bump steer?
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
http://www.naxja.org/html/techarticl...psteertech.htm
Bump-steer is the result of poor (and sometimes unavoidable) suspension geometry. The reaction is described as the vehicle steering off the intended track, on it's own (without driver input), when encountering a bump. Sometimes it is described by a driver as the vehicle needing constant direction correction whenever driving over a bump, "I hold the steering wheel straight but the vehicle steers to the right every time I hit a bump."
http://www.bakerprecision.com/longacr17a.htm
Bump Steer
Bump steer is the toe-in or toe-out of the front wheels as the suspension goes from normal ride height through full bump (suspension system moves up) to full droop (suspension system moves down). Measurement is usually limited to 3" up and 3" down from ride height. It is specified either by a graph or measurements at 1", 2", and 3".
Bump steer affects handling much as setting toe-in does. All cars have a certain designed-in pattern of bump steer. Check with your chassis builder for his normal pattern. If you have a factory chassis, you'll probably want to try to minimize bump steer as a starting place.
Bump-steer is the result of poor (and sometimes unavoidable) suspension geometry. The reaction is described as the vehicle steering off the intended track, on it's own (without driver input), when encountering a bump. Sometimes it is described by a driver as the vehicle needing constant direction correction whenever driving over a bump, "I hold the steering wheel straight but the vehicle steers to the right every time I hit a bump."
http://www.bakerprecision.com/longacr17a.htm
Bump Steer
Bump steer is the toe-in or toe-out of the front wheels as the suspension goes from normal ride height through full bump (suspension system moves up) to full droop (suspension system moves down). Measurement is usually limited to 3" up and 3" down from ride height. It is specified either by a graph or measurements at 1", 2", and 3".
Bump steer affects handling much as setting toe-in does. All cars have a certain designed-in pattern of bump steer. Check with your chassis builder for his normal pattern. If you have a factory chassis, you'll probably want to try to minimize bump steer as a starting place.
Safety Car
Joined: Sep 2006
Posts: 4,129
Likes: 1
From: Pasadena CA
Mine has it pretty noticeably when cornering and hitting a bump -feels like the whole front end shuffles sideways momentarily. I'm not sure how badly all C4s have this problem
Part of the problem is the transverse leaf springs as shocks can be transmitted from one wheel to the other, unlike with a coil-over setup
Part of the problem is the transverse leaf springs as shocks can be transmitted from one wheel to the other, unlike with a coil-over setup
Thread Starter
Pro
Joined: Sep 2003
Posts: 655
Likes: 2
From: Rio Vista ca.
Thanks for the info and links, Redeasysport. Most of the info is over my head but I think I kind of understand the symptoms. Sometimes when I hit a bump the steering wheel "wiggles" for a while then settles down. Thought it might be a wheel balance problem but it doesn't happen at just one speed and sometimes not at all. The roads around here are in poor repair with a lot of bumps and uneven surfaces. I need to take it on the freeway and check it out. My mechanic said that all of the suspension parts seemed ok but I don't know how hard he looked. It seems from the info in your post that steering wheel wiggle is not a symptom of bump steer.
Thanks, Kim
Thanks, Kim
Thread Starter
Pro
Joined: Sep 2003
Posts: 655
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From: Rio Vista ca.
Not really sure but I think it wiggles left to right. I do get a little going off in another direction after a pretty good bump but nothing severe. The car only has 56K on it so I'm thinking it's probably not worn suspension parts. Wheels and tires are pretty new and have been balanced and I had a 4 wheel alignment done about 4 months ago.
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
There is also body flex on top of bump steer that make these cars feel unstable on bumpy roads. From what you say it may very well be normal 'Vette response to crappy roads.
Burning Brakes
Joined: Apr 2005
Posts: 960
Likes: 0
From: CA
I had those symptoms, but it seems as the body flex, old shocks, worn suspension, worn tires all contribute. As I replaced shocks, rotated tires, and made some other minor adjustments the effect has faded to a degree. Not all C-4 came this way from the factory, and the effect has to be at different levels depending on component age. Any opinions?
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Team Owner
Joined: Mar 2001
Posts: 30,845
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From: Boston, Dallas, Detroit, SoCal, back to Boston MA
Early and late mode C4's have a different suspension geometry, the later ones are call zero scrub.
Alignment can fix that darty feeling, both early and late.
Stock the cars have positive camber, they lean OUT
Set them to about 1/2 a degree in all around, about 1/18 toe in on the front and 1/8 toe out in the back, and you'll really like the way it drives from day to day.
Alignment can fix that darty feeling, both early and late.
Stock the cars have positive camber, they lean OUT
Set them to about 1/2 a degree in all around, about 1/18 toe in on the front and 1/8 toe out in the back, and you'll really like the way it drives from day to day.
Pro
Joined: Jul 2002
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From: Lynchburg VA
Melting Slicks
Joined: Feb 2006
Posts: 2,753
Likes: 14
From: backwoods upstate ny
suspension designers refer to upward movement of the wheel relative to the chassis as "bump"...downward motion is "rebound"
lengths of suspension components cause changes of wheel tracking as the wheel moves up in "bump" or down in "rebound" and these tracking changes are highly desireable to prevent loss of operator control during cornering (bump steer "out" of the outboard wheel is prefered to "0 bump" or "in") , but may cause undesireable tracking inputs when road surface variations (aka bumps) are encountered during straight line operation.
note that wheel movement in bump or rebound during cornering is produced by "body roll"...so "body roll" is not all "bad"
lengths of suspension components cause changes of wheel tracking as the wheel moves up in "bump" or down in "rebound" and these tracking changes are highly desireable to prevent loss of operator control during cornering (bump steer "out" of the outboard wheel is prefered to "0 bump" or "in") , but may cause undesireable tracking inputs when road surface variations (aka bumps) are encountered during straight line operation.
note that wheel movement in bump or rebound during cornering is produced by "body roll"...so "body roll" is not all "bad"
Last edited by redrose; Jul 4, 2008 at 11:55 AM.
