Mid corner bump causes car to "jump" sideways
Thread Starter
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Joined: Sep 2007
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Mid corner bump causes car to "jump" sideways
Quick question - I have a 99 FRC with the stock Z51 suspension and run-craps. On spirited drives, if I hit a bump or large expansion joint in the middle of a corner the car feels like it "Jumps" sideways about 3 feet. It catches and hangs on, but it does not inspire confidence !
I didn't even notice these bumps in my Porsche. Is this a function of the run-flat tires, the shocks, or both?
Any help is appreciated.
I didn't even notice these bumps in my Porsche. Is this a function of the run-flat tires, the shocks, or both?
Any help is appreciated.
Team Owner
Joined: May 2003
Posts: 23,480
Likes: 9
From: East Meadow NY
St. Jude Donor '12
Run flats will definitely contribute. You can also skim this short thread if you have lowered the car.
http://forums.corvetteforum.com/c5-t...ump-steer.html
http://forums.corvetteforum.com/c5-t...ump-steer.html
Safety Car
Joined: Nov 2000
Posts: 4,549
Likes: 30
From: Shenandoah Valley Virginia
There are a bunch of things that could cause that type of feeling - Is the turn off camber ? How old are your tires, what type, condition and inflation pressure ? How many miles on car and what is condition of shocks and suspension in general ? Are you braking, driving steady state or accelerating through the turn ?
Team Owner
Joined: May 2001
Posts: 36,836
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From: Dear Karma, I have a list of people you missed.
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15-'16
There are a bunch of things that could cause that type of feeling - Is the turn off camber ? How old are your tires, what type, condition and inflation pressure ? How many miles on car and what is condition of shocks and suspension in general ? Are you braking, driving steady state or accelerating through the turn ?
Also, lose the plastic anti-rollbar end links you have too. You may even have a broken/loose one anyway.
Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
My guess would be lowered and the rear shocks are bottoming out on the bump. Hitting the bump stop will cause it to do that.
Another thing not really mentioned is the aligmnent. When was the last time the car was properly aligned?
Another thing not really mentioned is the aligmnent. When was the last time the car was properly aligned?
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 328
From: CA.
St. Jude Donor '13-'14-'15
What is the MPH entering the corner ?
Is the bump before you hit the brakes ? during ? or after ?
The Z51 suspension is great for hi speed cornering-But may be too stiff for corners less than 70 MPH--Does your car have a suspension mode switch ?
Many reasons for a sidways hop---I assume you mean the "rear" hops sideways ?
As mentioned you may have broken or just even the plastic end links--You need the metal ones--Also other mods that aid in keeping the car flat in the corners is installing poly bushings ( they claim it adds 25% to the bar) and a solid trans mount --that reduces rearend to fame flex
Also is the bump clear accross the track ? or does it hit only the left or right sides ?
Is the bump before you hit the brakes ? during ? or after ?
The Z51 suspension is great for hi speed cornering-But may be too stiff for corners less than 70 MPH--Does your car have a suspension mode switch ?
Many reasons for a sidways hop---I assume you mean the "rear" hops sideways ?
As mentioned you may have broken or just even the plastic end links--You need the metal ones--Also other mods that aid in keeping the car flat in the corners is installing poly bushings ( they claim it adds 25% to the bar) and a solid trans mount --that reduces rearend to fame flex
Also is the bump clear accross the track ? or does it hit only the left or right sides ?
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Thread Starter
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Joined: Sep 2007
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Thanks for the replies - answers to the questions:
The car is "new to me" - the last owner did not put many miles on it over the last 8 years
Steady state or mild acceleration - think "corner out" - yes, the rear is what is stepping out.
100k miles.
Tires are not original but are several years old. (5+) 35 psi.
All original suspension with plastic end links, NOT lowered. No suspension mode switch.
Not sure on the last alignment. What are good alignment specs for the street ?
Side note - I have 10+ years of DE experience with the Porsche club of America. This is not a "driver induced" condition.
Thanks !
The car is "new to me" - the last owner did not put many miles on it over the last 8 years
Steady state or mild acceleration - think "corner out" - yes, the rear is what is stepping out.
100k miles.
Tires are not original but are several years old. (5+) 35 psi.
All original suspension with plastic end links, NOT lowered. No suspension mode switch.
Not sure on the last alignment. What are good alignment specs for the street ?
Side note - I have 10+ years of DE experience with the Porsche club of America. This is not a "driver induced" condition.
Thanks !
Last edited by brrgrr951; Jan 28, 2013 at 10:17 PM.
Drifting
Joined: Jan 2000
Posts: 1,660
Likes: 9
From: Charlotte, NC
I had a similar issue during a curved on-ramp for my morning commute. Once I replaced the stock shocks with the Bilsteins, it no longer did this.
Depending on the mileage of your car, the shocks probably need to be replaced anyway. It is not a bad install.
Depending on the mileage of your car, the shocks probably need to be replaced anyway. It is not a bad install.
Melting Slicks
Joined: May 2008
Posts: 2,606
Likes: 355
From: Rochester New York
How many miles are on the car? Have you had any work performed recently? If your alignment is off or eccentric bolts are loose that would definitely send you off a bump. Tires would not do that unless they are rock hard but even still, they would more loose traction than shoot you a few feet.
Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
Probably mostly the old shocks and the higher tire pressure but the GY run flats really aren't the greatest tire and any older high performance tire won't have the same grip as new tires.
As far as alignment. I'd say around 0.4* camber, 0.1* toe in and around 7* caster in the front with maybe 0.2* or 0.3* more in the right side to compensate for road crown. 0.1* toe in is per corner = 0.2* toe in total per axle.
Don't accept "within spec", you want the numbers to be equal all around. As an example, the caster spec will likely have a 1* range and you certainly don't want a wheel on each side set at opposite sides of this "within spec" range.
As far as alignment. I'd say around 0.4* camber, 0.1* toe in and around 7* caster in the front with maybe 0.2* or 0.3* more in the right side to compensate for road crown. 0.1* toe in is per corner = 0.2* toe in total per axle.
Don't accept "within spec", you want the numbers to be equal all around. As an example, the caster spec will likely have a 1* range and you certainly don't want a wheel on each side set at opposite sides of this "within spec" range.
Last edited by lionelhutz; Jan 29, 2013 at 06:57 PM.
Burning Brakes
Joined: Jun 2012
Posts: 1,240
Likes: 1
From: Orange County California
Shocks
Tires
Alignment
Ride height
all of these things not setup right can cause that, also, even if all else is set up well, you can get Bump Steer in these cars....I'd check the alignment and ride height issues, and look into getting a Bump Steer kit put on
as a side note, DRM makes an alluminum steering rack bushing that makes steering and curves a lot better, as do poly bushings on all the control arms.
if you want to get anywhere near the steering response of your porsche, you will want poly bushings on the control arms and the upgraded steering rack bushing to go along with a good alignment and tires.
Tires
Alignment
Ride height
all of these things not setup right can cause that, also, even if all else is set up well, you can get Bump Steer in these cars....I'd check the alignment and ride height issues, and look into getting a Bump Steer kit put on
as a side note, DRM makes an alluminum steering rack bushing that makes steering and curves a lot better, as do poly bushings on all the control arms.
if you want to get anywhere near the steering response of your porsche, you will want poly bushings on the control arms and the upgraded steering rack bushing to go along with a good alignment and tires.
Thread Starter
Advanced
Joined: Sep 2007
Posts: 66
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Thanks for the replies. I'll stick to the basics - Tires, Shocks, Alignment - and go from there.
C6 Z06 shocks, a good set of non run flat tires and a good alignment will probably do the trick.
I'm not expecting to match the steering feel of the Porsche, but this "bump steer" is downright evil. I wouldn't even think about tracking the car with it handling the way it is right now ! The good news is I can pick up a whole set of C6 Z06 shocks for the price of one Porsche brake rotor. lol.
C6 Z06 shocks, a good set of non run flat tires and a good alignment will probably do the trick.
I'm not expecting to match the steering feel of the Porsche, but this "bump steer" is downright evil. I wouldn't even think about tracking the car with it handling the way it is right now ! The good news is I can pick up a whole set of C6 Z06 shocks for the price of one Porsche brake rotor. lol.
Tech Contributor
Joined: Jan 2004
Posts: 20,030
Likes: 289
From: Wylie TX --> Less is More, except under the hood !
My Coil overs fixed that. 
No jumping when hitting a bump going around a turn.
No more body roll, no front end dip at a stop light, and more comfortable ride overall.
Better handling was a bonus.
Toque
No jumping when hitting a bump going around a turn.
No more body roll, no front end dip at a stop light, and more comfortable ride overall.
Better handling was a bonus.
Toque
Burning Brakes
Joined: Jul 2010
Posts: 959
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From: Peters Colony Republic of Texas
The Lizzard
Tech Contributor
Joined: Dec 1999
Posts: 32,910
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Shocks are most of your issue. I had the same feeling in my 02 ZO6 and upgrades to C6 ZO6 shocks fixed the issue..
What suspension do you have now?? Base/Z-51/etc???? Re read your post... Z51 Metal endlink upgrade!! YES!!!!
Shocks, decrease your cold tire pressure to 29 rear 30 front add the metal end links and get a performance alignment.
Bill
What suspension do you have now?? Base/Z-51/etc???? Re read your post... Z51 Metal endlink upgrade!! YES!!!!
Shocks, decrease your cold tire pressure to 29 rear 30 front add the metal end links and get a performance alignment.
Bill
Burning Brakes
Joined: Jun 2012
Posts: 1,240
Likes: 1
From: Orange County California
they are like a box of lego's
you can build them in endless variations, and create a truly world class performance machine for comparatively little $$$ if you choose to.
I'd go with Bilstein Sports for the shocks, great price and better performance driving than the C6Zo6 shocks, but not so aggressive as to make the car uncomfortable.
Heel & Toe
Joined: Sep 2002
Posts: 22
Likes: 9
From: Abingdon MD
More info please.
I can understand how better shocks will keep the rear wheels from bouncing off the ground.
And I dont doubt the sway bar changes help, but I dont understand how. Is the idea to get more weight on the inside tire? That way the car is less upset by a bump because it has better weight distribution? With a lesser sway bar wont the outside tires dig more?
Or are we just talking about replacing old worn out bushings and links to get the handling back to normal?
I can understand how better shocks will keep the rear wheels from bouncing off the ground.
And I dont doubt the sway bar changes help, but I dont understand how. Is the idea to get more weight on the inside tire? That way the car is less upset by a bump because it has better weight distribution? With a lesser sway bar wont the outside tires dig more?
Or are we just talking about replacing old worn out bushings and links to get the handling back to normal?
