Bumpsteer and shims
10-24-2022, 08:58 PM
#1
Advanced
Thread Starter
Bumpsteer and shims
I have 10mm dropped my GTS 4.0, now have a bit of bumpsteer. Is there any relationship between tie rod shim thickness and dropping? I mean, a 10mm drop means 10mm of a gap to fill with tie rod shims?
10-24-2022, 09:10 PM
#2
Did you get a wheel alignment done when it was lowered? Ride height affects your alignment so it might need to be reset.
10-24-2022, 09:24 PM
#3
Advanced
Thread Starter
10-24-2022, 09:50 PM
#4
When I added shims to increase the neg. camber on my GT4, it made a big difference (in a bad way) to front bump-steer. For reasons I do not fully understand, adding LCA shims changes the geometry in such a way that it adds bump-steer. It took an RSS Bump Steer Kit to resolve it.
As far as lowering, it has the same neg. effect on bump-steer. Anytime you deviate from the arms being in perfect alignment on a strut suspension, you will alter the bump-steer.
As far as lowering, it has the same neg. effect on bump-steer. Anytime you deviate from the arms being in perfect alignment on a strut suspension, you will alter the bump-steer.
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GentianGentleman (10-25-2022)
10-24-2022, 11:10 PM
#5
Strut suspension and its pivot points don't have perfect geometry, so camber and toe angles will change as it sweeps through the range of travel.
TPC has a good graph of how the rear toe deviates through the range of travel. I am not sure how the front is affected but it will have its own profile. Lowering by 10mm will change where the cars zero point is on this graph, since it's not perfectly linear you could be making bump steer better or worse.
https://www.tpcracing.com/wp-content...2/download.png
Lowering will also naturally add some more camber so my guess, is that to preserve the same alignment you had before you will probably want to remove some shims.
TPC has a good graph of how the rear toe deviates through the range of travel. I am not sure how the front is affected but it will have its own profile. Lowering by 10mm will change where the cars zero point is on this graph, since it's not perfectly linear you could be making bump steer better or worse.
https://www.tpcracing.com/wp-content...2/download.png
Lowering will also naturally add some more camber so my guess, is that to preserve the same alignment you had before you will probably want to remove some shims.
Last edited by Reedy; 10-24-2022 at 11:16 PM.
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GentianGentleman (10-25-2022)
10-25-2022, 02:29 AM
#6
Burning Brakes
When you start adding a bunch of front shim, that off centers the wheels and and causes a 3d change in the suspension. If you can recenter the wheels in the wells with the solid adjustable LCA middle bushings "caster pucks", that at least brings it back to more of a 2d issue on the suspension geometry.
10-25-2022, 05:55 PM
#7
Intermediate
Hold on a sec - on a GTS (unless you've upgraded) you've got single piece lower control arms, so there shouldn't be any shims involved. I assume you've lowered the car by way of lowering springs? Nevertheless, due to the limitations of strut-based suspensions outlined above, you'd pick up a minimal amount of additional negative camber and (I think) a very minimal amount of added caster, but neither of those should be causing the kind of adverse effects the GT4/Spyder crew get when adding too many shims to their 2-piece LCAs.
Do you happen to know the spring rates of the lowering springs you've installed? They may not be a great match to the stock dampers and that could be causing the odd sensation you're experiencing. Do you notice any difference with the shocks set to the hard/sport setting?
Do you happen to know the spring rates of the lowering springs you've installed? They may not be a great match to the stock dampers and that could be causing the odd sensation you're experiencing. Do you notice any difference with the shocks set to the hard/sport setting?