Bottom Line
Vive Lo Resistance
Tire designs and innovations no longer leave truckers deciding between good grip and good fuel mileage with today's low-rolling-resistance tires

By Paul Abelson, senior technical editor

In this era when fuel economy is king, you have to know what causes fuel mileage to decrease and make changes in the areas where you can.

One of those areas is your tires. They are the greatest contributors to the rolling resistance that accounts for one-third or more of fuel use. In addition to a desire to save money on the fuel, the regulatory environment with the impending federal fuel economy regulation and California’s “SmartWay” rule are driving up the need for fuel efficiency from tires as well.

Not too many years ago, good traction and low rolling resistance were considered opposite characteristics. Shallow rib tires that were barely a half-inch deep provided the least rolling resistance and unfortunately also the least grip. Deep lug tires, often with treads a full inch deep, offered the best traction on all but the smoothest roads, but also had high rolling resistance.

But a lot has changed.

Tires do not support the truck. They contain the air that does. While holding the air, tires must also allow vehicle forces to act on the roadway in the form of acceleration, braking and cornering.

In order to accomplish all of those tasks, tires must flex in a controlled manner.

Tires flex more in the sidewalls for cushioning and remain relatively stiff at the tread surfaces for stability. Shoulders provide the transition between the sidewalls and tread while holding the tire together. Under the tread are belts that keep the tread stable for traction while allowing it to conform to the road surface.

Without enough flexibility, traction will be broken and skids will occur.

Too much flexibility allows tire components to move against each other creating friction. As we all know, friction creates heat and that heat, in turn, destroys tires.

When designing tires, engineers strike a delicate balance dictated by the way the tire will be used.

Tire engineers are continually working on new casing designs and new combinations of materials for treads and belts. Tires have 20 or more components with each made up of as many as 15 ingredients. The way the ingredients are mixed, kneaded or coated can affect their performance and tire properties such as rolling resistance.

Rolling resistance equates directly to fuel use, since any motion results from forces that get their energy from the fuel your truck burns.

For example, tires flex, causing internal components to rub against each other. This creates friction that generates heat.

Engineers design tires to manage flex in a controlled manner to limit heat from friction to around 100 degrees above ambient temperatures. Internal tire temperatures will reach more than 200 degrees with a properly inflated tire in the desert during summer. If underinflated by 20 percent – say 80 psi instead of 100 psi – flexing increases significantly and results in internal temperatures of more than 250 degrees.

Sustained heat over a prolonged period weakens the bonds that hold components together. That leads to tread separation and “road gators.” Even if the tire doesn’t blow, its life will be shortened. And the increased friction consumes additional fuel.

That’s why maintaining tire pressure is critical.

Wide-base tires
Flexing, especially at the sidewalls and the shoulders, consumes energy – which in turn burns more of your fuel. To cut the sidewall and shoulder flexing in half, Michelin and Bridgestone introduced wide-base tires to replace duals. Michelin’s X-One and Bridgestone’s Greatec have two sidewalls and two shoulders in place of four. The configuration is about 4 to 5 percent more fuel efficient because of lower rolling resistance.

Initially, drivers were reluctant to try wide-base tires, fearing loss of “limp home” ability and scarce supply if repairs or replacements were needed. Over the past decade, the two suppliers built infrastructure and many fleets tried and adopted the tires.

Meanwhile, Goodyear brought out its Fuel Max Technology, a combination of component innovation involving polymers and rubbers, reinforcement fillers and manufacturing techniques. The result was dual tires with similar rolling resistance reductions.

Last year, Goodyear introduced wide-base tires with DuraSeal Technology to address fears of being stranded with a flat. Both the drive and trailer tires incorporate their well-proven gel-like inner liner that seals punctures up to 1/4-inch in the repairable area of the tread. Instead of losing limp-home capability, the tires let you run home.

There are cautions and considerations when using wide-base singles. When mounted on axles designed for duals, loading on outside spindle bearings is increased. With 2-inch offset rims to get full track width, excessive inner-edge tire wear has been noticed with lightweight axles. In some cases, axle capacity should be reduced by as much as 3,000 pounds.

Tire pressure is critical with wide singles. If under-inflated, excessive wear will occur. The tires are also more speed sensitive. They can handle 75 mph structurally, but centrifugal force distorts center ribs. That results in scuffing of shoulder ribs. These problems are not seen when driven at 65 mph or less.

TMC has a task force studying operational and maintenance issues with wide-base singles. LL