Tracking Down the Cause of Tire Wear

Question: The tires on my 1990 Mercury Sable are wearing out badly on the shoulder.

I’m told it is due to under-inflation. I noticed at the tire shop that it seems to be a common problem on the castoff pile. The sticker on my door frame says to inflate the tire to 35 pounds per square inch.

The tire man and my service mechanic steadfastly refuse to do that. They point to the printed statement on the tire that says the maximum inflation is 35 pounds.

Judging from the stacks of different sizes and brands of castoff tires, it looks like this is a widespread problem.

Who is right?

--R. W. M.

Answer: The problem is almost certainly not with the inflation level. The 35-pound limit should be used only when carrying heavy loads or in connection with long road trips in hot weather.

An under-inflated tire tends to wear more along the shoulders and less on the crown or center line, and an over-inflated tire tends to go bald first on its crown. But in either case, the inflation must be grossly wrong to cause major wear problems.

Your problem is most likely caused by what tire designers call steering axle wear, in which a tire suffers accentuated wear on one of its shoulders. It frequently ruins tires with plenty of usable tread on the crown.

Steering axle wear results from scuffing of the tire in low-speed driving and when turning the wheel while trying to move out of a parking space. When a car is stopped and you turn the wheel, the tire scuffs along the pavement because it is not rotating.

Because the outward edge of the tire moves the most and the center moves very little, the shoulders scuff the most. The problem is worse on heavy cars.

Low-speed driving tends to accentuate this wear pattern. On a left turn, for example, the right front tire bears greater force and weight than any other. The force of the turn causes the tire to lean outward, and that puts additional wear on the outer edge of the tire.

Tire engineers say drivers generally tend to turn harder and faster in left-hand turns than in right-hand turns. As a result, the right front tire’s outer edge wears out first.

Also, you should make sure that the car’s front end is properly aligned. If you have an improperly adjusted front-end camber, which is the angle at which the tire meets the road, you certainly end up with premature shoulder wear.

As front-end designs have improved in recent years, cars tend to hold their alignment much better than in the past. Nonetheless, if your tires are wearing unevenly, you need to check the alignment as well as the front struts.

If you do significant highway or freeway driving, the wear will even out. But city drivers have a tough time with uneven tire wear. If you suspect that your shoulders are wearing prematurely, it would help a great deal to rotate the tires more frequently, putting the front tires on the rear axle.

Ford recommends tire rotation on its Mercury Sable every 15,000 miles, starting after 6,000 miles. If you have followed this schedule and still have problems, then consider rotating the tires every 10,000 miles and see if this helps.