GXP Tire Noise/Rotation

[bocoogto]

The Grand Prix GXP's have Bridgestone 255X45R18 tires in front and 225X55R18 tires in back. Mine has 14,000 miles on it and they are just beginning to make noise from cupping. Tire dealer says I could rotate lefts to right but it won't help cupping problem or noise. They were quiet until about 12,000 miles.

Any suggestions?

[joe92k1500]

The Bridgestones on my 05 GXP were always a little noisy. Did they check the alignment? These cars have a good amount of negative camber dialed in and anymore will result in bad tire wear. The alignment can help the noise but once the tires wear incorrectly not much you can do.

[bocoogto]

Pontiac dealer also says I should rotate the right front to left front and right to left with the rears, also. He said some Cadillac models have the same arrangement and several customers have had good luck with this method.

I rotated them today and carefully examined the tires for cupping, etc. I could see no evidence of abnormal wear hence no reason I can see to have alignment checked. It gave me a chance to clean the inside of the rims, which are very visible on these forged aluminum wheels. You don't realize how huge these tires are until you take them off the car.

[CrazyHorst]

If you look carefully at a cupped tire, it has a sort of "leading edge"....a gross exaggeration would be the profile of a circular saw blade. When the "teeth" contact the pavement that's where the noise comes from.

So....if you cause it to reverse-rotate it should quiet down IMHO.

Cupping is a strange phenomenon...some vehicles have it *bad* and others you can drive all the tread off the tires without rotating a single one and never have a hint of the noise. Personally I think it has to do with some sort of natural-frequency type of excitation (mechanical amplification if you will), that's perfectly "geared" to the tire's OD, and so the oscillations repeat in the same areas of the circumference over and over.

[tblake]

I thought it was caused by mis-alignemnt... HA HA

[LC-x]

Theres basically five things that can result in cupping of a tire -

1- Defective tire (unlikely if more than one is doing it)

2 - Lack of rotation

3 - Alignment /Chasis issue

4 - Weak Suspension,excessive bounce (the road pounds on the tire until the belts break)

5 - prolonged incorrect tire pressures

Also i've come across some tires that just don't work with some vehicles.

[doctorhrdware]

I have a question, why do you have the 255 in front which is a bigger tire, than the 225 that is in the back. Wouldn't that cause some handling problems.

[joe92k1500]

The GXP's come stock from the factory that way. Its to reduce under steer.

[bocoogto]

Yes, the GXP's do come factory-equipped with larger tires and wider rims on the front than rear. The reason is for traction, not handling. That being said, Pontiac would have simplified things for owners if they had put the larger tires and rims on the back, too. Cadillac does the same thing. Corvette also does it, but being RWD, larger tires on the rear.

I've now driven a few hundred miles on the rotated tires as described. There was absolutely no cupping or abnormal wear visible on any of them. The noise I noticed is still present at about the same level. It's not a really offensive noise, but noticeable since the tires were very quiet for the first 10K miles.

Yes, we've all owned cars and/or trucks that developed noisy tires over time. I've owned a few that NEVER had the problem--even without rotating the tiresl. Front wheel drive cars always need tires rotated. Rear wheel drive not so much. Alignment with newer cars is much more stable than it was on old cars. That's a good thing, since alignment costs as much as one new tire now.

[doctorhrdware]

I never knew that, but that does make sense though.

[BNaylor]

Actually it doesn't make sense. The oddball tire and wheel combo to include improved suspension was based on GM/Pontiac engineers running a computer simulation as validation and inadequate real world testing on the street and at the track. The oddball tire and wheel configuration was for major traction issues and possibly to help alleviate the excessive torque steer.

[bocoogto]

All '05-'07 GXP V8 Grand Prix's used this different rim/tire configuration in front and rear. I'm not aware of any other Pontiac built in past years that did this. My previous comment was that it would have made sense to put larger tires on the rear, also. Normal tire rotation would be possible if that would have been done.

But, on the other hand, most of the exotic performance cars in the world today use different sized tires and rims in front and back. The '08 Corvette uses 18" rims with 245 width tires in front and 19" rims with 325 width tires in back on some models. Nearly all European performance cars and the Chrysler Viper use similar arrangements.

The comment above that Pontiac engineers used "computer simulation as validation and inadequate real world testing" to arrive at the combination seems presumptuous. No one but Pontiac engineers know how and why it was done. I would believe that cost savings played a part in that decision.

All front wheel drive cars with the amount of power the GXP V8 has exhibit large amounts of torque steer. Larger tires in front, in some cases, make that worse, not better. My father-in-law's '70 Olds Tornado with 455 powerplant was a prime example of this. We put larger tires on the car when the originals wore out and worsened the torque steer.

[BNaylor]

Originally Posted by bocoogto

But, on the other hand, most of the exotic performance cars in the world today use different sized tires and rims in front and back. The '08 Corvette uses 18" rims with 245 width tires in front and 19" rims with 325 width tires in back on some models. Nearly all European performance cars and the Chrysler Viper use similar arrangements.

The GXP is not an exotic car like mentioned above so you are comparing apples to oranges. No comparison with rear wheel drive.

Originally Posted by bocoogto

The comment above that Pontiac engineers used "computer simulation as validation and inadequate real world testing" to arrive at the combination seems presumptuous. No one but Pontiac engineers know how and why it was done. I would believe that cost savings played a part in that decision.

Obviously you did not do your homework before purchasing one. The comment is factual and the same deduction reached by Car & Driver, Road & Track and Edmunds. The decision was based on solely a "computer simulation" and had nothing to do with "cost savings". Piss ass poor engineering which is typical of GM lately.

Explain to me why there are so many on the used market. And don't tell me poor fuel economy. Regardless, it is going the way of the dodo bird.

It is your car and problem based on a poor purchase decision so suck it up and live with it.

Originally Posted by bocoogto

All front wheel drive cars with the amount of power the GXP V8 has exhibit large amounts of torque steer. Larger tires in front, in some cases, make that worse, not better.

I'll bet I have larger amounts of torque steer in my GTP than your GXP considering it puts out at least 350 whp. However, I compensate by using drag radials not wider tires plus have years of practice and experience with it.

[cissna99]

Guys you should not rotate these tires! At All! side to side back to front. at least on the 08

[BNaylor]

Originally Posted by cissna99

Guys you should not rotate these tires! At All! side to side back to front. at least on the 08

I agree but for all '05-'08 GXP models you should not rotate the tires for obvious reasons and the annotation is in the owner's manual.

Plus in case anyone never noticed it there is no spare emergency wheel/tire. Just the air pump in the trunk.

BTW - This is an old thread.