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Rotational Education

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Unraveling the mysteries of tire rotation takes a bit of doing. Just about all of us

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are nonchalant about rotating tires. We advertise free rotations for life if you buy tires from us. We unbolt ‘em, rotate ’em and rebolt ’em. From a marketing point-of-view, that’s a good idea.

Just be certain you don’t sell yourself short. There’s more to know about tire rotation than you may have considered. Consider wheel offset. Back in the late 1980s and early 1990s, F-body Camaros came with different offset wheels front and rear.

The difference was about 7mm or 1/4-inch. A 1990 IROC-Z used the same size wheels (16×7) and same tire size front and rear (255/40ZR17, the plus one option), with the same outside diameter. The wheels shared a common bolt pattern, but the offsets were different.

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Why? GM replaced its rear drum brakes in favor of disc brakes. To accommodate the larger brakes, a wider offset wheel was selected. Only the well-schooled knew the difference, but when these tires were rotated with their original wheels, the difference could be felt by discerning drivers.

Dynamically speaking, the customer came in with a vehicle that handled one way and left after a simple tire rotation with a vehicle that handled another way.

Sizes and Directions Matter

Different sized tires front and rear are another issue. Many exotics require special care. Porsche 911s and Boxsters, Lotus Esprits, some Mercedes-Benz models and the Corvette, are among those with different size front and rear tires and wheels. This eliminates cross rotation and same-side front to rear rotation. In some cases, rotation isn’t an option, so if there’s odd wear, only a new tire will do.

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Directional or unidirectional tires have been around for a while, so most of us are accustomed to these tires. Still, great care must be taken when rotating a directional tire.

These tires and their tread designs have been engineered to enhance straight-line acceleration by reducing rolling resistance. They are also said to provide shorter stopping distances. Mount them backwards and these advantages are lost. Look first for an arrow on the sidewall indicating the tire’s intended direction of travel. It must always point the same way or you’ve just made a mistake.

When a directional tire is mounted with the arrow pointed backwards, noise levels can increase, a change in handling characteristics can occur, hydroplaning performance can drop, and the potential for lower treadwear climbs. While these symptoms won’t be noticeable at first, problems later on can lead to customer dissatisfaction.

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Without demounting, directional tires can only be rotated one way: Front to rear on the same side of the vehicle. That’s it.

Perfect Symmetry

What about asymmetrical tires with the tread pattern changes across the face of the tire? Inside tread blocks are small and compliant to deliver traction and dissipate water. Outside elements are larger to provide stability during cornering. These areas may even feature differing tread compounds.

While these tires are distinctive in appearance, a busy day in the shop could result in a relatively easy mistake. When rotating asymmetric tires you must take a close look at the sidewall. You are looking for this phrase: "Mount this side in."

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Since these tires are not directional, they can be cross rotated in the traditional "X" pattern. Just make sure you follow the sidewall instructions.

Asymmetrical tires mounted incorrectly result in a marked reduction in steering response and handling firmness.

‘Forcing’ Rotations

There is really one reason for rotating tires: Front and rear tires wear differently. Why? Different types of suspension systems, improper alignment, weight distribution and drive wheels (i.e. FWD, RWD, and 4WD).

Tires will assume the wear pattern the suspension system causes them to have. It’s like water conforming to the shape of a container.

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Remember that every suspension system has a unique camber/toe change, geometry designed specifically for that particular vehicle. Some suspensions are designed with an emphasis on handling, others on ride.

Generally speaking, when the front suspension system is a MacPherson strut type and the rear features a short-long arm system, the rear tires will wear more in the center, the fronts more on the shoulder. While this is not absolute, it occurs on more than 90% of the tires you’ll encounter.

We have talked about forces before, but it’s worth a review. Tractive forces allow the vehicle to move forward. Braking forces cause the vehicle to slow down, and cornering forces allow the vehicle to change direction.

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Each force acts in a different direction pulling tread blocks in that direction. This results in treadwear. The type of treadwear is dependent on the percentage of time the tires are generating each force and the amount of force.

The rear tires of a FWD vehicle don’t generate accelerating forces and do very little cornering. For this reason, and because the majority of the weight is at the front, it’s not uncommon for rear tires to outlast three sets of front tires.

Quite the reverse is true for front tires on a FWD vehicle. They must generate all of the accelerating, braking and cornering forces. The wear tends to be greater and takes place much more rapidly than the rears. Experts say tires on a FWD vehicle should be rotated at least every 6,000 to 8,000 miles.

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On front-heavy RWD vehicles, the rear tires generate high tractive and moderate corning and braking forces. Up front, the tires generate no tractive forces, but do experience heavy cornering and braking forces.

This sharing of duties explains why tires on RWD vehicles tend to experience even wear front and rear. Wear patterns are more dependent on suspension type and weight distribution than dynamic forces. Again, the advice is to rotate at least every 6,000 miles.

Full-time 4WD vehicles tend to wear tires more evenly. That’s because all four tires are being asked to generate all of the forces. However, many 4WD vehicles split their time between two- and four-wheel-drive.

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In such cases, wear patterns and rates are dependent on the amount of time the vehicle is in each drive mode. In some cases, rotating tires at 4,000 to 5,000 miles provides better results.

Keep in mind that an improperly aligned vehicle driven for too long will result in excessive shoulder wear (camber) and tread element feathering (toe). When motorists don’t check inflation pressures and operate their misaligned vehicles too long, it’s way too late for tire rotation to save the day.

The only way for drivers to get maximum tire life and the best possible ride is to inspect tires regularly and rotate them at least every 6,000 miles.

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Help your customers by looking for these common culprits. Correct any misalignment and inflation problems immediately. A little preventative action will pay off for you and your customers.

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