## Tire Rolling Resistance, Its Impact on Fuel Economy, and Measurement

Standards Presented to the California Energy Commission

by Tim J. LaClair, Ph.D.CEC Workshop on Fuel Ef ficient Tires
 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 Rolling Resistance Basics

 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, F RCEC Workshop on Fuel Ef ficient Tires
 Loss contributions: Rolling Resistance Basics Aerodynamic Drag
Tire/ground and tire/rim friction (<5%) Hysteretic losses 80 to 95 % 0 to 15%CEC Workshop on Fuel Ef ficient Tires 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 W e Go?", 1992)CEC Workshop on Fuel Ef ficient Tires RR Impact on Fuel Consumption

e Resistanc Rolling in Reduction % n

Consumptio Fuel in Reduction % Factor Return   Tobetter 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): e Resistanc Rolling in Reduction % Factor Return   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 vehiclesCEC Workshop on Fuel Ef ficient Tires RR Impact on Fuel ConsumptionCEC Workshop on Fuel Ef ficient Tires
 In response to CAFE and a sense of environmental stewardship, the tire and rubber industries have responded to dramatically reduce rolling resistance since 1980: RR Impact on Fuel Consumption

150 175

50 75 100 125 150

1975 1980 1985 1990 1995 2000 YearRolling Resistance Index

(Lowest Michelin F R high-volume construction at 80% of the T&RA 1.8 bar load, 2.6 bar, 80 kph)CEC Workshop on Fuel Ef ficient Tires
 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 Rolling Resistance Measurements
range of speeds for emissions certification and fuel economy testing  Therefore, for coastdown and performance modeling, auto manufacturers require component- level data, including F R , as a function of speed  The new method measures the dependence of F R on speed, in addition to load and pressure, which is its primary benefitCEC Workshop on Fuel Ef ficient Tires Rolling Resistance Measurements  Using the rolling resistance model and coefficients  ,  , a , b , and c deduced from the J2452 coastdown data:



#       f t t t

cV bV a L P MERF 0 d 2  

 For North America, the natural choice of velocity cycles are the EPA Urban and Highway cycles used for emissions certification and fuel economy measurement 

## 

 f

t t t MERF
0 dCEC Workshop on Fuel Ef ficient Tires 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 measuredCEC Workshop on Fuel Ef ficient Tires 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 vehicleCEC Workshop on Fuel Ef ficient Tires 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 vehicleCEC Workshop on Fuel Ef ficient Tires 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