# Stop and swap

Tire rotation requires only minor, infrequent downtime, but pays big dividends.

During the 15-plus years Jim Green has been visiting fleets as a Yokohama Tire field engineer, he’s learned the biggest problem with tire rotation: “Basically no one does it.” Greg James of Bridgestone Firestone agrees. “Years ago we tried to recommend rotation to maximize rotation to maximize mileage, but no one would ever actually do it,” says James, engineering manager for commercial products. “It is difficult to verify the cost benefits.”

It’s difficult only in the sense of being tedious. If you take time to measure tread depth, observe wear and schedule rotations, there’s plenty of money to be saved.

The art of rotation starts with legal limits. Federal Motor Carrier Safety Regulations require truck owners to remove steer tires when the tread is down to 4/32 of an inch, and 2/32nds for the drive tires.

The logical rotation pattern for steer tires is from side-to-side, say Dan Guiney, a Yokohama Tire service manager, and others. The left front steer tends to wear down quicker because of the crown of a road, Green says. So by the time crowning grinds one tire down to 4/32-inch, the other still might have 7/32 inch.

“When a trucker purchases a new pair of steers, he very well may be tossing out a tire with a bit of life in it,” Green says.

Suppose a new tire costs \$300 and the casing is sold for \$50, for a net cost of \$250, Green says. If that tire starts with 18/32nds of tread, it has 14/32nds of usable tread life. Divide \$250 by 14 to get the cost for each 32nd of tread: \$17.86. Multiply that number by 3 – the 3/32-inch difference over the life of the tires if not rotated – to get the cost of wasted tread.

“The removal of a right steer tire with 7/32nds of remaining tread could be considered wasting \$53.58 worth of usable tread life,” Green says.

While the steer tires require only a side-to-side switch, patterns vary for rotating drive and trailer tires. Rotating these tires is important not only for maximizing tread but also for minimizing realignment and other maintenance problems.

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James says there are two potential benefits of drive tire rotation: “One is to erase any uneven wear in a high-mileage operation and the second would be to equalize the rate of wear between the front and rear drive axles.” Tires on the rear drive axle wear down faster than the forward drive axle because the torque is not distributed evenly between the two axles, he says.

Green recommends watching the rear drives more closely for tread wear. If an owner notices a difference, a front-to-back rotation could prevent uneven wear, he says.

The maximum recommended tread depth difference between matched dual tires is a quarter inch (8/32), Green says, citing the Rubber Manufacturers Association’s Care and Service of Truck and Light Truck Tires manual. That avoids excessive irregular wear and damage to the tandem drive axles.

“A variation of 4/32 of an inch in tread depth at the top and bottom of the tire – that little bit of difference may result in damage to your differential,” Green says.

X-type rotation patterns are common for drive and trailer tires. For drive tires, such patterns alleviate differential damage and reduce heel-and-toe wear patterns and uneven wear that occurs when the front tires have greater tread depth than the rear tires. Heel-and-toe wear occurs when a set of dual tires are mismatched or not inflated uniformly. While one tire has flat, constant road contact, the other tire “scuffs” along. This causes the lugs to wear unevenly, with more force being put on the lug’s toe, the portion last leaving the road, than on the heel, which hits first.

Green says X rotations allow the owner to stem problems in “all eight tires at one time, which will eliminate the possibility of having the difference between the duals exceed 1/4- inch.”

Doug Jones, an engineering support manager for Michelin, says X-type rotations aren’t as good as rotating between different types of axles.

“When possible, tires should be rotated to another axle on the vehicle, run in the opposite direction of rotation, and exchanged ‘inner-outer’ if in dual mounting,” Jones says. This would mean, for example, rotating a drive tire to a trailer or steer axle position.

However, “Directional tires and wheels should maintain the proper direction of rotation. When mounting any directional tire, ensure that the directional arrow points toward the direction of travel during the original tread life.” Directional tires are commonly marked with arrows dictating which side of the vehicle they should be mounted on.

Heel and toe wear can be repaired only by switching the direction of rotation, says Paul Blackiston, a senior development engineer with Goodyear Tire & Rubber Co.

“You could switch sides, or move the rear tires to the opposite front positions,” says Blackiston. “You could also take the rear axle tires and put them on the front axle.”

Owner-operators who own a trailer often use a system of rotating an entire set of drive tires, after a certain amount of wear, to the trailer.

“Wear a set [of drive tires] down to 11/32nds, then move them to a trailer position,” James says. “They could be run for a couple of years more, since most trailer tires start at 11/32nds.” You can follow rotation patterns for trailer tires similar to those for drive tires.

Tire rotation evens out tread depth and prolongs the life of the tire. If it’s practiced on a routine basis, the time and labor invested is more than made up for by savings in tire costs.

FREQUENCY OF ROTATION
Tires should be rotated only once, halfway through their life, recommends Eugene Johnston, a Bandag business development manager. Regular measurements of tread wear can indicate when a tire is halfway through its life.

More frequent rotation would be called for if you see irregular wear, which can result from an alignment or inflation problem, or from mismatched tires, Johnston says.

Paul Blackiston, a Goodyear senior development engineer, says that dual tandem axle tires should be rotated around 125,000 miles, but that estimate could vary depending on the pattern of wear.

Rotate steers a couple of times during the life of the tires to make up for damage and uneven wear cased by the vehicle’s geometry and crowned roads, says Greg James of Bridgestone Firestone.

“Normally, the left steer tire wears down as much as 25 percent faster than the right,” Blackiston says, “so it would be advantageous to flip from side to side around 50,000 miles.”

MONITORING TREAD WEAR RATE FOR MAXIMUM PERFORMANCE
For those who want to be a little more scientific in their rotation schedule, regular measurements of tread wear rate can be a handy guide, says Eugene Johnston of Bandag.

That rate can be “highly variable, depending on road surface, loads carried and the type of tire,” he says.

Tread measurements made regularly also allow you to calculate cost per 32nd inch. Suppose a tire costs \$300 and gives you 14/32nds of usable tread. Divide 300 by 14 to get a little more than \$21 per 32nd inch.

Over time, if you change tire models and keep similar records, you can compare which tires give you the slowest wear rate, and the lowest cost per 32nd inch. Even if the cost is a little more, a tire that lasts longer might well be the best buy.
–Lance Orr

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