Rolling Resistance Reduction

Background

The rolling resistance of a tire is the force required to move the tire forward, and represents nearly a third of the tractive forces on a vehicle. The force is directly proportional to the weight load supported by the tire, and the ratio of the force to the weight load supported by the tire is called the rolling resistance coefficient (RRC). The higher the RRC, the more fuel needed to move the vehicle.

Tires are of two construction types: bias-ply and radial-ply. Bias-ply tires have been largely phased out of the light-duty truck and car markets except in certain rough-duty applications, but still retain some market share in the medium-duty and heavy-duty commercial truck and bus markets. In general, bias-ply tires have significantly, higher RRCs than radial tires. The RRC of radial tires has also decreased over time owing to improvements in materials and design.

The primary source of tire rolling resistance is internal fiction in the rubber compounds as the tire deflects on contact with the road. Reducing this “hysteresis loss” has typically involved a trade-off with other desirable tire attributes such as traction and tread wear, but advances in tire design and rubber technology have brought significant reduction in rolling resistance without compromising other attributes.

This evolution of passenger car and light truck tires maybe divided into three phases:

· The first radials (generation one), which used a type of synthetic rubber, 37 had 20 percent to 25 percent lower rolling resistance than bias-ply tires, and became available during the late 1970s.

· The second phase (generation two), using new formulations of synthetic rubber,38 achieved an additional 20 percent to 25 percent reduction in rolling resistance over generation one radials, and became available during the mid-1980s.

· The third phase (generation three), which adds silica to the tread compounds, achieve an additional 20 percent reduction, and has recently become available in limited quantities.

In addition to changing the tread materials, RRC reductions can be realized by changing the shape of the tread and the design of the shoulder and sidewall, as well as the bead. The type of material used in the belts and cords also affects the RRC. For example, DuPont has suggested the use of aramid fibres to replace steel cords and monofilament replacement of current polyester multifilament to modify stiffness. Aramid yams have been available for over a decade, and their use can cut rolling resistance by 5 percent.40 Polyamide monofilament have been recently introduced that improve the tire sidewall stiffness and reduce rolling resistance by about 5 percent. These new materials also contribute to reducing tire weight (by as much as 4 kg/tire), which provides secondary fuel economy benefits and improved ride.

The rolling resistance values of current OEM tires are not well documented. Anecdotal evidence from experts states that most normal (i.e. not performance-oriented) tires have RRCs of 0.008 to 0.010 as measured by the Society of Automotive Engineers (WE) method.41 Performance tires used in luxury and sports cars, and increasingly in high performance versions of family sedans, use H- or V-rated tires that have RRC values of (SAE) 0.012 to 0.013. Tires for compact vans have RRC values of 0.008 to 0.009 while four-wheel-drive trucks and sport utilities feature tires with RRC values (SAE) of 0.012 to 0.014.

Potential for Rolling Resistance Improvement

Most manufacturers OTA interviewed had similar expectations for tire rolling resistance reduction over the next decade. The expectation was that an overall reduction of 30 percent was feasible by 2005, resulting in normal tires with an RRC of 0.0065 (if the current average is 0.009). Most also believed the H-rated or V-rated tires would have similar percentage reductions in rolling resistance so that they would have RRCs of 0.009 to 0.01 by 2005. Very similar percentage reductions in RRC for light truck tires were also expected. A 30 percent reduction in rolling resistance can translate to a 5 percent improvement in fuel economy, if the design is optimized for the tire. Manufacturers were unwilling (or unable) to estimate additional RRC reductions in the post-2005 time frame, possibly owing to their unfamiliarity with tire technologies in the research stage at this time.

These 30 percent reductions are expected to be achieved with virtually no loss in handling properties or in traction and braking. Manufacturers suggested that some loss in ride quality may occur because of the higher tire pressure, but this could be offset by suspension improvements or the use of semi active suspension systems. However, manufacturers expected noise and tire life to be somewhat worse than those for current tires. Both of these factors are highly important--noise may represent a special problem because the improved aerodynamics and, possibly, electric drivetrains of advanced vehicles will reduce other sources of noise.

An optimistic view for the 2015 time frame suggests that RRC values as low as 0.005 may be achievable. Such low rolling resistance tires have already been built for electric cars. Auto manufacturers believe that such tires are not yet commercially acceptable because prototypes have suffered from losses in handling, traction, and durability. Tire manufacturers have expressed the view that technological improvements during the next 20 years could minimize these losses, and an RRC of 0.005 could be a realistic goal for a “normal” tire in 2015, as an average, which implies that some tires would have even lower RRC values.

Only two auto manufacturers discussed other components of rolling resistance, including brake drag and wheel/drivetrain oil seals and bearing loss. Brake drag accounts for 6 percent of total rolling resistance, while bearing and seal drag account for about 12 percent of rolling resistance, with the tires accounting for the remaining 82 percent. The use of highly rigid callipers, pads, and shoes to avoid brake pad contact with the rotor when the wheels are spinning can reduce brake drag by as much as 60 percent. Bearing and oil seal relative friction can be reduced by:

· Downsizing bearings and reducing preload

· Using low-tension oil seals

· Using low-viscosity lubricants

Manufacturers anticipate that these frictional losses can be reduced by 20 to 25 percent by 2005. A composite analysis of total rolling resistance suggests that a 25 percent reduction is possible by 2005, and up to 40 percent by 2015, if new tire technologies are successful There is some disagreement among engineers about the effect such reductions will have on vehicle fuel economy, with some asserting that the 25 percent reduction in resistance would translate into no more than a 3 percent fuel economy increase, and the 40 percent reduction into a 5 percent fuel economy increase. OTA is more optimistic than this; we conclude that the projected reductions in rolling resistance may yield as much as a 5 percent improvement in fuel economy by 2005 and an 8 percent improvement by 2015 for an optimized vehicle design.