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Cedric
08-30-2004, 03:59 AM
I'd like to know how bump steer (unwanted steering of wheels due to changes in toe-in when hitting bumps) happens and why it affected a number of low-volume sports cars. How can it be prevented and where do those so-called rose joints come into play?

I hope someone can provide me with a decent answer. Matra, I am looking in your direction...

Matra et Alpine
08-30-2004, 04:42 AM
I'd like to know how bump steer (unwanted steering of wheels due to changes in toe-in when hitting bumps) happens and why it affected a number of low-volume sports cars. How can it be prevented and where do those so-called rose joints come into play?

I hope someone can provide me with a decent answer. Matra, I am looking in your direction...
damn, I'm getting to feel like the old guy in the village everyoen brings their problems to when all I want is there daughters :)

So, new rule, from now on when I help, I'm expecting an offer to drive your car(s) on track near you. OK ?
( Warning, I work for a multinational and travel extensively - though for family reasons I've got that under control just now :) ) So THIS is one 'bill' you may have to pay out on :) !!!!

When suspension travels up and down it alters the angles of the front wheels to the road. With front wheels it is advisabel to build in some caster to assist the wheels to stay in the straight-ahead position - otherwise you'd CONSTANLY have to steer the car on the freeway :) Making that caster angle the same as the wheel moves up and down is difficult. So that is the most common cause for 'bump-steer'. Making it the same across ALL suspension travel AND steering travel is near impossible. So somewhere every car exbibits some aspect of bump-steer.

Next as suspension moves up and down, some designs change the angle of the wheel - the camber. The change in camber changes the point where the suspension angles meet the ground and also the contact patch on the road and this is a secondary cause for bump-steer.

I'm afriad it's too complex to go through the different ways that king-pin versus strut versus wishbone versus trailing arm changes angles and hence contributes to bump-steer. If the basic summary I've give above doesn't answer then I don't mind going over a couple of specific cases of cars you've experienced it in and why it happened. BUT if you want to go over ALL the possibilities then sign up to any of the automotive design courses in the UK Universities and after 4 years you'll have all the answers :)

henk4
08-30-2004, 04:54 AM
damn, I'm getting to feel like the old guy in the village everyoen brings their problems to when all I want is there daughters :)



This is an intriguing typo: Do you want three daughters or do you want their daughters? :)

Matra et Alpine
08-30-2004, 04:59 AM
This is an intriguing typo: Do you want three daughters or do you want their daughters? :)
difficult choice :)
OK, THEIR daughters :)

henk4
08-30-2004, 05:09 AM
difficult choice :)
OK, THEIR daughters :)

one after one I suppose :)

UK CARS
08-30-2004, 05:15 AM
difficult choice :)
OK, THEIR daughters :)
I want your daughters, lol :D

Cedric
08-30-2004, 07:23 AM
The early Marcos cars, which I believe had double wishbone suspension, are known to be affected by bump steer. I'd like to know why, cos I always thought a wishbone set-up commonly used in racing cars was quite favourable.

Matra et Alpine
08-30-2004, 07:24 AM
I want your daughters, lol :D
You may laugh guys but he's serious !!

With a 17 and 19 year old anyone who tries is in trouble :)
And the worst won't be from me .... remember they're used to being around good cars. So you better come knocking with some serious vehicle and prolly has to be in the excess of 100K ( real money, not MM :) ) or you'll get the fastest bruch off of your life :)

UK CARS
08-30-2004, 12:56 PM
You may laugh guys but he's serious !!

With a 17 and 19 year old anyone who tries is in trouble :)
And the worst won't be from me .... remember they're used to being around good cars. So you better come knocking with some serious vehicle and prolly has to be in the excess of 100K ( real money, not MM :) ) or you'll get the fastest bruch off of your life :)

I was hoping the Mini would impress :(

VtecMini
08-30-2004, 01:06 PM
If that doesn't work, my dad has a Maxi that he's trying to get rid of. It's got 4 doors and a bigger back seat. :D

UK CARS
08-30-2004, 01:16 PM
whens matra going to show off his great work then,
im looking foward to meeting them, lol :D
Im also looking foward to showing them my shed too, HAHAHA

Matra et Alpine
08-30-2004, 01:36 PM
The early Marcos cars, which I believe had double wishbone suspension, are known to be affected by bump steer. I'd like to know why, cos I always thought a wishbone set-up commonly used in racing cars was quite favourable.

With double wishbones the steering arm has to pivot on the same length as the pivot to ball joint. This assumes equal length wishboes and not configured for anti dive. THEY make it all too copmplicated to explain in text :(

So with equal wishboes the wheel camber will stay stable assuming the wishbones are a reasonable length. But equal wishbones aren't good for body roll during cornering maintaining tyre contact. Again, another topic needing a drawing medium :(

Anyway, the Mini uses unequal wishbones and offset balljoints to provide the best compromise in the small space. The steering arm is connected through an offset arm BELOW the plane of the top wishbone. So now there are 3 varying components affecting the geometry.

The bump steer comes from 2 things with Mini kits.
First, the use of wider wheels. The Mini geometry was designed for narrow tyres. Wider tyres and differeing offsets, as stated before, can combine to introduce some vicious bump steer.
Secondly. Kits often operate the Mini supension away from the optimum designed for. So engine in the front and high front end weight bias is the setup the suspension expects. Take the engine away and it is no operating under different conditions. So typically the suspension is lower, this affects the way the steering rack alters the steering as the wheels go up and down.
So bump steer.

sutton4481
08-30-2004, 02:46 PM
whens matra going to show off his great work then,
im looking foward to meeting them, lol :D
Im also looking foward to showing them my shed too, HAHAHA

Are you trying to get banned again? :D