Bought a 2005 M6 Z51. The owner put on 285/35/18s on the front and 305/30/19s on the back. Huge difference between that and stock for this car. I just replaced the suspension with the O'Connell Stage 1 package with shocks and sways.

I have had a 2006 M6 Z51 back about 5 years ago and I don't remember that one being this twitchy/unstable/erratic(name your adjective). Could it be these oversize tires? I have read about some active handling anomalies when you mess with the tire sizes but this is just plan old terrible handling. I was thinking alignment as well but it tracks pretty straight on the highway. Just goes nuts when you get on it a bit.

Any help would be appreciated. Anyone in the South Florida (Boynton) area want to meet up and let me borrow your regular sized tires for 15 minutes so I can determine if these tires are the culprit.

Thanks
Mark

 

 

was it erratic before the new suspension parts?

 

yes it was since I bought it about 2 months ago.

 

 

 

Stock sizes are 245/40/R18 and 283/35/R19. It looks like you have about the same stagger.
My guess is an alignment issue. If you don't track the car find a shop that will set the alignment to EXACT base specs (some folks use pfadt). Again, you want it exact, not "within specs". If it is a track car have the shop set it to EXACT Z51 specs (tire wear will be more aggressive).

 

Did you verify hubs, ball joints, and tie rods are all good? If those are fine, then it's alignment.

 

If these are on stock wheels, the wider tire effectively being squished onto a narrower wheel will allow for lateral movement (squishing from side to side) as you drive/corner. The front tires on my C7Z are 285s and during slow tight cornering they feel like they are binding and then hopping along. I believe GM put a notice on '17+ models stating this was normal and not to bring it in for repair. I would swap out for stock sizes, at least the fronts. I think you will be happy you did.

 

Tire OD sizes are fine so your not throwing codes, but with such wide tires on oem rims, it balloons the tires out on the rims ,so you only end up with center tread contact, and this cause problems in itself.

On stock rims for track use, 255 front, 295 rears is about as wide as you want to go on the oem rims, so tires are not ballooning on the rims.

As for new suspension, it needs to be adjusted to start with, including a new alignment on the car as well.

Hell, you don't state what springs you have on the car to start with, and if you still have the FE1 on the front, its going to cause problems with the suspension not balanced to begin with.

Lastly, you don't state what tires, and if you tension up at tight as a Z06, you need tires that have the amount of grip as the Z06 tires to start with, or your going to be all over the road at speeds isntead.

So to sum it up, everything starts with the amount of tire grip to start with, then you work inwards with a balance set up on that amount of grip, including setting up the corner balace, suspension adjustments as well.

So tires too wide for the oem rims causes them to balloon on the rims to start with to reduce the grip amount to start with, and not know the rest of the specs on the suspension and it's set up number specs, the real problem isntead.