1. Scope--This SAE Recommended Practice is applicable to pneumaticPassenger Car "P" Type, Light Truck Metric, and Light Truck HighFlotation tires, or similar tires approved by bodies other than Tire Rim Association. The methodology is applicable within normaloperating ranges of vertical load and inflation pressure, and forvelocities between 115 km/h and 15 km/h (71 mph and 9 mph) during arelatively short duration event such as a coastdown. This procedure isapplicable only to operation in the free-rolling mode at zero slip andcamber angle for ambient temperatures between 20°C and 28°C(68°F and 82°F) and for surfaces with diameters of 1.2 m (48in) diameter or greater. Details regarding the equipment, tires, andtest methods used specifically for validation of this document areincluded in Appendix A. Two basic measurement methods covered by thisdocument are as follows:

1.1 Force Method--Measures the reaction forces at the tire spindle andconverts it to rolling resistance. The chief advantage of this methodis that the only parasitic losses in the measurement are tire spindlebearing losses and aerodynamic losses associated with rotation of thetire and its wheel. The main disadvantage is that the measured spindleforce can contain large errors caused by load misalignment andload-spindle force interaction ("crosstalk"). Elimination orcompensation of these effects is necessary. A minor disadvantage isthat the loaded radius of the tire must be measured in order to convertspindle force to rolling resistance.

1.2 Torque Method--Measures the torque input to the test machine andconverts it to rolling resistance. The advantage of this method is thatthe measurement is direct: rolling resistance is the net torque dividedby the test surface radius. The main disadvantage is that parasiticlosses contained in the measurement include rotational test wheellosses as well as tire spindle losses. Hence, the parasitic losses arelarger than those of the force method and can be of the same order ofmagnitude as the rolling resistance itself. Accurate measurement andcompensation for these losses is necessary. Speed hunting oscillationin the drive motor can also induce large measurement errors due to thehigh moment of inertia of the test surface.

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