffer, hence the suspension may not move enough to cause problems that would exhibit themselves on a road car, and it may not be such an issue that the car is a bit darty/tramliney, 'cos racetracks don't have quite so many potholes and lorry-ruts.

I ask again, though, how did you change the roll centres? Assuming the G33 is double wishbones at both ends, the only sensible way I can think of is to move the chassis suspension pickups, in conjunction with new wishbones?

Assuming you changed the front, this will introduce bump steer unless you re-set the rack height to suit.

Are you sure that the toe settings are correct - the other obvious cause for your symptoms would be toe out (particularly at the rear, and particularly if the new bushes were deflecting enough to allow excessive toe changes under hard acceleration)?

Did you have the car corner weighted as part of the changes?

Roll centres were changed as per other G33 owners by machining a little off the rear upright and adding spacers to lift the front wishbones (see www.g33.co.uk). Rack height is interesting point, but the rack is fixed and all joints are rosejoints.

I am wondering about toe settings as the outer edge of the front tyres are notably scrubbed after about 20 miles on the road.

The car was previously corner weighted, and I think I will have it done again as it was pre the suspension changes. (the changes were as a result of replacement following an accident caused by snap oversteer)

The car didnt previosuly feel like this, previous set up was done by a race car specialist although set up was with a road bias even though I was doing a little hill climbing with the car.

Thanks again, your comments are useful

[Sam_68]

blueg33 said:

Rack height is interesting point, but the rack is fixed and all joints are rosejoints.

Well, I can guarantee that the modifications recommended by that website will have changed the bump steer characteristics to some extent.

Half-arsed doesn't begin to describe it! I could write an essay, but just for a start the guy seems to be suggesting that the mere fact that the G33 has an inclined roll axis is a fault and that only cars with a horizontal roll axis will handle properly. This is so mis-conceived that I can't even begin to comprehend it.

[blueg33]

Sam_68 said:

blueg33 said:

Rack height is interesting point, but the rack is fixed and all joints are rosejoints.

Well, I can guarantee that the modifications recommended by that website will have changed the bump steer characteristics to some extent.

Half-arsed doesn't begin to describe it! I could write an essay, but just for a start the guy seems to be suggesting that the mere fact that the G33 has an inclined roll axis is a fault and that only cars with a horizontal roll axis will handle properly. This is so mis-conceived that I can't even begin to comprehend it.

The roll centre mod is a mod recommended by the original designer of the G33 a guy called Duncan Campbell. Roll centres were not sorted before the car went into production (similar process to TVR ie customer development!). I think the article is an overview.

Frankly , all I want to do is get the car sorted. Others have done the roll centre mod and they seem fine.

[Sam_68]

blueg33 said:

Frankly , all I want to do is get the car sorted. Others have done the roll centre mod and they seem fine.

Well, at risk of repeating myself, I can absolutely guarantee two things will result from the suggested modifications to the front geometry, if they are done in isolation:

• You will change the caster angle (hence the mechanical trail).
• You will introduce (or change the amount of) bump steer.

Both can be corrected by means of further modifications (ie. correcting the bump steer by means of changing the rack geometry and re-setting the caster by whatever means), but if you've made the roll centre modifications in isolation, then you will have affected the steering feel, accuracy and stability.

Sorry, but there's no soft-and-fluffy way to put it; we're talking about basic, fundamental geometry here - if you change one thing in isolation it will impact on other factors.

You will also have altered the weight transfer characteristics (but then that's the aim of the modification...) and the camber gain geometry (a relatively minor consideration in comparison to the bump steer and caster issues). The new bushes might also muddy the water by changing the amount of toe deflection you're experiencing, too.

Your best bet is to take the car to someone who knows what they're doing(eg. Steve Guglielmi)and get them to correct the bump steer and re-check the rest of the geometry as a start; you can decide whether the caster has had an unacceptably adverse effect afterwards, maybe (the effect of increasing the height of the front upright - ie. the distance between the wishbone outer pickups - will be to reduce the caster, which will reduce steering feel/weight and self-centring to some extent, but lifting the top ball joint by 6mm will only change the setting by a fraction of a degree and you might find that you can live with this).

I have also heard good things of Neil Garner up at Kemble Airfield, nearer to you, but I've never used him personally so can't really give a recommendation. I live within spitting distance of you, so I'd have offered to help myself, but I don't have the necessary gear for checking bump steer or corner weights these days - I was using it so infrequently that I sold it off some time ago. Contrary to the advice on the Ginetta website, you need a bit more accuracy than getting your mate to jack the wheel up and down while you check for bump steer visually, incidentally!

[blueg33]

I agree with you about not doing it myself even with a mate. The Powerstation in Ashchurch set up the car previously, but I was thinking of going to Steve Gugliami. I have used Neil Garner for servicing etc but havent discussed geometry with him.

Cheers

nick