Have a steering pull problem you can't straighten out? Or a wheel that just won't balance? Or a weird vibration that won't go away? Blame it on the tires!
The tires are often the scapegoats for various steering and ride complaints that seem to defy diagnosis. Sometimes the tires are to blame, sometimes not. Other times, the tires are the last thing that's considered when trying to solve a steering or NVH (noise, vibration, harshness) mystery. The fact is certain kinds of tire imperfections can cause steering and vibration problems. The trick is knowing how to identify and diagnose these conditions.
Most tire manufacturers today have a pretty good grip on quality control, and perform extensive tests to weed out defects that may occur in manufacturing. Tires are a hand-built product that requires skilled labor. Mistakes sometimes happen, though, and tires that are less than perfect sometimes slip through the screening process and end up on somebody's car or truck. So if you've just installed a new set of tires and created a steering pull or vibration problem that wasn't there before, it's logical to assume one or more tires have some kind of defect.
The more difficult situations to diagnose are those where an existing set of tires may be causing a problem. The underlying cause may be a manufacturing defect that took time to surface, or it could be the result of structural damage caused by a pothole or improper mounting.
Anytime you suspect a possible tire-related problem, therefore, the tires and wheels should be carefully inspected for obvious defects or signs of trouble. Start by looking for obvious tread or sidewall defects such as cuts, bulges, ripples, bruises, scrapes, missing chunks of tread, separations, cracks, etc. Uneven tread wear may indicate misalignment, under or overinflation or a tire construction defect.
Unfortunately, the kinds of tire defects that can cause a steering pull or vibration may not be visible to the naked eye because they're inside the tire or are difficult to see. This includes defects such as off-center belts, misplaced belt splices, damaged belts or cords, or beads that are in different planes as well as variations in sidewall stiffness (force variation).
When the belts are positioned as a tire is being built, they must be almost perfectly centered. According to some sources, if the belts are off center by more than one millimeter, it may be enough to cause a steering pull in some applications (some tires and/or vehicles are more sensitive to this kind of problem than others). The position of the outermost belt is the most important because it has the greatest effect on the directional stability of the tire.
A tire with an off-center belt will lead to one side because the location of the belt shifts more weight to one side of the tread than the other. This condition is known as a "conicity" problem, and occurs because the tire behaves as it if were cone-shaped. A cone always rolls in a circular path towards the pointed end. A conicity pull will therefore always be directional (either to the left or right), and can be reversed by switching the front tires from side to side, or by mounting the offending tire backwards on the rim.
Conicity problems can also arise when the beads are not in the same plane. The resulting difference in sidewall height and loading will create the same unbalanced steering forces that make the tire want to roll towards the shorter side.
Conicity problems are usually most noticeable on the front wheels, but may affect tracking and induce some rear axle steer on vehicles with independent rear suspensions. Vehicles with less caster also seem to be more sensitive to conicity probably because high caster angles have a stabilizing effect on the steering.
If you're trying to diagnose a steering pull and suspect a tire may have a conicity problem, rule out the other possibilities first. Check inflation pressure in both front tires and make sure it is equal on both sides. Low pressure can cause a pull towards the side with less pressure. Compare tire sizes and tread wear on both sides. Measure tread wear, too, because the steering will pull towards the side with the "shorter" tire (the one with the most tread wear). Also, check ride height and make sure it is within specs and is equal on both sides. The steering will usually pull towards the low side. Check for brake drag. A frozen or sticky caliper can cause a pull towards the side that's dragging.
Next, switch the left and right front tires to see what effect it has on the steering. If the steering still pulls in the same direction, the problem isn't the tires. Check for camber or caster misalignment, or the presence of a rear axle thrust angle. If the direction of the pull is now reversed, one or both front tires may have a conicity problem.
The next step would be to swap the front and rear wheels on one side (right or left) to see if it eliminates the pull. No change would tell you it's the other front tire that's causing the problem. Swapping the other front tire to the rear should eliminate the problem as long as the offending tire remains in the rear. But if somebody rotates the tires later on and the bad tire ends up back in the front again, the steering pull will return. Replacing the offending tire will get rid of the problem altogether, and may be necessary if the tire causes problems in the rear, too.
Another way to reduce the effects of a conicity problem is to increase the inflation pressure of the tire. But the tradeoff may be a harsher ride and increased center wear in the tread.
Sunday, September 21st, 2008 AT 12:37 AM