I'm getting some kind of shimmy through my steering wheel when driving at highway speeds. I've thought of a few things that might be contributing to the problem – improperly torqued lug nuts, tire imbalance, or alignment. Could it be anything else?

I've got stock RX-8 wheels on my FD... what is the proper torque on these things? The RX-8 spec or the RX-7 spec? I'm driving on Pirelli P Zero Rosso tires at 36 psi.

 

Probably need to balance your tires.

 

*Bad tire, ply separation.
*Do the RX8 wheels have the same center bore as the FD (67.1)? If not, are you using hub-centric rings?
*Worn tie-rod ends.

 

Thanks, guys. I'll ask the garage to check the tie-rods and tire ply as well.

The RX-8 wheels use the same center bore (67.1) as the RX-7:

Wheel size: 18 x 8.0
Offset: +50 mm
Bolt Pattern: 5 x 114.3
Hub Bore Size: 67.1 mm
Lug Nut Thread: 12 mm x 1.5

 

Tire pressure seems a bit much...
Howard Coleman suggests... "when you get your car re-aligned make certain that they set tire pressure at 30 psi front, 27 rear and then check that the ride height is equal side to side. (since you have diff springs). differing ride height changes camber which will cause the car to not run true."

 

A shimmy isn't likely alignment, more like worn or unbalanced tires. Older tires can be worn to an extent that they really never fully balance out, and you'll get a subtle vibration.

If you've been running around at 36 psi cold for a while, it could be that they are wearing funny.

 

FYI, flat-spotted tires will do this really bad. Rotate your tires and see if that fixes the problem. If you got tires that are really old and maybe cupped a little or are starting to separate, same thing. I'm not discounting the un-balanced tire thing either as all of the above will cause this.

Usually you don't have this problem with ABS cars flat-spotting unless you've gone into a full slide which I've definitely done on the Auto-X track several times.

 

First thing first whenever a steering wheel shimmy problem occurs. Balance your tires. Rotate them IF necessary ONLY after the problem has been resolved.

Then proceed with front end visual inspection if it does not cure the problem. 95% of my customer complaints about front end shimmy while driving is improper wheel/tire balancing or weight fell off.

-AzEKnightz

 

Sometimes I don't drive my car for a week or two, so a flat spot is definitely possible. Also, the car may have been sitting for weeks or months at the dealership before I bought it. I think the rubber is fairly new... the tread looks fine and there isn't much wear visible on the rubber. The recommended RX-8 tire pressure is 32 psi, so I'm definitely running it higher than spec.

At the beginning of May, they asked me to press the brakes hard at track school, so it's possible that a slight irregularity was created on hard braking then (the instructors were surprised the car didn't have ABS).

I was thinking of rotating my tires anyway at the next oil change, so I'll ask them to check the tire balance first, and proceed with the rest afterward. The other interesting bit is that the shimmy will go away for a while... I'll try to monitor the behavior more closely the next time I drive on the highway.

 

flat spotting from sitting isnt to severe like what you get from locking tires . I doubt it happened from sitting to long , if it was after driving the car for a day it should be gone .

Do you still have abs??

 

No, I think the ABS must have been disabled. I've never felt the vibration through the pedal, even on hard braking at the track school.

 

most likely unbalanced tire or warped rotors. Does the brake peddle shake at all when you press it?

 

No, the brakes seem to be steady.

 

Get your tires balanced. Dont let anyone else change your oil.

 

Maybe unbalance tires or slightly bent rim.

 

I drove the FD for a couple hours on the weekend and the vibration went away after a while, at all driving speeds. It must be a flat spot thing... also, the asphalt nearby is a bit rough, accentuating the problem.

The extra unsprung mass of the RX-8 wheels and the short tire sidewalls probably don't help, either.