Beware run-flats

Inflation pressure is the critical variable that allows pneumatic tires to carry loads; transmit driving, braking, and cornering forces; and act as an integral part of the vehicle suspension system. Consequently, inflation maintenance is essential to consistent and predictable vehicle performance. But tire maintenance on light-duty consumer vehicles is less reliable than that practiced by the commercial

Inflation pressure is the critical variable that allows pneumatic tires to carry loads; transmit driving, braking, and cornering forces; and act as an integral part of the vehicle suspension system. Consequently, inflation maintenance is essential to consistent and predictable vehicle performance.

But tire maintenance on light-duty consumer vehicles is less reliable than that practiced by the commercial vehicle industry, and emergency tire changes on passenger vehicles are no longer considered routine tasks by many drivers. As a result, some car and tire manufacturers have pursued the development of run-flat, or extended mobility, tires. Most of these premium designs have significantly reinforced sidewalls and are self-supporting to a degree with very low, or negligible, inflation pressure.

Will the next step include run-flat tires for larger (Class 4-8) over-the-road trucks? My guess is probably not. Studies have shown that modern radial truck tires initially inflated to 100 psi lose pressure very slowly, absent any puncture or unusual tire/rim sealing irregularities. Most commercial maintenance programs include tire checks scheduled frequently enough to avoid running at significant under-inflation, although the actual execution of inflation checks and visual inspection for cuts/punctures requires diligent follow-up.

Several engineering differences between light-duty passenger tires and larger truck tires are also noteworthy. First, truck tires are designed for much higher inflation pressures and loads. The challenge of engineering tire sidewalls so robust that they could support these loads absent internal inflation pressure is daunting, and probably not possible with current materials.

Secondly, bead seats at the tire/rim interface are more severely tapered on truck tires. This allows reliable and consistent bead seating concentricity of the higher pressure and volume truck tires, but precludes the use of bead retaining humps to maintain the load bearing at very low or negligible inflation pressures.

Bead retention could be accomplished by using clamping rings or other mechanisms, but these are cumbersome, labor intensive, and add considerable weight. Another approach would be to use separate support structures attached to the rim and sized to restrict tire deflection in the event of inflation loss. But this type of mechanism is difficult to service and adds weight.

Retreading would also be impacted by the adoption of run-flat truck tires, especially they included substantially heavier, reinforced sidewalls. Revisions in processes and equipment would likely be required.

NHTSA has recently published a Final Rulemaking that mandates the fitment of low-tire-pressure warning systems on new light-duty vehicles. Some run-flat passenger tire designs make it more difficult for a driver to recognize when a puncture has occurred, so a low-tire-pressure warning system is considered by some to be an essential system component if run-flat tires are used. For now, vehicles over 10,000 lb. GVW are excluded from this requirement.

Current run-flat light-duty tire technology does not appear to be readily adaptable to larger over-the road truck tires, and significant increases in cost, labor, weight, and overall complexity could be expected to accompany any effort to force the adaptation of current materials and processes. We need to do everything we can to avoid on-highway air loss incidents. Disciplined inflation maintenance and visual tire inspections are crucial. Reducing the need for run-flat truck tires may be the best approach to avoid having to deal with them.

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