dylanc

So after some good feedback I got a PPI done on a lovely gray 96' in Columbus with 43,000 miles. The car was described as excellent, and the shop (northend wrench) described the condition as "better than very good, just below concourse."

He had a few little things, like the A/C needs a charge. But here is the question.

He noticed a slight tracking issue when the car went over a bump. Probably due to the work when lowering the car. Said this may or may not be noticeable to a normal driver under normal circumstances. But he drives autocross often, and noticed it. He thought there was an issue with "the tierods pointing up." His words. Not exactly sure what that means - anybody have any ideas or experience with such a thing?

cgfen

research "bump steer".
this is an effect caused by lowering the ride height of a car and changing the suspension geometry.

not a big deal for most people, unless it precipitates a crash..........
big deal for serious track-folk.

have fun.

Craig

dylanc

Gotcha. Bump steer. Did a quick tour through the archives. Hard to find a simple straight answer though.

Is it a function of ALL lowered M030 Row cars? Or does it mean the lowering was done incorrectly? Or too low without compensation?

johnsjmc

To minimize bumpsteer on a lowered car the tie rods are best if parallel to the lower control arms. The very expensive RS uprights move the outer end upwards and I believe reversed the direction the ends attach from stud pointing up/down to stud pointing down/up(can,t remember which).to maintain the desired rack angles. The earlier torsion bar cars often used spacers under the rack mounts to lower the rack to achieve the same thing.If lowered to ROW height the 993 bumpsteer should still fall into an acceptable range after all thats the height the suspension was designed for. If lowered below to RS height then bumpsteer will increase. You may or may not notice it . I suppose there is a chance the tech is referring to the tierods being installed upside down in the standard uprights but I think unlikely as the taper in the upright wouldn,t properly fit to allow that.

Bill Verburg

Bump steer can never be eliminated, but it can be minimized.

The cause of bump steer is the different arcs traversed by the steering arm and the suspensions A-arm. Since the inner pivots of these arms cannot be co-located and they are different lengths, the outer ends will always go through different arcs. The difference in arcs causes them to pull on each other causing toe changes which can also be felt through the steering wheel and is an issue in a turn where the suspension is heavily compressed on one side and unloaded on the other, but will also be felt when one wheel goes over a bump compressing that one corner. The change in toe steers that wheel, in the turn example the other wheel is steered in the opposite direction

To minimize bump steer you want the steering arm to be as neutral as possible, normally this will be parallel to the ground, if the steering arm points up or down it usually means that the bump steer is out of the minimized zone into a regime where the steering is more obvious to the driver

here is the bump steer(vospur) plot for the front(top) & rear(bottom) of a normal 993


ON a lowered car the tie rods tend to go up to the steering arm on the wheel carrier, to restore geometry RS uprights are installed, these have shorter wheel carrier arms so the tie rod is back to a neutral orientation.

here is a pic of a 993RS(aka EVO) wheel carrier on the left and a standard 993 wheel carrier on the right, the blue arrows point to the wheel carrier steering arm

Paul M

I'd be wary of the little thing like "A/C needs a charge."

If it needs charging because the evaporator is leaking, you're looking at a $2k repair.

bb993tt

^^ Where you come up with this stuff, virtually instantaneously, never ceases to amaze me, Bill! And I thought phelix on the TT board was good. He's a bloody piker in comparison.

dylanc

Wow Bill. Thank you for that. So I may assume that with this car, the 993RS wheel carriers were not installed, thus the steering arm mis-alignment.

Changing the wheel carriers would (or would not) be considered standard when lowering the car?

As a stand alone repair to fix the bumpsteer - what am I looking at, ballpark?

johnsjmc

Last I heard the RS uprights were more than $1200 and you need new wheel bearings and $600 for tierods. It,s not normally done until lowered below ROW height.

dylanc

So the RS uprights are not really necessary unless lowered BELOW ROW height.

Is there a standard test to determine if the car is at or within acceptable height with stock uprights? Or is this just something the mechanic has to eyeball?

johnsjmc

The shop manual will have measuring points and distances for the front and rear of the car at the 3 "standard" heights US,ROW and RS. I,m sure the info is available here somewhere. You can also get measurements to the fender lip or rocker panel but I believe the official method is to a suspension mounting point

bart1

Streather's book also has good diagrams and measurements for setting suspension height. Mine is about ROW sport -10 and corner balanced. Bump steer is not terribly objectionable. YMMV.

Bill Verburg

Bill Verburg

Memory like an elephant, except I can't remember where my glasses are half the time

dylanc

Allright. As always - thank you all for the help. I'm going to get a second opinion on the issue tomorrow.

- Dylan