Bump Steer?

[hcolon]

I have a 99 Torch Red six speed with(unfortunately) FE-1. The car seems to steer itself on bumpy roads or when changing lanes or creases on the roads(like soft shoulders). I notice it's better with traction control off but it still happens. Is this normal? Someone told me that the goodyear EMT's make the car hip-hop, but...??
Also happens when applying brakes only over bumps and road creases. Any suggestions anybody?
thanks

 

[FrankM]

some of this normal due to width of the tire.
I have had a different brand, ( i now forget the brand) that did this intrusivly, it went away when i installed diffferent brands.

 

[danl72]

I have the Goodyear EMT's and this happens to me as well sometimes. The car will pull itself to the right, but it is not bad. I had it happen with another car before so I am used to it if and when it happens.

 

[99WhiteC5Coupe]

hcolon,

My 1999 C5 coupe (A4) will "jump" or "hop" when I am driving, enter a sweeping curve (such as on the expressway) and hit bump or rough surface.

My C5 has the base suspension and has done this since the car was new. I drive the car very carefully and do not "push the limits" when I drive. I have found the car's tendency to move when I hit a bump in the road very annoying and dangerous.

1999 White C5 Coupe

 

[hcolon]

bump steer

Frank M, Danl72 and 99 White C5 Coupe, thanks for the reply. I feel better now as I thought perhaps I was the only one with this condition. Perhaps when the tires wear down I will consider non-EMT's.....I was thinking the base FE-1 cars do this, cause a buddy of mine has a 2000 with Z-51 and the car seems to handle alot better. I lowered the psi to 26 cold pressure on the factory EMT's and it seems to aleviate the problem a little. The EMT's are hard!
I noticed Eckler's and other Vette stores sell the "bump steer kit".
Will this help you think?
Herbie Colon

 

[motorman]

bump steer

if you lower your car you may run into bump steer because the angle if the tie rods are now changes in relation to the angle of the A frames. they sell bump steer kits that raise/lower the tie rod ends.

 

[hcolon]

Motorman, so the bump steer kit is for lowering the C5? any recomendations on how to keep the car from zig-zag on the road, besides different tires?

 

[Magic]

bump steer

I don't know about a C5 but I did have this problem on my 1985 C4 and a Porsche 944

Our bumpy roads in the UK cause havoc with bump steer.

One word "CASTER"

Caster is the angle looking side on of the top and bottom ball joints, the less caster the quicker, lighter and more responsive the steering but this increases bump steer. The more caster the heavier and less responsive the steering

ie. 1 degree = quick sharp steering but more bump steer
7 degrees = slower steering and no bump steer

GUARANTEED TO WORK!

I use 7 degrees and with the strength of the power steering on corvettes you won't notice the weight.

 

[motorman]

bump steer

the definition of bump steer, "the toe in/toe out changes with the up and down movement of the suspension". that is bump steer. wander on the road my not be bump steer if you have not changed the geometry of the suspension parts.

 

[hcolon]

Thanks Magic! Is this something I can do at the dealer? Who can work on this. Does anybody know of someone in MIami?
Herbie

 

[hcolon]

Motorman, I haven't done anything to the car. It does zig-zag sometimes left, sometimes right. But if your on a good flat clean surface, the car goes perfectly straight. Weird.....
Herbie

 

[vettepilot]

hcolon,
The condition that you are speaking of, wandering, following the crown of the road, wheel ruts etc. is pretty much the norm for wide, low profile, high performance tires. The stiffer side wall construction of the runflat tires probably accentuate this, so I really don't think that changing to a non-runflat tire will completely alleviate the condition. It may make it a little better, but then you have to think about your contingency plan for a flat tire since you would then have no drive away capability.
My base suspension 91, my ZO7 93, and now my 02 ZO6 all had and have a tendency to want to climb the gentle crown of the roads. You stated that it is fine on a smooth no crown road, that tells me that you PROBABLY have nothing wrong with your suspension, steering, tires, or alignment. Obviously it's up to you what you want to do about tires, but myself, I would stay with the OEM type EMT for that car.
vettepilot

 

[hcolon]

Thanks Vettepilot. That seems to be the normal consensus so far so I think I will just live with it. Just the same, I am going to check the alignment, just in case. I really love the car alot and just want what's best for my baby. Thanks for your input!
P.S. ON another note, do you think I should upgrade to Z-51 sway bars on my base FE-1, or the Z06 bars? Is there a difference?
Herbie

 

[vettepilot]

I think they are both the same, I'll have to look it up in the specs later, but that would improve your cornering by keeping the car flatter through the turns. To complete the modification you might look into different shocks also. I know the ZO6 FE4 suspension has different valving, even between the 01 and 02 ZO6. Off the top of my head I "think" the Z51 suspensions also have different valved shocks, but I can't swear by it with out looking it up.
None of these modifications will improve the wandering or climbing of the wide tires, but they may help make the car itself feel more stable even during these excursions because it will feel more solid than the base suspension and dampen out the rocking and rolling sensation.
vettepilot

 

[hcolon]

Vettepilot, thanks again for the reply. I beleive I saw the part numbers for the Z-51 bars on the C5 Registry, along with the end links, bushings etc. The shocks were listed there also, but there was no mention of the Z-06 components. I have attached it here as a HTML file, which I don't know if it will open, but give it a try. If it doesn't, I will sent it again as a text file. I am going to take your advice and change out the shocks as well. If you find out the part numbers for the Z-06 bars and shocks, that would be great to know because the I can make a final determination. Like I had mentioned previously, a buddy of mine has a 2000 with Z-51
and his car drives great! I would love it if my car drove like that.
Your right in your assesment, It was solid cornering, and the steering was firmer, it just felt generally better. Anyway, if you could look up that info and post it here on this thread that would be cool. What do you think about the Bilstein's?
Herbie

 

[hcolon]

Vettepilot, here are the part numbers for the C5 suspension, listed by component......
Herbie

 

[vettepilot]

Herbie,
The part numbers didn't come through. There was nothing there.
Re. what do I think about Bilsteins, I had the electronically adjustable Bilsteins on my 93 ZO7, I finally replaced the rears at 110,000 miles, the fronts were still going at 146,000 when I traded that car in for the 02 ZO6. I think they are pretty good. There are others (shocks) out there that probably do just as good a job. But I don't know about their longevity, most shocks seem to become weak around 35 - 40 K miles, but the Bils keep on going, I think they are advertised as being good to 100,000 miles. That's not too shabby, especially if you drive high annual mileage like I did, you don't have to change shocks every 18 months.
Did you check into the spring rates for the Z-51 suspension, I can't remember if I mentioned it in my earlier posts, but I'm quite certain the spring rate is higher over the base suspension also.
vettepilot,
Stan (not to be confused with Stan A also found on the forum)

 

[rjsmith]

Re: bump steer

the definition of bump steer, "the toe in/toe out changes with the up and down movement of the suspension". that is bump steer. wander on the road my not be bump steer if you have not changed the geometry of the suspension parts.

You know your business well.

According to Fred Puhn in "How To Make Your Car Handle" (HP Books, 1981) on page 23:

"Toe change is called roll steer if caused by body roll, and bump steer if caused by vertical suspension movement".

He elaborates on page 93:

"A car with bump steer in the front suspension will be unpredictable in a turn and unstable during braking. The car is very sensitive to toe-in changes. With bump steer, toe changes can happen with every dip in the road or when you hit the brakes. Both toe-in and toe-out errors give terrible handling and should be avoided in the front suspension.

Cars with independent rear suspension can also have bump steer at the rear."

On page 90 he offers a solution which matches your solution:

"On the front suspension, the bump steer can be adjusted by changing the height of one end of the tie rod".

The solution offered by "magic" of the U.K. to increase positive caster, according to Puhn, does not eliminate bump steer per se, but does induce more self-centering force to the steering, causing the wheels to return to their original direction after a transient.

I think the original subject of the thread ("Bump Steer") is unintentionally misleading. Your deduction that wander on the road may not be bump steer if the suspension geometry has not been altered is certainly approriate.

I think what the original poster (hcolon) experienced (especially when hitting a bump in a hard corner) is that the tire contact patch leaves the ground, causing a momentary absence of traction at that corner of the car. In that very brief instant, the car seems to be heading for the outside of the corner (which it actually is). The springs eventually bring the contact patch back down onto the road and the shocks dampen the spring oscillation, keeping it down, thus restoring traction on the most heavily loaded tire, but for that brief instant the sphincter tightens appreciably.

BTW, are you near Camp Delmont?

Ray

 

[motorman]

camp delmont

i have no idea, what is camp delmont? we raced circle track both on the local level and at places like daytona so i have been used to working out bump steer.

 

[hcolon]

Vettepilot, hope you had a great weekend. Do you know how to send a attachment on this thing? I hit the browse buton on the reply post and attempted to attach a text file but? If you have a fax nuber I will fax it to you . I think the springs that you mentioned are here also. I will try resending the attachment here again, if It doesn't work, let me know....
Thanks again, Herbie