Rick's Free Auto Repair Advice

Replace 4 tires on AWD

Do you really have to replace 4 tires on an AWD vehicle?

Well, do you want to destroy your $3,000 drivetrain?

You have two choices when you damage a tire beyond repair on an AWD or 4WD vehicle. You can replace 4 tires or you can buy a brand new tire and have it shaved down so the tread depth falls within the allowance rolling radius difference set by the car maker. Get it? Two and only two choices. If you buy just one tire and add it to your AWD with worn tires, the unequal circumference difference will cause the AWD differential systems to work overtime and overheat and fail. That’ll cost you a bundle!

Why does one new tire damage the drivetrain?

If the axle between a left and right tire were solid, the axle would break during a turn because the outer wheel rotates more than the inner wheel. Older cars with rear wheel drive incorporate a differential mechanism that allows one wheel to rotate more than the wheel on the opposite end of the axle. When car makers started building front wheel drive cars, they often incorporated the differential mechanism right into the transmission.

What’s so special about an AWD and 4WD vehicle? Well, not only do the outer tires rotate at a different rate than the inner tires on a turn, but the front tires rotate at a different rate than the rear tires. If the front and rear differentials were linked by a solid shaft, it would break because of the difference in rotation between front and rear tires. To avoid that, car makers instead a center differential between the front and rear differentials.

What happens if you install one new tire on an AWD or 4WD vehicle?

You cause the front, rear and center differentials to compensate for the different rolling radius FULL TIME. Differentials were never designed for full time operation. They were designed to compensate for different rotation rates only during turns. So installing a brand new tire on these vehicles can destroy those very expensive components.

Different rotations from two tires

Let’s compare two 225/45R17 size tires. A typical new tire tread depth is 10/42.” You drive your AWD vehicle, accumulate some miles and the tread wears down to 8/32. Then you get a flat tire than can’t be repaired.

If you buy one new matching tire, you’ll have a rotation difference. In this case the new tire’s diameter is 24.97,” while the worn tires measure only 24.84.” The new tire will have a circumference of 78.44,” while the worn tires are only 78.04. Doesn’t sound like much difference, does it? But as you drive the new tire will rotate 835 per mile while the worn tires will rotate 839 times per mile. It’s those extra revolutions every mile, every day that can add up to drivetrain destruction.

Car makers know that all four tires don’t wear at the same rate, so they build some tolerances into their mechanisms. How much? Well, that depends on the car maker.

What maximum allowable difference in tread depths?

It varies between car makers and it’s sometimes listed in your owner’s manual, but here are a few examples:

Audi states: “The rolling radius of all four tires must remain the same” or within 4/32-inch (1/8″) of each other in remaining tread depth.

Subaru states that tires must be within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.
Tire rotation is just as critical on an AWD vehicle

If you own a vehicle with all-wheel drive it’s imperative that you religiously follow the manufacturer’s tire rotation schedule. Tire Rotation All Wheel Drive vehicles is critical. The whole reason behind tire rotation is to distribute the tire wear evenly to all the tires. That’s always important, but especially in an all-wheel drive or 4-wheel drive vehicle. A difference in tire circumference from one tire to the next can fry the all-wheel drive components in your car, truck or mini-van. In fact, Chrysler has informed its dealers that tire rotation and tire circumference is critical to the proper operation of the Power Transfer Units on all 1996-2001 Town & Country, Caravan, and Voyager vehicles. They are experiencing a much higher than normal failure rate because of this problem. The uneven rotation is actually toasting the guts of the all-wheel drive unit. You must use identical BRAND tires with equal amounts of wear for all 4 tires. Tires of different brands, even brand new and of the same listed size, may differ in outer circumference. So DO NOT MIX used and new tires or tires of a DIFFERENT brand.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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