MCI often receives calls from operators asking why their coach tires are wearing oddly or faster than expected. The reasons are generally as varied as the operators themselves. For example, a line-haul or charter operation will generally see different types of tire wear than a transit operation; tires on over-the-road coaches run hotter due to speed and the types of road surface, while a transit coach will see more scuffing and sidewall issues due to more frequent turning. If you pay close attention, the tires will tell you a lot about your coach's alignment.
Understanding that tires will wear only where the tire contacts the road surface will help determine the root cause of abnormal wear. Improper inflation wear is perhaps the most misdiagnosed reason for alignments, as well as being the easiest to maintain and correct.
Another wear pattern often misdiagnosed as an alignment issue is cupping, or dips in the tire-tread pattern. This is generally seen as an irregular pattern caused from rebound wear when the tire is leaving and re-contacting the road surface. Balance, worn shock absorbers or loose suspension components are generally the cause.
Nevertheless, wear will often point to an alignment issue. Alignment wear issues arise when the tires are not in straight-ahead full contact with the road surface. The most common wheel angles associated with wheel alignment are CASTER, CAMBER and TOE IN, each with their own concerns.
CASTER is an angle of the steering pivot points as viewed from the side of the wheel. The forward or rearward tilt of the kingpin designates the caster angle. Caster is generally not considered to be a tire-wearing angle but can cause stability issues or cause the vehicle to pull to one side, requiring constant correction from the driver. As an example, the front wheels on a shopping cart have negative caster, causing them to follow the direction of travel.
On MCI coaches, caster is not adjustable. If the caster is out of specifications, this represents worn or damaged steering or front-axle mounting components
A small amount of positive camber is generally preferred, as it places the majority of the rolling weight on the larger inner wheel bearing
Like caster, camber is nonadjustable on solid-beam axles. If out of specifications, components should be inspected and replaced as needed. They may be worn or damaged.
Excessive toe-in or toe-out conditions result in a "feathered" or sharp edge on the tire treads as the rubber is scrubbed from one point to another. Toe is adjusted by turning an adjusting sleeve attached to the tie rod(s) on the steering knuckle. During travel, the tires will have a tendency to toe outward, and a small amount of toe in allows for the tires to travel straight ahead. At ⅛" of toe on a set of steer tires, the tread-to-road contact can scrub up to eight feet in a mile of travel
Want more information? MCI Service Bulletin 2782 contains MCI-approved procedures for equipment setup and usage. You can also visit your nearest MCI Service Center or call MCI Technical Support at 800-241-2947.
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