Tire Wear? Check the Axles

Q: We’re seeing excess tire wear on one of our trailers, and our drivers are complaining about the vehicle dog-tracking. We’ve checked the wheel alignment on the trailer, but we’re not seeing immediate results. Any suggestions?

A: Sounds like you’re on the right track in addressing the alignment. I’d make sure your shop is doing a thorough inspection of the vehicle.

A properly aligned and maintained vehicle can extend tire mileage significantly compared with a vehicle whose alignment and maintenance are sloppy. Proper alignment also makes vehicle handling predictable and driver control easier, and improves fuel economy.

There are four significant components of vehicle alignment: toe, camber, caster, and axle parallelism. Of the four, axle parallelism is the cause of most alignment problems.

On a properly aligned vehicle, all the axles should be perpendicular to the frame and parallel to each other, and all the wheels should track the front wheels. Tandem drive axles that are not parallel or axles that are not perpendicular to the chassis centreline have a definite effect on steer tire wear.

Rear tandem axles that are not perpendicular to the frame but are parallel to each other create a “thrust angle” that tends to push the vehicle off course. The driver feels the vehicle pulling in the direction the drive axles are angled and must steer in the opposite direction. Steer-axle tires are constantly subjected to “scrubbing” or side forces as they correct the direction of travel, which results in fast and irregular wear.

One indication of tandem thrust is having both steer tires feather in the same direction as opposed to toe, which causes feathering in opposite directions. Irregular wear will also be apparent on the drive tires.

An out-of-parallel condition on trailers, known as “dog tracking,” is apparent as you follow the vehicle down the highway, since the trailer appears to be traveling at an angle to the tractor. The driver feels the vehicle “wandering” and must make constant steering corrections to keep the vehicle straight. Tires on a trailer with this condition are dragged sideways a few feet for every mile of operation.

This can add up to thousands of miles of sideways drag over 100,000 miles in severe conditions. “Tandem skew” or “scrub angle” occurs when tandem axles are not parallel to one another or perpendicular to the chassis centreline.

Drive axles will fight each other and the driver. Trailer axles with tandem skew will be dragged in different directions. This condition causes excessive and rapid tread wear on all tires of a vehicle. The steer tire on the same side of the truck on which the drive tires are closest together will wear into an out-of-round condition.

Drive axle misalignment will create irregular wear on drive tires as well as steer tires, and is the most common cause of alignment-related irregular wear.

Trailer axle misalignment will produce irregular wear on trailer, drive and steer tires and is the second leading cause of alignment-related irregular wear. Steer axles are least susceptible to alignment related irregular wear.

Tandem axles should be parallel to each other within 1/8 inch and all axles should be perpendicular to the chassis centreline within 1/8 inch.

Axle parallelism, otherwise known as tracking, is one of the most common causes of fast tire wear and also one of the easiest to correct.

Failure of the wheels to track is usually due to the following:

o Uneven axle spacing or the lack of parallelism between axles.

o Broken spring leaf.

o Worn springs.

o Loose “U” bolts.

o Bent frame.

o Improperly adjusted torque or locating rods.

o Excessively worn torque or locating rod bushings.

Besides checking these components, ensure that fifth wheels are well-greased so they do not bind up. Also check pintle hooks for wear regularly. This should all be part of a regularly scheduled PM.