Basic Methods

The force, torque, and power methods of measurement are all in common use and should yield the same test results. Effects of steering, traction, surface texture, and non-steady-state tire operations are excluded from the recommended practice because they are still in the research stage.

Force Method

The chief advantage of the force method is that the only parasitic losses in the measurement are tire spindle bearing losses and aerodynamic losses associated with rotation of the tire and its wheel. The main disadvantage of this method is that the spindle force measured can contain a severe error caused by load misalignment and load-spindle force interaction (“crosstalk”). Elimination or compensation of these effects is necessary. A minor disadvantage is that the loaded radius of the tire must be measured in order to convert spindle force to rolling resistance.

Torque Method

The torque method has the advantage that the measurement is direct: rolling resistance is the net torque divided by the test wheel radius. The main disadvantage of the torque method is that parasitic losses contained in the measurement include rotational test wheel losses as well as tire spindle losses. Hence, the parasitic losses are larger than those of the force method and can be of the same order of magnitude as the rolling resistance itself. In addition, speed-hunting oscillations in the drive motor can introduce errors.

Power Method

The advantage of the power method is that no force or torque transducer is necessary; a voltmeter and an ammeter to measure the electrical energy input to the drive system are sufficient. The chief disadvantage of the power method is that electrical losses are included in the measurement in addition to all the other parasitic losses of the system. Thus, the parasitic losses are even larger than those of the torque method. In some laboratories, special control may be needed to prevent line voltage fluctuations from creating power surges which can distort the readings.

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