January 2014, TruckingInfo.com - WebXclusive
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.
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.