tire cupping

[GMR911]

Hi, noticed while I was driving at about 70 mph that I was getting a slight shimmy in my steering wheel. I looked at my tires and saw that both tires on the inside were cupping. The car was aligned about 6000 miles ago and the tires have about 10000 miles on them. What are the reasons for this cupping, Thanks Jerry

[mkossler]

The cupping is happening because you have one of a few conditions. Either the toe-in is out of spec, or the front-to-rear alignment (e.g. net thrust angle) is. Balanced cupping of the inside tread indicates a (rare) toe-out condition. Cupping on the exterior tread indicates excessive toe-in. Cupping on the same side for both fronts indicates a four-wheel out-of-spec condition such as thrust angle.

Please note that very mild cupping is a normal condition present in all-season radial tires, particularly new ones, caused by flexing of the tread blocks longitudinally along the direction of roll. This wear-in actually serves to increase the life of the tire, as the tread block leading edges are "protected" by tread block directly proceeding.

In any case, the cupping occurs as you point the car in the intended direction, and the wheels constantly correct for the fact that they are not positioned (relative to each other) to go in that direction.

FWIW, total mileage has a tenuous connection to alignment condition. The style of driving, state of your suspension components, and state of your roads has the greatest impact. Less than ideal conditions in any of the three generally mean that an alignment is advisable every year. Any event, such as a pothole hit, curb hit, spinout, or replacement of any suspension components, make a new alignment mandatory.

Lastly, the vibration at 70 MPH is more likely an out-balance condition that has become noticeable due to tire wear. A high-speed dynamic balance will address that issue.

Good luck -

Matty K.

[mkossler]

I want to be clear that your condition may very well be due to worn suspension components. Worn ball joints, for instance will create a condition where the alignment is a generality at best. Always have your suspension checked when having an alignment performed, especially when exhibiting symptoms such as yours.

Cheers -

Matty K.

[GMR911]

Matty, thanks for the response- very informative. The tires are Pirelli P zero nero all seasons. The front susp was rebuilt completely 6000 miles ago- new turbo tie rods, ball joints, a-arms, sway bar bushings, shocks and bearings. For the last 2 sets of tires I have run all-seasons, but neither of those 2 sets wore like these. I plan to take it to the alignment shop tomorrow to get their opinion. Jerry