Empty Miles on Loaded Tires

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Empty miles can chew up more than your balance sheet. Most fleets avoid empty miles like the plague, but there are dedicated fleets and some specialty carriers out there running one way loaded and the other empty. It’s good on fuel, but hell on tires.

There’s some debate over the impact running empty or lightly loaded has on tire wear, but fleets than run a high ratio of light or unladen miles seem to report higher instances of irregular wear such as cupping, scalloping, and flat-spotting.

It’s not clear whether irregular wear is a direct result of light tire loading or collateral damage resulting from some other condition, but evidence suggests tires tend to perform much better under pressure than when lightly loaded.

"By far, the largest contributing factor to tire wear is improper tire pressure for the anticipated axle load," says Brian Buckham, program manager for axles, brakes, and wheel-ends at Hendrickson. "With tires typically pressurized for a loaded condition, trailers operating with a high percentage of empty or lightly loaded miles can see a decrease in tire life as a result of the tires being over-inflated for the reduced axle load."

A tire running under normal load will scuff away tread rubber at a predictable rate, but if there’s some other factor contributing to tire wear, such as an alignment issue or loose wheel bearings, the wear caused by the problem will be noticed sooner because the lightly-loaded wear rate is slower than the loaded wear rate.

In other words, the irregular wear would occur at the same rate on a loaded or empty tire, whereas the regular wear would be slowed. The irregular wear patterns become more pronounced sooner.

When empty at highway speed only the center
of the tire tread makes contact with the pavement.

This could lead to the assumption that running light or empty is the root cause of the accelerated wear.

Mike Beckett of MD Alignment in Des Moines, Iowa, disagrees with that assumption — to a point. He was once called upon to settle a dispute between a fleet owner, a tire maker, and a trailer maker. A large portion of the trailers in that fleet were burning through tires in about 25,000 miles, Beckett says.

"The fleet ran 250- to 450-mile headhauls and returned empty. Through all the finger pointing, the high percentage of empty miles incorrectly emerged as the principal culprit," Beckett says.

Upon inspecting the fleet, Beckett found higher-than-normal incidences of loose wheel bearings, improperly matched tires in dual assemblies, and improper inflation.

"Once all the problems were rectified — and they went to a quality tire — the fleet-average miles-to-pull leapt to 150,000 miles. Was it the high empty miles? No, it was poor maintenance," he says. "The fleet wasn’t taking the right steps to correct the problem."

CHANGING THE FOOTPRINT

Idaho Milk Transport is a textbook case of a high-empty-mile fleet. Based in Burley, Idaho, the food-grade tank carrier runs a lot of dedicated trailers loaded one way and back empty. Leased owner-operator Tony Head says he has switched to running a closed-shoulder drive tire to minimize cupping along the edge of his dual drive tires.

"I used to run lug tires, but the edges cupped out real quickly," he says. "And I see more center wear on the company’s wide-base single trailer tires."

Shawn Estes, IMT’s fleet services facilities manager, agrees with Head’s observations, and while he notes that proper alignment can solve some of the cupping problems, he says running light seems to make the problem worse.

"The unloaded tires just skip down the road without any weight to bear down on the tire," says Estes.

The tread surface is generally pretty close to flat, or square with the road, at operating pressure. Under load, contact with the pavement is pretty even across the tread face, but when empty at normal operating pressure, the center of the tread tends to be slightly higher than the shoulder. This is more acute at high speed, where centrifugal force also acts on the tread causing it to extend outward at the center.

What you have is an egg-shaped footprint, with the center of the tread having a slightly larger circumference than the edges. This increases the scuffing action because the edges are not revolving at the same rate as the center of the tread.

"In addition," notes Bridgestone‘s Guy Walenga, "an unloaded tire that bounces along the road actually slows while it is ­airborne causing a minor bit of scuffing each time it regains contact with the road."

BOUNCING ALONG:

Walenga says tire pressure has a role to play here, as does the damping effect of the suspension.

"Think of a fully inflated tire as a Super Ball," he says. "With relatively little weight to keep them on the ground, they’ll bounce like crazy at normal inflation pressure. The suspension can mitigate this, but only to a certain extent."

Correct tire pressure for the load would have a large influence on tire wear in theory, Walenga says, but he acknowledges that adjusting tire pressure downward is a task few drivers would undertake.

Tire balancing can affect how the tire tread contacts the pavement surface. Unbalanced tires will bounce as the heavier part of the tire revolves around the axle spindle, alternately lifting the tire off the pavement and slamming it down again as it rolls.

"Out-of-balance wheel assemblies resonate with the vehicle suspension frequency at around 10Hz (600 times per minute) at 600 rpm (65 mph on most truck tires). This resonance multiplies any imbalanced portion of the wheel assemblies by over 1,000 times," says Roger LeBlanc of Counteract Balancing Beads in Georgetown, Ont.

"This can cause axle hop at every revolution ­similar to hitting a bump at every revolution, and that’s how you get cupping wear."

And as Bridgestone’s Walenga just ex­plained, a tire inflated to working pressure for a full load will bounce like crazy when empty or lightly loaded.

Hopefully, you can catch a failed shock
before it begins to wear out your tires.

Low-spring-rate suspensions could also help to reduce irregular wear by maintaining better contact between the tire and the road.

"Air suspensions typically have a lower spring rate than leaf spring suspensions, and they maintain a relatively constant low spring rate whether the trailer is loaded or empty," says Jim Rushe, program manager for on-highway products at Hendrickson. "Air suspensions also have shock absorbers that dampen the suspension movement, further improving the suspension’s ability to maintain tire contact with the road."

Leaf springs provide a stiffer ride on an empty trailer resulting in a varying load on the tire as it encounters irregular road surfaces, Rushe adds. "Leaf spring suspensions do not have shock absorbers, but the leaf springs themselves provide some damping characteristics."

In many high-empty-mile applications, tires and suspensions run either fully loaded or empty, so "tuning" the suspension and tire pressure to work equally well at opposite ends of the spectrum is realistically out of the question.

"For air suspensions, the air pressure is used to maintain the trailer height, so changing air pressure would have little effect on the spring rate," notes Rushe. "Leaf springs can be designed to provide varying spring rates at different loads, but the amount the spring rate can be varied is limited."

Since there are no spec’ing options to help manage this unique situation, the duties seem to fall on the maintenance department’s shoulders. Beckett sees this issue cropping up constantly in his alignment shops. He suggests the jump-in point is careful tire matching.

"Mixing brands and models of casings in a dual assembly encourages irregular wear. They don’t all have the same sidewall flex," he says. "The more focus on matched brands, models, circumference, and inflation pressure, the better the tires will wear — even under these conditions."

And he says shock absorbers on air suspensions need particular attention. "That’s where all the damping takes place. If they’re failing, your tires will fail too."

Since your fuel costs will be lower in a half-loaded application, maybe some of the savings should be allocated to the maintenance budget.

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Jim Park was a CDL driver and owner-operator from 1978 until 1998, when he began his second career as a trucking journalist. During that career transition, he hosted an overnight radio show on a Hamilton, Ontario radio station and later went on to anchor the trucking news in SiriusXM's Road Dog Trucking channel. Jim is a regular contributor to Today's Trucking and Trucknews.com, and produces Focus On and On the Spot test drive videos.


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