Mismatched tire sizes means the shorter tire is chasing the taller tire down the road, and scrubbing away value with every mile.

Mismatched tire sizes means the shorter tire is chasing the taller tire down the road, and scrubbing away value with every mile.

Close enough for Rock & Roll; good enough for the women I go out with; close enough for government work ... all these expressions speak to our willingness to settle for something less than the ideal. In tire maintenance, the difference between "it'll do" and "ideal" can be costly.

In 2003, FMCSA did an extensive survey of truck tire pressures during roadside inspections. Inspectors found only 50% of the tires checked were within 5% of their recommended pressure. As well, FMCSA found that in dual tire assemblies only 20% of the tires checked were within 5 psi of the tire beside it.

That means about 80% of the tires in dual assemblies are at risk of premature wear due to scrubbing damage or premature failure due to over loading.

At the Annual Meeting of The American Trucking Associations' Technology & Maintenance Council in February 2012, Chris Flanigan of FMCSA's Office of Analysis, Research and Technology outlined the general financial implications of underinflated tires. He noted that that underinflation can increase the total annual operating costs per truck by as much as $600 to $800.

Flanigan said FMCSA didn't delve deeply into the implications of mismatched pressures across dual tires, and therefore had no figures to present. But other sources tell us that damage incurred through mismatching dual tires can cost fleets as much as 20% in tread life, wasting the equivalent of about $50 per tire in lifecycle value.

Mismatched in this context means a difference exists in either inflation pressure and/or diameter or circumference between the two tires.

When two tires are bolted together in a dual assembly, both must have the same circumference or diameter in order to cover the same distance as the assembly rolls along the pavement. If the diameter or circumference of the two tires are even slightly different, the smaller of the two tires will scrub along the pavement to make up the distance traveled by the larger tire. It's that scrubbing that kills tread life.

"While that may not sound like much, a diameter mismatch of just 5/16 of an inch, means the larger tire will drag the smaller one a distance of about 13 feet for every mile, or 246 miles for every 100,000 miles," says Guy Walenga, director of engineering for commercial products and technologies at Bridgestone.

At the same time, the larger tire is forced to bear an unequal share of the weight on the dual assembly, causing additional wear to the tread face of that tire and greater stress on its sidewalls. In effect, you are inflicting death by scrubbing on the shorter or underinflated tire, while overloading the taller or higher-pressure tire. Two birds with one stone.

Maintaining proper inflation pressure in dual assemblies can minimize the damage cause by mismatched diameter.

According to Matt Wilson, controls business unit manager at Hendrickson, the risk is with unequal loading across the tires. The harder/taller of the two does most of the heavy lifting, while the soft tire flexes and squishes its way to an early grave.

"Equalizing the pressure across duals increases helps prolong tread and casing life," he says.

Dual Dynamics' Crossfire tire pressure equalizers maintain a consistant pressure between dual tires and provide a visual pressure indicator.

Dual Dynamics' Crossfire tire pressure equalizers maintain a consistant pressure between dual tires and provide a visual pressure indicator.

Maintaining equal pressure across two dual tires on trailers is easy with an automatic tire inflation system, such as Hendrickson's TireMaax Pro, which also bleeds off excess pressure.

For drive tires, the Crossfire by Dual Dynamics of the Cat's Eye from Link Manufacturing equalize inflation pressure across the two tires while providing visual indication of the pressure and a single inflation point.

Right from the Start

When mounting two new tires in a dual assembly, chances are they will be the same brand and model of tire. It's safe to assume that both tires will have equal tread depth and if installed properly, the same inflation pressure. In other words, you're unlikely to see a mismatch between the two new tires in terms of circumference or pressure.

All bets are off, however, when mounting used tires in dual assemblies, especially on trailers.

[PAGEBREAK]A recently released White Paper called "The Butterfly Effect on Tire Maintenance" from specialty tool manufacturer Innovative Products of America Inc., suggests there is real room for improvement when remounting a pair of used tires moved from another wheel position to a trailer.

"Fleets often make the mistake of using either inflation pressure or tread depth as an indicator of a proper match," the paper notes. "In fact, these two variables are independent of the diameter as a whole and if used alone will yield undesirable tread wear and fuel economy results."

This could be especially true if different brand, models, or tread patterns are assembled together.

The Tire Comparator from IPA is a quick way to ensure the tires in a dual assembly are compatible.

The Tire Comparator from IPA is a quick way to ensure the tires in a dual assembly are compatible.

Even if two tires had the same tread depth, differences could exist in the tread underlay, the height of the sidewall or other places on the tire that will affect its circumference. Maintaining equal tread depth between the two tires may not be enough to ensure compatibility.

IPA offers a simple but clever solution to the challenge properly matching dual tires, the Tire Comparator. There are actually two versions of the caliper-like device, but both provide a clear visual indication of a mismatch condition. The tire tech simply checks the relative height of the two tires with IPA's device and makes a go or no-go decision.

The company also offers a tire inflator, the Mobile Tire Pressure Equalizer, that tops up inflation pressure in up to five tires at once, saving technician time and assuring equal pressure across all tires.

According to the IPA paper, the success of a tire pressure management system can fall back to the accuracy of the tire pressure gauge and the diligence of technician using it.

"When comparing two brand new tire gauges from the same manufacturer, accuracy often differs up to 3% right out of the box," IPA claims. "A gauge with +/- 3% accuracy will shift your reading by as much as 3 psi per tire at 100-psi target inflation."

While automatic tire inflation and tire pressure monitoring systems can assist in tracking problems on in-service vehicles, proper inflation before the vehicle leaves the shop as well as accurate gauges used during routine tire pressure testing as preventative maintenance measures can maximize your investment.

Fill four tires at the same time and all to exactly the same pressure with IPA's Mobile Tire Pressure Equalizer.

Fill four tires at the same time and all to exactly the same pressure with IPA's Mobile Tire Pressure Equalizer.

Consigning a tire from a previous wheel position to pre-graveyard trailer position need not mean giving up optimum mileage before retreading. But because trailer tires seldom get the attention they deserve, they are often given up for lost even as they are being bolted to the axle.

Mike Beckett of MD Alignment in Des Moines, Iowa, says there are no spec'ing options to improve the fate of trailer tires, so the duties seem to fall on the maintenance department's shoulders. Beckett sees the issue of mismatched tires cropping up constantly in his alignment shops.

"Mixing brands and models of casings in a dual assembly encourages irregular wear," he says. "The more focus on matched brands, models, circumference and inflation pressure, the better the tires will wear."

As tire costs continue to rise, along with fuel, it will become harder to ignore the living conditions of trailer tires. Lack of attention and basic maintenance can strip thousands of miles from even scabby old trailer tires, and increase the risk of a mission-crippling blowout.

"At about 1.9 cents per mile, tires are a significant maintenance cost for fleets," FMCSA's Flanigan reminds us. "The initial surveys we did in 2003 led us to conduct field tests of automatic tire inflations systems and tire pressure monitoring systems, and those tests proved our hypothesis that the cost recovery on those systems is there, and they provide ongoing cost savings and fuel economy improvements. In other words, they are worth the investment."

So, too, would be tools that ensure you're not killing both tires in a dual assemble through simple mismatching.

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