Z1 Performance

iTrader: (564)

Now that I am using a grippier street tire, and have had the car aligned, etc, I've noticed what I can only describe as a "shimmy" in the steering wheel immediately preeceding a bump in the road. It varies in intesity with the bump itself, though it never gets incredibly violent. I've never experienced this before, so am unsure exactly how to title it to you guys. From my reading up on a few different sites, it seems bump steer would be the applicable term, but I could be wrong. I do not hear any atypical clunks or noises (having coilovers and stiff walled tires, I have grown somewhat accustomed to these noises, particularly at low speeds). The only odd thing I find re: bump steer is the car has a great alignment on it right now, with about -2 camber, a tinge of toe out and castor that is well within spec.

Tomorrow is oil change time so we are going to give everything up front a once over...but it would help to know where to start.

Relative mods: Cusco sway, Cusco coilovers, Cusco a arms, S03's on Work Emotions (18x8.5 +30 with a 5mm spacer) and 245/40/18 S03's at 34 psi.

Thanks in advance!

adam

Kolia

iTrader: (2)

I just reallized I have no idea what the standard offset is on our car...

Here is a hint so you can orient your research on your issue: Scrub Radius

The Scrub Radius is the distance between the center of your tire contact patch on the ground and the extention of the axis of rotation around the steering axis. The longer this is, the longer the lever that pushes against the steering wheel whenever you hit a bump.

Wider tires usually increase that (moves the contact patch outboard), your offset might have changed also. More grip and stiffer sidewall will also transmit more of these forces. Lowered suspensions might affect your castor (which normaly works with you to keep the car going straight). Less castor means a steering more susceptible to road irregularities.

It's "normal" in that it's not a problem. It might keep you on your toes !

daveh

iTrader: (9)

It's probably due to tires and offsets. I notice this with my beefy race tires up front. Returns to normal with my stock tires.

FritzMan

If we're talking about the same thing (steering wheel feedback when hitting a bump) I found it got worse the stiffer the front bar was set. Like you I wasn't sure if it was bump steer but the term would match the effect nicely.

It's brutal with the Hotchkis full stiff, tollerable when at medium, and not an issue when the stock bar is used.

John

iTrader: (17)

I'm thinking the same thing. Since a stiffer bar "lessens" the independence of each of the wheels, when you hit a bump on one side, it's going to have an effect on the opposite side.

EnthuZ

iTrader: (2)

I had a similar "Bump Steer" issue with my Kumho MX 255/45 on 27mm offset 9" rims. I thought that the 30 mm increase in width & the 3 mm increase in scrub radius was causing it. BUT NO!! I recently switched to Falken RT-615's, size 255/40 17, and the bump issue has disappeared! I'm guessing that the more rounded outer edges of the tread have contributed to the improvement.

Z1 Performance

iTrader: (564)

I "think" we figured it out today - noticed when turning the wheel from a dead stop I heard a little clunk...put the car up in the air, and viola...bad inner tie rod end. I am praying the dealer has it in stock tomorrow and all should be right with the world

benzz280

iTrader: (2)

off topic but what kind of 5mm spacers are you using? Are they strong enough for the track? Are you selling them? thanks!

Z1 Performance

iTrader: (564)

just generic 5mm ones...does not need to be anything special for that small of a spacer. If it were larger, hubcentric would be the way to go.

not something we sell..any tire place will have them

These will be eliminated tomorrow anyway as I just got new front wheels with a lower offset...the spacers were for purely cosmetic reasons