When the rub wears off the tire, where does it go?

By Christopher W. Ferrone

I recall a great episode from Seinfeld where Jerry muses over the wear of the tires on his BMW, wondering, “Where does the rubber go that wears off the tire?”

This is truly an enigma. Who sees tire material from wear on the roadway? On the other hand, attend any Indy Car race and the small balls of tire rubber collecting on the track are very noticeable. As tires wear the material does in fact deposit onto the roadway, but it goes undetected because it happens so gradually.

It’s easy to take tires for granted. But, like Jerry, it’s good to stop and remind ourselves of all that goes on with a set of tires.

The mechanical function of the tire casing is to contain the air pressure that supports the vehicle load, assure the function of the suspension by absorbing road irregularities, resist lateral drifting and centrifugal force, and transmit the torque necessary to move and stop the vehicle.

A number of factors affect the condition of a tire: the nature of its application for line haul, stop and go traffic or mix of both; the number of axles on the vehicle, overall maintenance of the tire during its life cycle and especially inflation pressure.

Suspension and alignment

The major suspension and alignment adjustments control tire wear and condition. Technicians must monitor toe-in and toe-out, caster angle and camber angle. However, not all vehicles have a front axle that allows the control of these adjustments. Therefore it is of vital importance to know the type of front axle on all vehicles and the allowable adjustments.

The most controlling factor in tire wear may be its inflation pressure. For every 10 psi a tire is under-inflated, the life cycle of the tire is reduced by 10 percent. The amount of air in the tires affects the weight distribution between the wheels. An under-inflated tire does not carry its share of the load. This condition affects torque, traction, steering, alignment, braking and cornering, and may cause pulling from side to side.

A tread separation usually means the tread has separated from the casing, disconnecting from the outermost steel belt. The outermost steel belt has become detached from the lower steel belt and the rest of the casing. Tread separation can be the entire tread or simply a section of the tread. This usually occurs later in the life of the tire casings unless a traumatic event has occurred.

A hole or some sort of damage that starts in the sidewall is normally the cause of a sidewall failure. Curbing the tire, striking a sharp object, age cracking and other structural issues all contribute to sidewall failures.

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It is my belief that operators should never use retread or recapped tires on a motorcoach for any reason. When they experience failure they can damage the vehicle with the delaminated tread that is rotating with the tire casing, just as a circular saw would cut air hoses and suspension bellows during this type of failure.

The four to six year mark

Typically the life cycle for a tire is four to six years, which corresponds to the average useful life of a tire casing. Despite the condition of the tread, tires should be taken out of service when they reach the four to six year mark. The DOT number embossed on the side of the casing notes the age of the tire. The last three numbers indicate the week and year of manufacture.

Technical staff must understand what affects tire condition and respond proactively. They must routinely monitor wheel alignment, inflation pressure and suspension adjustments in order to maintain proper tire performance. Additionally, if the vehicle has a steerable tag axle, technicians must also monitor those components to ensure the expected life cycle for the tag axle tires.
Bus and coach operators should always refer to CFR 49, Appendix G for tire out-of-service regulations.

Christopher W. Ferrone is president of Americoach Systems Inc., Glenview, IL, an engineering firm specializing in transportation, technology, analysis and safety.