Q. The front tires on my car are wearing excessively on the outer edges. I’ve rotated and balanced them, but the wear has reappeared. I’ve been told by friends that it is the alignment, that I have bad struts, or that I take turns too aggressively. How can I tell which it is? How big a deal are each of these conditions?

Ann Fletcher,

Mountain View, Calif.

A. Ann, as you’ve heard from your friends, there can be several reasons for tires to wear irregularly.

Correct inflation pressure and wheel alignment are the two primary factors in determining proper tire wear. Before we look at them, let’s iron out some of the less likely causes.

It’s unlikely your driving habits are an issue unless your regular commute includes winding roads and you take them with vigor.

Tire balance – while important for operational smoothness and steering/suspension component longevity – isn’t likely to cause the wear pattern you’ve noticed. A severely unbalanced tire can hop across the road, causing a cupped or choppy wear pattern across the tire’s tread.

The same goes for struts, which is a shock absorber combined with a vertical suspension arm.

Inflation pressure is very important for proper tire wear. An underinflated tire will wear excessively on both the inner and outer edges, as well as generating destructive heat.

An overinflated tire wears excessively in the center of the tread, and can result in reduced traction. It’s important to inflate tires to the value that’s specified on the car’s door-post.

The last but not least factor to consider is wheel alignment. Of the several angles measured, camber and toe have the greatest effect on tire wear. Camber is the inward or outward lean of the top of the tire/wheel. A tire/wheel that leans outward at the top (positive camber) can cause greater wear to occur on the outer edge of the tire. The opposite occurs with negative camber – an inward lean.

Try this: With the car parked on a level surface and the front wheels pointing straight, stand 15 feet or so behind the car and carefully compare the outer edges of the front and rear tires (from top to bottom, crouching down as necessary). If more than the slightest difference is seen, either the front or rear camber might be incorrect.

Toe is the measurement of a tire/wheel’s direction, relative to its mate on the opposite side and the vehicle’s centerline. Even a slight misalignment can cause the tire to be scrubbed away – often most noticeable on one edge of the tread – as it rolls.

Try rubbing the palm of your hand inward and outward across the tread surface. If it feels rough in one direction and smooth in the other, a toe adjustment is called for.

My hunch is you’re due for a wheel alignment.

This procedure can pay for itself in reduced tire wear and better gas mileage. It can help even out minor wear irregularities.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-06-25-04 0619EDT