When a bus undergoes a safety inspection, among the critical components always checked are the tires.

While underinflated tires are considered a violation, there is no precise definition of exactly what constitutes an "underinflated tire."

Consequently, underinflation is subjective for officers conducting destination and roadside inspections.

In an effort to develop a standard for an "underinflated tire," the trucking industrys Technology & Maintenance Council has created a definition it plans to submit to the Commercial Vehicle Safety Alliance (CVSA) for adoption.

The Technology & Maintenance Council is the technical organization for truck equipment technology and maintenance professionals.

No OOS criteria

In most states, bus and truck safety inspectors use the CVSAs North American Standard Out-of-Service Criteria manual as a guide.

The manual defines a flat tire -- a tire that is 50 percent below the maximum tire pressure marked on the tire sidewall or has a noticeable (can be heard or felt) leak but it doesnt define an underinflated tire.

TMCs recommended definition is that a tire is underinflated if its hot inflation is less than 70 percent of the maximum inflation pressure thats molded on the sidewall of the tire.

In other words, any tire with a hot inflation pressure above 70 percent is not underinflated.

A tire with an inflation pressure at less than 70 percent to 51 percent below the stated maximum inflation pressure on the sidewall would be considered underinflated.

Tires inflated to 50 percent or less of the maximum inflation pressure stamped on the sidewall are considered flat.

For example, if the pressure written on the tire sidewall is 120 psi, it would be considered underinflated at 84 to 61 psi (70 to 51 percent of 120 psi).

Tread depth measurements

TMC also is recommending to CVSA guidance on where tire tread depth measurements should be taken. Once again, compliance officers have no guidance.

TMC is suggesting such measurements be taken between two adjacent major tread ribs where there are no tread wear indicators; measuring on an indicator will not provide a true measurement of tread depth.

Tread wear indicators are designated on the upper sidewall/shoulder of a tire.

CVSA is the organization comprised of local, state, provincial and federal officials from the U.S., Canada and Mexico that have responsibility for commercial vehicle safety programs and who perform vehicle inspections and have other safety-related responsibilities.