C3 bump-steer
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,052
Likes: 0
From: Vancouver BC, Canada
C3 bump-steer
I've really noticed that my Vette has a lot of bump-steer. Is this a common C3 observation? I have fairly loose steering, and I suspect the rag joint, but my car can feel very unsettled when the one front wheel encounters a bump, especially under braking. I can feel it wanting to pull to that side sharply.
Bump-steer is a suspension characteristic that occurs when a front wheel travels up/down forcing the steering angle to change. This is due to less than ideal tie-rod geometry and Ackerman angles which are of course a compromise on a production car. Ideally, the tie rod should be parallel to the lower A-arm in both planes, and have pivot points acting on the same radii as the lower A-arm’s.
What effect does the rack and pinion conversion have?
Bump-steer is a suspension characteristic that occurs when a front wheel travels up/down forcing the steering angle to change. This is due to less than ideal tie-rod geometry and Ackerman angles which are of course a compromise on a production car. Ideally, the tie rod should be parallel to the lower A-arm in both planes, and have pivot points acting on the same radii as the lower A-arm’s.
What effect does the rack and pinion conversion have?
Race Director
Joined: Aug 1999
Posts: 18,126
Likes: 174
From: WI
Re: C3 bump-steer (Turbo-Jet)
Turbo,
"bump steer" is a race car term and I doubt you can feel it. What you probably feel is a bad idler arm, bad tie rod ends , bad a-frame bushings or bad ball joints. Did I forget anything?? Oh yea bad shocks or shock bushings. ...redvetracr
"bump steer" is a race car term and I doubt you can feel it. What you probably feel is a bad idler arm, bad tie rod ends , bad a-frame bushings or bad ball joints. Did I forget anything?? Oh yea bad shocks or shock bushings. ...redvetracr
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
Re: C3 bump-steer (Turbo-Jet)
Turbo, on the steeroids design the tie rods are up there a bit higher than I have mine, so the Ackerman is probably imporved a bit...but I have NO bump steer now, and it tracks truck rutted roads VERY well now, I used to have to fight the wheel a LOT here in sandy road bedded Florida....
of course my inner tie rod ends are only about 4-5 inches apart...so I have LONG tie rods....see sig file....mine are of unequal length, but tires and steering are unaffected as they are 19 & 21 inches long....
GENE
of course my inner tie rod ends are only about 4-5 inches apart...so I have LONG tie rods....see sig file....mine are of unequal length, but tires and steering are unaffected as they are 19 & 21 inches long....
GENE
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Re: C3 bump-steer (redvetracr)
With my car sitting on a hoist that has rails for the tires to drive on. It puts the car in a normal position but gives you lots of room underneath. I put a jack under the center of the car, carefully measured the toe then jacked the car up inch by inch measuring the toe change as the car rose. At 4 inches of lift I had 2 inches of toe in.
I then did the opposite. I pulled the car down with a threaded rod and hook. The toe never change pulling the car down 3 inches.
To improve the geometry in lift I heated the pitman arm and bent in down and added an extension to the idler arm causing the inner and out tie rods to move on a more favorable arc.
I also totally rebuilt the steering box and you do not feel any movement over uneven pavement, ruttes or pot holes even with the 265 front tires.
I then did the opposite. I pulled the car down with a threaded rod and hook. The toe never change pulling the car down 3 inches.
To improve the geometry in lift I heated the pitman arm and bent in down and added an extension to the idler arm causing the inner and out tie rods to move on a more favorable arc.
I also totally rebuilt the steering box and you do not feel any movement over uneven pavement, ruttes or pot holes even with the 265 front tires.
Safety Car
Joined: Oct 1999
Posts: 4,101
Likes: 10
From: NorCal
Re: C3 bump-steer (Turbo-Jet)
I agree with redvetrcr that you probably don't have enough front or rear toe change ("bump steer") just going down the road to notice with a solid, stock suspension. My symtoms were the same as yours and it was almost all in the worn out rubber bushings. I replaced them with poly and fixed the problem. I will, however, put the VB smart strut on in the future to minimize rear camber change during rear suspension travel.
I'm sure the stock suspension could be improved using the advice from the others.
Hans
I'm sure the stock suspension could be improved using the advice from the others.
Hans
Le Mans Master
Joined: Oct 2000
Posts: 7,831
Likes: 5
From: Austin TX
Re: C3 bump-steer (Turbo-Jet)
Vette Brakes sells an "Anti-Bump Steer Kit".
I think it's basically a steel block that spaces the tie rods
away from the spindle arm.
I have no first-hand experience with it...
I think it's basically a steel block that spaces the tie rods
away from the spindle arm.
I have no first-hand experience with it...
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Re: C3 bump-steer (Steve439)
I think my inner tie rod ends were too high. The out ones were alot lower then the inners . Since the outers are fixed attached to the spindles I needed to lower the inners to match the fixed outers. To do this I modified the pitman arm and a longer idler arm drops the drag link to make all the joints on the same plane.
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,052
Likes: 0
From: Vancouver BC, Canada
Re: C3 bump-steer (norvalwilhelm)
I think my inner tie rod ends were too high. The out ones were alot lower then the inners . Since the outers are fixed attached to the spindles I needed to lower the inners to match the fixed outers. To do this I modified the pitman arm and a longer idler arm drops the drag link to make all the joints on the same plane.
I built an open wheel FSAE racecar with very good front suspension geomerty and ideal rack location. We had almost zero toe change over 8" of wheel travel, and the suspension is limited to 2" of travel once the shocks were installed.
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Advanced
Joined: Jan 2002
Posts: 52
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From: Tempe AZ
Re: C3 bump-steer (Turbo-Jet)
My car has the Anti-Bump Steer Kit type blocks on it but I do not have a base line to tell you if they made a difference although there is no noticable bump stear (and yes I could notice as I have driven street rods with very bad bump steer). My steering is currenly sloppy with some excess play that I was hopping to solve with a rebuilt box (already have poly bushinngs and other upgrades). I am running 275/50's on the front and it tracks groves pretty bad. I have also been considering the rack conversion (been watching the group purchase tread and other treads for comments)
Race Director
Joined: Nov 2000
Posts: 11,872
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From: Waterloo ontario Canada
Re: C3 bump-steer (MilesM)
MilesM I have 265-50-15's on the front and my car was bad for following groves in the pavement. It would dart all over the road. After rubuilding my steering box and that includes replacing the pitman bushings the car now ignores these ruts in the road. Recently I drove on a road that they had scrapped to repave and I drove for miles without a single twitch of the front end. With really tight boxes our old cars can steer as good as new cars. I compare my wife's new Supercharged Bonneville with it's rack and pinion steering to my 75 vet and mine steers as well as hers.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
Re: C3 bump-steer (norvalwilhelm)
Norvalwilhelm, I hate to disagree with you so strongly, but the front of my '72 with '92 tires/wheels was all nearly brand new, and still tight as new...urethane and all, box was snug as new, and rebuilt bushings etc also....it wandered all over with truck ruts...that symptom disappeared with the rack conversion....and nothing else changed, not even alignment...
set the toe, good to go...now the car tracks like on rails, no wandering, no troubles...only thing is, I do wish the wheel didn't feel so light to the touch, I suspect revalving the pump may reduce the assist level to something allowing more road feel.....someday to do that...
GENE
set the toe, good to go...now the car tracks like on rails, no wandering, no troubles...only thing is, I do wish the wheel didn't feel so light to the touch, I suspect revalving the pump may reduce the assist level to something allowing more road feel.....someday to do that...
GENE
Race Director
Joined: Nov 2000
Posts: 11,872
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From: Waterloo ontario Canada
Re: C3 bump-steer (mrvette)
I too replaced all 4 ball joints, 4 new tie rod ends, new idler arm, new power steering control valve and rebuilt box with all new bearings and seals. Still didn't stear right so out came the box again and this time I measured internal bushing clearances and found them to be .006. You want .001 or less. I had to hand machine all 3 bushings, polish the pitman shaft then hone the 3 bushings for a snug fit. After that I carefully shimmed the end play of the pitman arm for again close to 0 clearance. After that my car stear like new.
Regardless if you rebuild the front end if the pitman arm bushing are not replaced you do not get a rebuilt box. Just replaceing bearings and seals will not do it.
The article "Big block from hell" had a great article on this.
Regardless if you rebuild the front end if the pitman arm bushing are not replaced you do not get a rebuilt box. Just replaceing bearings and seals will not do it.
The article "Big block from hell" had a great article on this.
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,052
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From: Vancouver BC, Canada
Re: C3 bump-steer (norvalwilhelm)
Norval, I can tell you like to pay attention to detail. I guessed you must be an engineer, and then looked at your profile. Good guess. Do you work in the automotive industy?
Race Director
Joined: Nov 2000
Posts: 11,872
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From: Waterloo ontario Canada
Re: C3 bump-steer (Turbo-Jet)
Turbo Jet. I pay very close attention to detail and trust nothing. I have spent 35 years working with PHD students in engineering in a large university. We have to prove everything. I also have taught undergrad engineering labs for what seems like forever.
I have the time, the ambition and the equipment to do just about anything.
I have the time, the ambition and the equipment to do just about anything.
Heel & Toe
Joined: Jan 2018
Posts: 16
Likes: 2
From: Australia
Dont be mislead, Bump steer is very noticeable and if everything in your supension is in good order you will notice your car darting left and/or right when it hits a bump.
I have just carried out a bump steer analysis on my C3 and the inner ties rods need to move in about 2-3/8 (60mm) and up about (1 inch 25mm)
I have just carried out a bump steer analysis on my C3 and the inner ties rods need to move in about 2-3/8 (60mm) and up about (1 inch 25mm)
Race Director
Joined: Mar 2017
Posts: 11,710
Likes: 4,363
From: Virginia
Before this 23 year old thread gets closed, please consider contributing on this, more recent, thread.
https://www.corvetteforum.com/forums...ump-steer.html
https://www.corvetteforum.com/forums...ump-steer.html
Administrator
Joined: Jul 2000
Posts: 53,887
Likes: 6,162
From: About 1100 miles from where I call home.
Dont be mislead, Bump steer is very noticeable and if everything in your supension is in good order you will notice your car darting left and/or right when it hits a bump.
I have just carried out a bump steer analysis on my C3 and the inner ties rods need to move in about 2-3/8 (60mm) and up about (1 inch 25mm)
I have just carried out a bump steer analysis on my C3 and the inner ties rods need to move in about 2-3/8 (60mm) and up about (1 inch 25mm)
