Rolf Lockwood

Excessive tire wear got you down? Your trailer tracking like a dog? Sounds like you’ve got an alignment problem – or two. Even if you’ve just had your tractor aligned, if the trailer you pull doesn’t have properly aligned axles
you’ll probably get irregular wear up front. And even if the trailer axles have been aligned, you can still see dog-tracking if your tractor’s out of whack.

Fact is, a properly aligned and maintained vehicle – total vehicle – can extend tire mileage significantly compared with a vehicle whose alignment and maintenance are sloppy. Better yet, proper alignment also makes vehicle handling predictable and driver control easier. It also 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 – a bad one – 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 excessive toe-in, 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 usually very apparent since the trailer appears to be traveling at an angle to the tractor. You’ll feel the vehicle wandering and you’ll have to 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
straight-ahead miles in severe conditions.

‘Tandem skew’ or ‘scrub angle’ occurs when tandem axles aren’t parallel to one another or perpendicular to the chassis centreline.

Drive axles will fight each other and you at the wheel. Trailer axles with tandem skew will be dragged in different directions. This condition causes excessive and rapid tread wear on all tires. 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 this sort of wear, but they are affected.

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:

* Uneven axle spacing or the lack of parallelism between axles.
* Broken spring leaf.
* Worn springs.
* Loose ‘U’ bolts.
* Bent frame.
* Improperly adjusted torque or locating rods.
* Excessively worn torque or locating rod bushings.

Besides having these components checked, ensure that fifth wheels are well greased so that they don’t bind up. Also check pintle hooks for wear regularly. This should all be part of a regularly scheduled preventive-maintenance regime.

Rolf Lockwood

Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to

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