We 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.
In the accompanying image, you see examples of tires that are over-inflated, under-inflated and properly inflated. An over-inflated tire will show even, excess wear on the center portion of the tire. An under-inflated tire will show excessive edge wear, often mistaken for an alignment issue. A properly aligned and well-inflated tire will show even wear across the full road-contact surface.
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
CAMBER is an angle of the tires' contact with the road as viewed from the front or rear, resulting in an inward or outward lean of the tire. The TOP of the tire leaning in towards the vehicle is NEGATIVE camber, while an outward lean is POSITIVE camber. Camber can accelerate tire wear on one edge of the tread or the other due to the contact surface with the road.
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 non adjustable on solid-beam axles. If out of specifications, components should be inspected and replaced as needed. They may be worn or damaged.
TOE is the angle of each tire on the axle in relation to the other, compared to a straight-ahead position, as viewed from above. If the front or leading edge of the tires is closer than the rear edge, it is considered toe IN. If the front edge is farther apart than the rear, it is considered toe OUT. Keeping the tires in a straight-ahead position allows the tires to roll freely versus a SCRUB of the tread if they are pushed across the road surface.
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.
PERFORMING THE ALIGNMENT
The process of a wheel alignment measures each steering angle and allows the technician to make adjustments to bring each wheel into specification. Coaches should always be aligned on a three-axle machine. This verifies the angle of every axle based on a geometric centerline of the vehicle. These types of alignment machines verify all the axles at once and create an imaginary center line with the center of the vehicle, which helps correct issues like the one in this figure, known as a THRUST ANGLE alignment. If the thrust of the drive axle is trying to push the coach to the left, the driver has to constantly steer to the right to maintain a straight track. Someone watching the vehicle from the rear will see that it is driving at an angle instead of straight in the traffic lane.
In addition to performing them as part of a routine preventative maintenance program, alignments should be carried out for the following reasons:
Unusual tire wear
Replacement of any steering component (tie rod, drag link, king pin)
Any time major suspension work is completed on a coach (radius rod or bushing, ‘V’ link)
To schedule your alignment, please call Laing Truck & Trailer at 607-775-9776