Advanced Automotive Technology: Visions of a Super-Efficient Family Car

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
tradeoff 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.

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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,^39 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 fibers 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

37 Emulsion-polymerized styrene-butadiene rubber, or SBR, in particular.

38 Solution-polymerized SBR-based formulations.

(^39) Goodyear, “The Environmental Tire” September 1991.