1. Presented to the California Energy Commission by Tim J. LaClair, Ph.D.
Tire Rolling Resistance, Its Impact on Fuel Economy, and Measurement Standards
Presented to the California Energy Commission by
Tim J. LaClair, Ph.D.

2. Rolling Resistance Basics
Definition: "Rolling resistance is the energy consumed per unit distance and is equivalent to the scalar sum of all contact forces tangent to the test surface and parallel to the wheel plane of the tire" (SAE J2452)
Units are [J/m] or simply [N], but conceptually, rolling resistance is better comprehended as a loss per distance than a force
Rolling resistance is primarily due to viscoelastic heat dissipation in the rubber
Aerodynamic drag, friction in the contact patch, and friction with the rim also contribute to the total rolling resistance, FR

3. Rolling Resistance Basics
Loss contributions:
Aerodynamic
Drag
0 to 15%
Tire/ground
and tire/rim
friction (<5%)
Hysteretic losses
80 to 95 %

4. RR Impact on Fuel Consumption
Fuel energy is dissipated by many vehicle losses, including rolling resistance:
(National Research Council, "Automotive Fuel Economy: How Far Should We Go?", 1992)

5. RR Impact on Fuel Consumption
To better quantify and understand the contribution of rolling resistance to fuel economy, Schuring (1988) proposed the concept of the Return Factor (also referred to as energy ratio):
For passenger cars and light trucks, RF is typically between 1:10 and 2:10. This indicates that a 10% improvement in RR gives only a 1-2% improvement in fuel economy.
For heavy trucks, RF can be higher, with a typical range between 1:10 and 3:10, hence the fuel savings potential for trucks may be higher than for passenger vehicles

6. RR Impact on Fuel Consumption

7. RR Impact on Fuel Consumption
In response to CAFE and a sense of environmental stewardship, the tire and rubber industries have responded to dramatically reduce rolling resistance since 1980:
(Lowest Michelin FR high-volume construction at 80% of the T&RA 1.8 bar load, 2.6 bar, 80 kph)

8. Rolling Resistance Measurements
In North America, the current rolling resistance standard is SAE J2452, which superseded SAE J1269 in 1999
In 1995, EPA mandated a true load matching procedure which would enable dynamometer simulation of road forces across a range of speeds for emissions certification and fuel economy testing
Therefore, for coastdown and performance modeling, auto manufacturers require component-level data, including FR, as a function of speed
The new method measures the dependence of FR on speed, in addition to load and pressure, which is its primary benefit

9. Rolling Resistance Measurements
Using the rolling resistance model and coefficients , , a, b, and c deduced from the J2452 coastdown data:
For North America, the natural choice of velocity cycles are the EPA Urban and Highway cycles used for emissions certification and fuel economy measurement

10. Rolling Resistance Measurements
Using MERF the RR can be more accurately applied to fuel economy predictions for a particular drive cycle
However, there is no direct correlation between RR values measured with SAE J1269 and SAE J2452
Largely for this reason, SAE J2452 has been adopted slowly: transition to the new standard is still not complete
For replacement and OE markets there is a bit of a mixture of data between the two standards
Furthermore, for the replacement tire market RR is not a significant performance target and in many cases data has not been measured

11. Rolling Resistance and Other Performances
Tire design requires balancing performances since changes in design may change different performances in opposing directions
For the OE market, a tire is designed for a single vehicle and the characteristics of the tire are optimized for that vehicle only
On the other hand, for the replacement market, tires need to be designed to provide a balanced set of performances for a wide variety of vehicle types with different handling characteristics, etc.
For this reason, rolling resistance is frequently higher for replacement market tires than a tire designed for use with a particular vehicle

12. Rolling Resistance and Other Performances
Rolling resistance is affected by many factors, both in tire design and operating conditions:
Tire Mass
Rubber Formulations
Inflation Pressure
Speed
Ambient/Tire Temperature
Applied Drive Torque
Surface Roughness
Steer angle and camber/toe of vehicle

13. Possible Approaches for Obtaining RR Data
Obtaining RR data for which a direct comparison can be made will require a significant effort and cost
Two basic approaches may be envisioned:
RR testing could be performed, for several tires having a range of RR values, at different sites in a “Round Robin”. A correlation would be established between sites which would then be applied to allow direct comparison of measured data from the different sites
Alternatively, an independent laboratory could be contracted to perform all necessary RR testing
The first approach may be more cost effective since any existing data could be used, but the second approach may provide more reliable results