Aftermarket

Mismatching Dual Tires: A Sure-Fire Way to Kill Two Tires at Once

January 2014, TruckingInfo.com - WebXclusive

by Jim Park, Equipment Editor - Also by this author

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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.

Comments

  1. 1. Harvey Brodsky [ January 14, 2014 @ 02:05PM ]

    Anything Jim Parks writes is worthwhile reading. We need more folks like Jim to write articles that can save truckers money and headaches.
    Thanks, Jim, for another great article.

    Harvey Brodsky
    Managing Director
    Retread Tire Association

  2. 2. Ed Bluebaum [ January 14, 2014 @ 09:14PM ]

    Good article. The one point I cannot grasp is that the shorter tire is "dragged" to cover the same distance? The last time I noticed, the duals were bolted together and one could not drag the other, nor can it skip to catch up!

  3. 3. Jim Park [ January 14, 2014 @ 11:05PM ]

    Good question Ed. Here's an explanation. Let's assume the circumference of the taller tire is 100 inches -- to use a round number. But the shorter tire's circumference is 90 inches. For every rotation of the wheel, the taller tire will carry it 100 inches down the road. The shorter tire would normally travel 90 inches along the pavement, but because it is bolted to the 100-inch tire, it can't help by be dragged 10 inches to make up the same distance traveled. Hope that helps.

  4. 4. Fernando Dewes [ January 18, 2014 @ 05:12PM ]

    What is the sales Manager ou Sales director contact email of the dual dynamics Crossfire?

  5. 5. Carlton Biggs [ January 19, 2014 @ 09:15AM ]

    Interesting article and you are so right. I recently had two mismatched on a trailer and I watched them and I thought the same thing you are telling. The smaller one wore faster and eventually had a blow out.

  6. 6. Gary Kendall [ February 01, 2014 @ 05:10PM ]

    Sounds good on paper. But, I just cannot see how you are "dragging" the smaller tire when it is bolted solidly to the larger one. Now, if the tire is slipping on the wheel, that would be a different story. Put a chalk line across both tires and I guarantee you the chalk line will still be in the same place after any number of revolutions. Of course, I retired from driving over 5 years ago, and things do change.But you are going to have to prove this theory to me. Maybe the smaller tire is "growing" because of less load compared to the mating tire?

 

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