Kicking today’s tires

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Bridgestone/Firestone says its cap & base tread construction allow for cooler running and longer tread life.

There’s little doubt that modern, commercial-vehicle tires are better all around than those
of yesteryear. The bonus is that they can save you money – and in more ways than one.

Fueling mpg
“Generally speaking, a standard commercial tire manufactured today is more fuel-efficient than tires labeled as ‘fuel efficient’ 10 years ago,” notes Greg McDonald, engineering manager of national accounts commercial products for Bridgestone Firestone North American Tire.

They have less rolling resistance, McDonald says. That has been accomplished through advanced rubber compounds that run cooler, and new casing designs that incorporate advanced materials and weigh less.

Michelin specifically credits its co-extrusion process for optimizing its rubber compounds. Co-extrusion, explains Michael Burroughes, product manager for Michelin Americas Truck Tires, is the ability to finely tune the volume and profile of two or more rubber mixes at the extruder head to deliver a seamless, semi-finished component. “This technology is used in not only the tread area, but also the sidewall rubber,” he says. The reported results are cooler running temperatures and, in some cases, a further reduction in rolling resistance.

Of course, the best way to realize a maximum mpg contribution from tires is to keep them properly inflated, thus ensuring that belts and sidewalls don’t undergo any more energy-wasting flexing than necessary. And while there are several onboard tire pressure monitoring and inflation systems on the market (see CCJ, May 2006, page 74), Goodyear has taken a step toward preserving air pressure with its DuraSeal technology, which allows a tire to self-repair small punctures.

Also, bear in mind that, when spec’ing drive tires, rib treads can inprove fuel economy by 2 to 4 percent over lug treads, according to the Technology & Maintenance Council. But traction needs and safety also must be factored into the decision. Either way, while deep tread on drive tires traditionally has been associated with a fuel-economy penalty – due to increased “tread squirm” and the energy it uses – that may be changing.

“Not too many years ago, a 26/32nd tread depth was considered deep,” recalls McDonald. “This progressed to 28/32nds, and then to 30/32nds. Currently, there are on-highway drive tires, including the Bridgestone M726EL, with 32/32nds tread depth. What’s interesting is that it’s more fuel-efficient than the 30/32nd M726 that it replaced.”

Long live the tread
According to Tim Miller, marketing communications manager for commercial tires at Goodyear Tire & Rubber, one of the most important tire advancements is the reduction of irregular wear. This, he says, has been accomplished “through innovations in tread designs, and controlling the shape of a tire’s contact patch on the pavement. For example, our G395LHS steer tire features a unique groove that redistributes pressures across the tire’s footprint.”

As weapons against irregular wear, Miller also cites the development of more specialized tires for specific applications, and improved manufacturing technologies to make tires more uniform and reduce variations from tire to tire. Goodyear’s Impact manufacturing process is one example.

Not only are tires developed for specific applications, they also are optimized for specific axle positions: steer, which is usually a deep, rib design for water evacuation, and can be used in all positions; drive, which can be either a deep rib or lug pattern for better traction under adverse conditions; and trailer, which is usually a shallower-tread, free-rolling rib pattern.

Bridgestone/Firestone’s McDonald agrees that a better understanding and application of tread design features – like sipes that relieve stress at the edge of ribs; equalizer ribs that retard the development of erosion-type rib wear; and tear-resistant side grooves that provide protection against shoulder edge wear and, in some applications, solid shoulders – have enhanced resistance to irregular wear.

McDonald adds that, “better control of manufacturing tolerances have resulted in tires that are more uniform (rounder), have more ‘near perfect’ footprint shapes, and are less likely to have any manufacturing anomalies” that can initiate irregular wear. Plus, “There are advanced tread designs and compounds that are more specialized for the application.”

If there’s a downside to all these advancements, it’s that they don’t come cheap. So tire companies are working on creating new, man-made materials to replace raw materials that have been escalating in cost, says Peggy Fisher, president of TireStamp. These include steel and petroleum-based products, including synthetic rubber, carbon black and chemicals. “Due to increased worldwide demand,” Fisher says, “the costs of these materials are never expected to go back to the levels where they were five years ago.”

Tire trends
So where are commercial tires heading? “Low-profile tires continue to prosper,” says Goodyear’s Miller, who notes that the 295/75R22.5 tire size is now 41 percent of the commercial tire market. Since lo-pro tires have shorter sidewalls than traditional-size tires, fleet owners can spec taller trailers while maintaining the same overall height. This is beneficial for fleets that typically “cube-out” – that is, trailer volume is filled with freight before legal maximum weight is reached. In many cases, a lo-pro package with a taller trailer allows more freight to be carried per trip.

For operations that “gross-out” – or reach maximum gross weight before filling the trailer, as in many bulk-haul applications – lo-pros also can offer an advantage in that they weigh up to 14 pounds less each than standard tires. Lo-pro tires could allow an extra couple of hundred pounds of product to be loaded.

On the downside, since lo-pros have a smaller overall diameter than standard-profile tires, they make more revolutions per mile. Not only can this have an adverse effect on tread and bearing life, but – unless a gearing change is made at the transmission or rear axle – higher engine speeds are needed to maintain a given road speed. In that case, fuel economy can suffer.

Another trend that seems to be growing is the use of wide-based single tires – such as Michelin’s X One or Bridgestone/Firestone’s Greatec – in lieu of traditional, dual-wheel assemblies. The main advantage to this concept, according to Bridgestone/Firestone’s McDonald, is that a weight savings of up to 1,200 pounds per vehicle is possible. While Bridgestone/ Firestone feels that this is a good option only where reduced vehicle weight can result in a definite, consistent increase in payload that provides additional revenue to the fleet, there arguably are other potential advantages, such as reduced center of gravity and attendant handling improvement, and improved fuel economy.

The latter seems possible, since a wide single tire has a smaller footprint than a pair of duals, and there are only two sidewalls at work, so there’s less material to flex, hence less rolling resistance. And unless the tires in a dual assembly are matched perfectly – in size, inflation pressure and degree of tread wear – their loaded radii will differ. In that case, one tire wants to travel farther per revolution than the other. The tires fight each other, consuming tread and fuel. Obviously, that can’t happen with wide singles. Yet another plus is that there are fewer tires to mount, dismount, inspect and rotate, which reduces maintenance downtime.

However, a major consideration has been tire trouble on the road. In the past, if a wide single blew on a single-drive-axle tractor, you had problems. But tire manufacturers are addressing these concerns with more robust designs and better product distribution.

For example, the Greatec uses a new, 90-degree Waved Belt, which allows for greater strength and better footprint control than a standard belt. In addition, a new innerliner adds tiny, long, thin particles that reportedly reduce the normal seepage of air through the tire by about 40 percent; and a new ply end construction that has the casing ply end wrapping back around the bead rather than ending away from it. Finally, a tire-within-a-tire concept called Aircept, which currently is being used in Europe, is being evaluated for the North American market.

Michelin also has made improvements to its X One. InfiniCoil technology, a metallic cord, is a proprietary bi-modulus design that is wound spirally around the casing. Each X One tire has about one-fourth of a mile of the InfiniCoil cable sandwiched between traditional steel belt packages, according to Michelin’s Burroughes. “The technology provides a consistent contact area, whether the vehicle is loaded or unloaded, and makes a key contribution to the reduction of the heat that plagues conventional wide-based tires.”

What’s next with wide singles? Burroughes believes that future vehicles will be developed around the technology. This, he says, “will provide new and unprecedented degrees of freedom to both trailer and power manufacturers to open frame rail spacing, lower centers of gravity and improve the stability of their products.”

Tireside reading
Tires come in a variety of diameters, widths and load capacities, and this information can be found on a tire’s sidewall. For example, a tire marked “295/75R22.5” has a section width (not tread width) of 295 millimeters. This also may be expressed in inches, especially on taller-profile tires.

The 75 is the tire’s aspect ratio. That’s the ratio of the tire’s section or sidewall height to section width. This tire’s sidewall height, then, would be 75 percent of its section width. Normally, aspect ratio is found only on low-profile tires. On regular-profile tires, whose standard aspect ratio is understood to be 90 or 100, the aspect ratio is not included in the tire size designation.

The R indicates that the tire is of radial construction, as the vast majority of modern tires are, and the 22.5 is the diameter in inches of the wheel to be used with the tire. On tires manufactured after May 2003, actual maximum load (as opposed to load range) and inflation pressure (in U.S. and metric units) also are indicated.