When is GT3 bump steer adjustment really needed?
 

After doing some research on GT3 bump steer I have found that many Cup Cars (and in fact some RSRs) were delivered without adjustable bump steer at the toe links. The early Motorsport toe links are not bump adjustable. eg this RSR:
http://www.flickr.com/photos/apex944...7623932622070/

So I have some questions to any of you who have actually plotted out the bump steer position while lowering a GT3 for track use.

A) Does anyone have a plot of the bump curve for a 6 or 7 GT3 that they can share?

B) At what point does the GT3 really need to address bump steer adjustments when lowering? IOW, how much can one lower it before bump steer needs to be addressed?

Mainly concerned about the rear #s here.

Thanks in advance.

I am currently running
F 102
R 117

Didn't use the spacers.

I don't have my plot but did have it adjusted through the whole range for my ride height.

i dont know much about RSR. but 996 cups are not delivered with bump adj toe links.
in fact thrust arm busing, control arm busing, upper dog bones are NOT monoball and are NOT adj.

How much was your car lowered and how much did you move the tie rod down?

my car is as low as 911slow's

Proper bump steer on the rear suspension is set with adjustable upper links (dogbones). If you spend enough time=$$ you can get the rear bumpsteer to almost 0, yes less than 1mm of toe change over a range of 2" of compression and 1" extension (Kinematic toe). The front is a different story, you are limited to useing tie rod heights (if useing monoball ends only) and thrust rod length(if adjustable by means of rod length like RSR style or offset bushing). The problem is a lot of guys have the adjustable upper links, but they are not setup properly (usually stock length), or someone doing the alignment screwed with them and now you have a car with diabolical handling, but you've been told the car had been bumpsteered, and all they did is try to get your rear tie rod as level as possible. Hope this helps
B

So, is this bump steer adjustment info (curve and upper control arm lengths -if installed) so classified that no-one is willing to share?

This is one of my winter projects... Gotta make a bump steer gauge 1st...

Few people seem to bother with the effort...

Ray

Mee too...and at the same bankruptsy I'll learn how to do the complete alignment at home.

R 107
Fr 101.6

Bump steer adjusted

Mk

What I'd like to know is these two things:
-upper link lengths, if they are adjustable
-shim thickness used between wheel carrier and toe link. OR, even better, the distance from wheel carrier's bottom (where toe link attaches to it) to toe link's centerline. See pic for clarification:

I would imagine bump steer becomes an issue for the guys that run super squishy soft springs or still have rubber in the suspension both of which allow for a ton of travel. I am running 1400/1500 springs with everything monoball and don't have any bump steer issues as far as I can tell. Car feels super solid compared to a stock 6GT3. I have the 997 PMNA cup rear toe arms that can be shimmed for bump steer. F98 R121 running 997GT3 tire sizes which are 1" taller than OEM in the rear. Fix the big issues and then worry about the little ones...

Hi powdrhound,

Quick thoughts:

The following is just my opinion - worth even less than you are paying for it...

When you start to get into springs that stiff, you are not allowing your suspension to do its job - ie provide compliance to deal with irregular surfaces. The stiff springs will hide any bump steer issues because you will not have any compliance - things will not move, so no bump steer.

I am currently running 1000/1100 with 275psi Motons and I think the car could probably loose ~ 100 - 200lbs of spring and some canister pressure ( have not factored the wing in)...

My suspension *should* be able to accommodate the travel and manage the bump. The reason these springs are so big is that this is what the car came with. With my current ride height, I am running out of shock travel which is why the canister pressure is so high.

I am not saying that your setup won't work, just that it works because you are effectively limiting the travel.

thoughts?

Ray

That's what I was getting at Ray. Soft springs will allow the car to wallow resulting in a lot of suspension travel exaggerating any bump steer issues. What I have learned is that if you have quality shocks you can run high rates without any downsides. The key is a quality shock in my opinion.

I've run my car with 270/550 on H&R coilovers, 600/800 and 700/800 on JRZ RS1s, and finally 1100/1200 and 1400/1500 on JRZ RS Pros with 175psi canister pressures. It took a lot of convincing by our local suspension guru for me to go that high but I'm glad I did. I know it goes contrary to all you read about on these forum from the many self proclaimed suspension experts but all I have to say is that it works. It made a believer out of me. All else unchanged, my car turns in the best lap times with 1400/1500 and there is no issue with traction even on the bumpy sections. Believe it or not, with my current shocks my car feels less harsh that it did with 700/800 and single adjustable shocks. My 1400/1500 set up actually still feels a bit soft and ideally I'd like to go to 1700/1800 as that was rate that was originally figured for my car an optimum track set up. My car is by no means light as it weighs 3200. For comparison, a 6Cup is 600 lbs lighter than my car and runs similar rates. One of the local track rats went from his reenlist 700/900 GT3 set up to straight 6Cup rates on his Motons and likes it. He picked up time.

Your car is 500 lbs heavier than a Cup and your springs are about 50% softer. I don't see your springs as overly stiff at all. I would say your main problem seems to be the fact that you are running out of shock travel as you say. That will kill your grip on bumpy surfaces and it's a huge problem that you are trying to mask with overly high canister pressures. Softer springs will only make that worse. Bump steer is irrelevant here. Like I said in my original post, fix the big issues first, then the little ones. Just my $0.02...and I stayed at a Holiday Inn last night...