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Treadwear Tales

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Tires always tell a story. The danger is misreading the story and coming to the wrong conclusion about what might be causing the tread to wear prematurely or unevenly.

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All tires wear as miles accumulate, and all treads eventually wear down to the point where replacement becomes necessary to maintain safe driving, traction and handling. Bald tires with little tread left provide marginal grip on rough surfaces, and can easily hydroplane, skid or spin on damp or wet roads.

If the cords are showing, there’s a serious risk the tire may fail due to a blowout. And we all know what that can do to a vehicle’s stability and handling.

In basic terms, bald tires are unsafe tires that have reached the end of their road. Replacement is required when the wear bar indicators in the tread are showing. The eight horizontal bars, which are evenly spaced every 45Þ around the tire’s circumference, will be flush with the tread when tread depth is down to 1/16th-inch.

So how long should a set of tires last? That’s a question for which there is no simple answer because of all the variables that affect tire wear. For many premium tires today, the wear bars may not show until the tire has rolled up 60,000, 70,000, 80,000 or more miles. But for others, the tread may be worn out at a much lower mileage.

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The longevity of the tread depends on the wear resistance of the rubber compound, the design of the tread and the stiffness of the belts. The higher the UTQG treadwear index number on the side of the tire, the longer the theoretical tread life relative to other tires. Theoretically.

But treadwear also depends on tire balance, maintenance (rotating every 5,000 miles or so, and proper inflation pressure), wheel alignment, road conditions and driving style.

No tire is going to realize its maximum mileage potential if it is out of balance, never rotated from one wheel position to another to equalize wear, is chronically underinflated or overinflated, or is out of alignment.

As the vehicle accumulates miles, play in the steering and suspension will gradually increase to the point where alignment will be adversely affected – and that includes rear alignment and the front on most FWD cars and minivans, as well as RWD vehicles with independent rear suspensions.

The kind of roads the tires are driven on and everyday road hazards can also shorten the life of any tire. So, too, can an aggressive driving style that is constantly spinning the tires or pushing them to the limits of adhesion when cornering, braking and accelerating. Hard driving will burn off rubber faster than a Congressman can spend your tax dollars on a pork barrel project.

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When inspecting a set of tires, the first thing to look for is obvious defects or tire damage: cuts, bulges, ripples, bruises, scrapes, missing chunks of tread, separations, cracks, etc. A damaged tire is unsafe and must be replaced.

Some defects, though, can’t be seen with a simple visual inspection because they are inside the tire. This includes damage and defects that can cause a steering pull or vibration such as damaged belts or cords, off-center belts, misplaced belt splices, or nonconcentric beads. These types of problems might be indicated if the driver has complained of a pull or vibration.

Next, check all four tires for even treadwear. On FWD cars and minivans, the front tires will usually show more wear than the rear tires if the tires have not been rotated.

If both front tires show heavy shoulder wear on either the inside or outside edges, the vehicle likely has a toe problem (front or rear). Radial tires, especially on lighter vehicles, tend to develop shoulder wear when toe is off. They may also have some feather wear, but this is more typical for bias ply tires.

If the front tires are toed out, the shoulder wear will be on the inside edges. If toed in, you’ll see more wear on the outside shoulders. Underlying causes include worn tie rod ends or a bent steering arm.

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If the front tires show toe wear but front alignment is within specifications, the vehicle probably has some misalignment at the back wheels. Any time the front wheels are steered off-center, it caused the wheels to toe out slightly which can accelerate toe wear.

If only one tire shows heavy shoulder wear, the vehicle may have a camber problem. Underlying causes may include a sagging spring (measure ride height on both sides), a bent strut or spindle, a mislocated strut tower or engine cradle, a collapsed control arm bushing, or a bent control arm.

If both front tires show shoulder wear, caster can sometimes be a factor, too. Though caster doesn’t affect tire wear directly, it does make the wheels lean or tilt when turning. This changes camber (“camber roll”), and if it changes a lot it may accelerate shoulder wear. This type of condition is common on vehicles like Mercedes and BMW that have high caster settings.

The rear tires on FWD cars often develop weird wear patterns when the tires are not rotated. Rear toe misalignment is the most common cause. The scrubbing action that results can produce a heel-and-toe or sawtooth wear pattern on the rear tires. We’ve also seen rear tires on FWD cars that have developed a horizontal wear pattern in several places because of toe misalignment.

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Rear toe wear may be barely perceptible to the naked eye, but is often rough enough to produce an annoying vibration at medium to high speeds that feels like a bad wheel bearing. Rotating the tires once a wear pattern has developed is usually a waste of time because it may take too long (if ever) to counter the abnormal wear. Better to replace the tires.

If the tread is worn more in the middle than the edges, the tire may be overinflated or mounted on a rim that is too narrow for the tire. If both shoulders of the tread are worn more than the middle, the tire may be underinflated, or showing normal wear as a result of hard cornering.

Irregular wear that only occurs along one strip of a tire may be the result of a manufacturing defect.
Scalloped wear (dips or cups in the tread) would tell you the tire is bouncing as it rolls along because it is out-of-balance, out-of-round or the shocks are weak. 

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