Run-Flats For Trucks?

Tires for passenger cars have become so dependable that many drivers take for granted the day-after-day performance and trouble-free miles they deliver.

This fact was driven home to me recently when a friend related that neither of his teenage daughters had ever changed a flat tire. The two teens had a combined five years of driving experience!

One unfortunate consequence of this reliability is that periodic visual inspections and simple inflation pressure maintenance have become less common, resulting in tires that are simply ignored until they either wear out, run underinflated for an extended period of time, or become flat, usually from a puncture. This type of consumer neglect certainly played a contributing role in the widely publicized tire problems allegedly experienced by some SUV drivers in recent years.

Since 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, inflation maintenance is essential to consistent and predictable vehicle performance.

Maintenance of tires on light duty consumer vehicles tends to be much less reliable than is common practice with commercial vehicles. And emergency tire changes on passenger vehicles, although not frequently required, are no longer considered a routine task as they once were.

This, and several other considerations, have caused some car and tire manufacturers to pursue the development of run-flat, or extended mobility, tires. Most of these premium designs have significantly reinforced sidewalls and are self-supporting even with very low, or negligible, inflation pressure.

Some tire/vehicle combinations are capable of traveling 100 or more miles, at reasonably restricted speeds, following an air loss.

No widely applicable industry performance standards for run-flats exist yet, but some engineers are referencing so-called 80/80 design targets (80 kilometers at 80 kilometers/hour), following deflation.

Run-flat tires are generally heavier than conventional tires. At the same time, the overall weight savings possible from not needing a spare tire/wheel assembly tends to offset that factor. Also, the space normally occupied by the spare can be utilized for other purposes, especially in smaller cars that have limited cargo space.

While development of run-flat tire technology for light-duty vehicles is gaining some momentum, 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, approximately 1 to 2 psi per month absent any puncture or unusual tire/rim sealing irregularities.

Most commercial vehicle maintenance programs include tire checks scheduled frequently enough to avoid running at significant under-inflation, although the actual execution of inflation checks and visual inspections still leaves a lot to be desired.

Can Concept Work?
Several engineering differences between light-duty passenger tires and larger truck tires point to some reasons why commercial run-flats may not be in our future.

First are the much higher inflation pressures and loads typical of truck tires. The challenge of engineering tire sidewalls so robust that they could support typical truck tire loads absent inflation pressure is daunting, and probably not possible with current materials.

Secondly, bead seats are more severely tapered (with an approximately three times higher angle) on truck tires vs. passenger and light truck designs. This allows reliable and consistent bead seating for higher pressure and volume truck tires, but precludes the use of bead retaining humps that help maintain the load bearing bead/rim interface at very low or negligible inflation pressures.

Bead retention could be accomplished alternatively by using clamping rings or other mechanisms, such as those found on some off-road tires or on top fuel drag racing wheels, but these are cumbersome, very labor intensive, and add considerable weight.

Another approach would be the use of separate support structures attached to the rim that would be sized to restrict tire deflection in the event of inflation loss. This type of mechanism is used in some military applications, but is also complex, difficult to service, and adds weight.

Another issue is that current light-duty run-flat tire designs have higher spring rates, which adversely affect vehicle ride and noise levels. In current run-flat applications, tire and vehicle engineers have worked jointly to overcome these issues on individual car models. But this is a costly, time-consuming process and requires integrated engineering relationships not currently in place in the heavy truck industry.

Second and Third Lives
Retreading would also be impacted, especially if the design chosen included substantially heavier, reinforced sidewalls. Revisions in processes and equipment would be required, but the tire would be the biggest concern.

Currently, modern truck radials are highly retreadable, offering cost-per-mile incentives for operators to preserve the structural integrity of tire casings. With current passenger run-flats, however, there is no real provision for repair let alone retreading. A punctured run-flat, in most cases, is targeted for the scrap pile.

In a heavier application, reinforced sidewalls or sidewall inserts would take quite a beating while the tire is deflated, and would likely be damaged beyond repair.

NHTSA has recently published a final rule that mandates the fitment of tire pressure warning systems on new light duty vehicles. Vehicles over 10,000 pounds GVW are, for now, excluded from this requirement. The very success of some run-flat passenger tire designs makes it more difficult for a driver to recognize when a puncture has occurred, and the requirement of a low tire pressure warning system is considered to be an essential component if run-flat tires are fitted.

In summary, current run-flat tire technology does not appear to be readily adaptable to larger over-the road truck tires. Even with some still-unknown technology, the significant increases in cost, labor, weight, and overall complexity would likely accompany any new fangled run-flat configuration. Those dollars-and-cents increases alone would preclude run-flats.

The trucking industry would seem well advised to get its own house in order by doing the best job possible to maintain tires and avoid on-highway air loss incidents. Disciplined inflation maintenance and visual tire inspection programs alone would save any fleet thousands of dollars – and hundreds of headaches – each year. And they would greatly reduce the need for expensive run-flat truck tires.