longdaddy

loaded in the turn, drive over seam/dip in the pavement, the back end hops sideways - i can feel/hear the back tire sort of skip. don't know how to describe it better. it's not too bad, I can stay on the gas and things don't seem to get too disrupted by this but it feels pretty weird and i keep wondering would this swing me around if I go a little faster through those corners . thought something was up in the rear suspension, went over it today, everything looks good.

is this what they call bump steer? the car is significantly lowered on LG coilovers/T1 bars.

another theory is that rear springs are too stiff...

Short-Throw

Bump Steer occurs when there's no movement from the steering wheel and your front wheels steer themselves (from hitting bumps on the track). From what your describing it sounds like another issue.

Too stiff a spring will not allow the shock to do it's job. Are your LG coilovers adjustable? If so you can start by adjusting your bump/rebound in the front.

Has your car been aligned recently and corner balanced?

Bill32

No, Bump Steer is a change in toe when a wheel moves up or down (bump or droop). For measurement purposes, static ride height would be the starting point and considered 0. As the suspension moves up (bump) the measurement of the change in toe in (or out) is bump steer.
Bump Steer happens because the A arms and tie rods are different lengths and travel thru dfferent arcs.

It's probably not the cause of what you describe since the chassis set (loaded) in the corner. Spring rate, tire press,etc. can affect this.

Bump steer is termed that way because when the suspension moves up, it's called bump (when it moves down, it's called droop).
It has nothing to do with hitting bumps.

AU N EGL

too stiff suspension and your bouncing around.

You might be surprised how soft some race car suspensions are set up to ride over bumps and and gaters without upsetting the car

longdaddy

corner balanced and aligned 3 days earlier, shocks are non-adjustable

gkmccready

I've been warned that if I lower the car "too much" I need to look at a bump steer kit. It's not clear what too much is, but I suspect it's when you get down near 4" to the frame rails as that's about an inch lower than stock.

longdaddy

that's what I read as well. I am sitting at about 3 3/4 between floor and frame rail in the front, a little higher in the back.

Bill32

Ok, then you have changed the suspension geometry somewhat. The kits are designed to reduce the bumpsteer by adjusting the length of the arms and/or shimming the tie rod higher or lower.

When the kit(s) are installed you need a bumpsteer gauge to measure the toe change in bump and droop. Its a fairly involved process, the springs and shocks have to be disconnected. The A arms have to be held in the static ride height position with a jack to zero the gauge, then readings and adjustments are done. When complete, the alignment should be rechecked.

vette6aut0x

And sway bar/bars

longdaddy

so my issue could be related to bump steer? getting some conflicting messages here. the car tracks fine in straight line or long sweeping turns, even going over rougher surfaces. it's the sharp, hard, lower speed turns where this happens.

longdaddy

yes there are some tracks (i.e. Reno-Fernely) where I miss my stock setup.

trackboss

If you bottom out your suspension you can get an effect like you describe, but it takes a lot to do that. Tires can also be a big part of it. Way back when I had the factory run flats(they have gotten much better since then) on my car it was all over the place. Almost felt un-controlable at times. Replacing them with quality tires made a world of difference.
It is very possible your outside rear tire is going toe out in your situation which can be because of bumpsteer, but also because of bushing compliance.

Bill32

Good point.

Bill32

Though Fernley's basically closed down, we're working on 3 dates this year for Time Trials & PDX. Possibility of this happening is about 80%, IMO. We are working on the contracts though.

ZR1 MK

Since you state "significantly lowered" I predict you have a Bump Steer issue. Since Bill32 gave you the proper description of Bump Steer, you can check, adjust if neccessary and at least rule this out as the problem.

DaveL82

Because you've already loaded the suspension in the corner significantly the suspension becomes very stiff so the ability to absorb a surface change is redcued. Traction decreases so car hops sideways. This I believe is the main cause so it might be best to try a lighter spring unless car is on the bump stops at this point. Do you know how much travel is left when you experience this (o ring or tie wrap on shock shaft to show max suspension travel).

In regards to bump steer I would think the best approach would be to measure ground to reference point on rear spindle/a-arm. Then remove shock, set height to reference point and then let suspension compress to see if toe does change and if so in what direction.

longdaddy

i'll test to see if I am bottoming out the shock

mgarfias

I've you've lowered, you need to goto a stiffer spring to absorb the bump in a shorter amount of deflection. Why not raise the car up a bit instead?

jtmck

Unless the LG shocks are shorter, you may be bottoming the shock.

Also, lowering the car takes handling away. (Even 1/2 inch) The roll centers can go below ground and move sideways quite a bit at your ride height. This will show up in the middle of the turn. If you have stock pickup points, the best handling Corvette is at stock ride height.

Jim M.

Lan.Jet

When I was testing the LG’s I found this would happen if you run them to low or to high. The shock bodies are shorter than stock and you will never get enough adjustment out of them to run a stock height. I found the best setting is about 18 threads from the top to the top of the ring that holds the spring not the lock nut. And 20 in the rear. It will work out to be about 4” of ground clearance.