Some brands of tires are noted for broken belts so having two would be common and rotating them might not help. Run the car in gear on a hoist and spin the other tires by hand, and watch the tread and wheels. If you see one front wheel with lateral runout, suspect debris between the wheel and rotor or between the rotor and hub. If you see TWO wheels with lateral runout, one on front and one on back, bent wheels is more likely the cause.
Besides the obvious broken tire belts that most alignment specialist know to look for, you must also look at the inner grooves of the tread, not just the outer part that contacts the road surface. If you see that inner part of one or two of the grooves rise in one spot as you spin the tire, the belt broke slowly over a longer period of time and the outer part of the tread that everyone looks at wore smooth and flat from driving on it. That can be a tricky one to find. Another way to approach it is to rotate just the two tires on the left side, then the two on the right, with a test drive in between. That way, if there is a bad tire on the front and the rear, at some point they will both end up on the rear and you won't feel it in the steering wheel.
If you see lateral runout in the wheels when spinning the front ones by hand, recheck them on a wheel balancer. If you see it there too, look for debris on the mounting surface where it contacts the rotor. This is more common with cast wheels and the vibration started right after the tires were rotated. It's a good idea to check along the lip where the wheel weights go with a dial indicator. There was a rash of truck wheels supplied to Dodge in the mid '90s that caused a vibration due to.045" or more of lateral runout. That was too little to be seen without a dial indicator. That runout had to be measured in four places after removing the tire. To the eye those wheels looked fine, but replacing a few took care of the customers' complaints.
If nothing shows up with the wheels or tires, use the dial indicator on the front rotors, both on the center where the wheel bolts to them and on the braking surfaces. Any small amount of runout will tug back and forth on the caliper which will tug on the steering linkage. That will often not be felt in the brake pedal, just in the steering wheel.
Friday, June 24th, 2011 AT 5:57 PM