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Tire Rolling Resistance Measurement: Force, Torque, and Power Methods
Rolling resistance is a key parameter influencing tire efficiency and vehicle fuel economy. The SAE J1270-2017 information report provides essential background and guidance on measuring rolling resistance for passenger car, light truck, and highway truck and bus tires. This article covers the three primary measurement methods, important equipment considerations, and practical insights from the standard to help engineers achieve accurate and comparable results.
1. The Three Primary Measurement Methods
The force, torque, and power methods are all in common use and should yield the same test results when applied correctly. Each method has distinct advantages and disadvantages, primarily related to parasitic losses and measurement complexity.
| Method | Principle | Parasitic Losses Included | Key Advantage | Key Disadvantage |
|---|---|---|---|---|
| Force | Measures spindle force and loaded radius | Spindle bearing and aerodynamic losses | Lowest parasitic losses; direct force measurement | Requires compensation for load misalignment and crosstalk; needs loaded radius |
| Torque | Measures input torque and divides by test wheel radius | Test wheel rotational losses, spindle losses | Direct measurement of rolling resistance net torque | Parasitic losses can be as large as rolling resistance; speed-hunting oscillations possible |
| Power | Measures electrical power input to drive system | Electrical losses plus all mechanical parasitic losses | No force or torque transducer needed; uses simple electrical measurements | Highest parasitic losses; sensitive to line voltage fluctuations |
🛠️ Engineering Insight: The test wheel curvature (typically 1.708 m diameter) increases tire deflection compared to a flat surface, resulting in slightly higher rolling resistance values. When comparing results from different test machines, this curvature effect should be considered.
2. Equipment and Environmental Considerations
Proper test equipment and environmental control are crucial for repeatable and comparable rolling resistance measurements. Key factors include the test wheel surface, rims, alignment, and ambient temperature.
- Test Surface: A medium-coarseness abrasive surface is recommended to simulate dry, well-maintained public roads. The surface must be periodically renewed to avoid contamination and wear.
- Test Rims: Rims should represent original equipment and conform to TRA specifications. Rim width and contour can affect rolling resistance, so they must be reported with results.
- Inflation Pressure: Three types are defined: base (maximum load pressure), capped (rising with temperature as in real-world use), and regulated (constant pressure via rotating union).
- Ambient Temperature: Rolling resistance is temperature-sensitive. All data must be referenced to 24 °C (75 °F) to enable reliable comparisons across laboratories.
⚠️ Common Mistake: In the force method, failing to correct for load misalignment can introduce severe errors. Angular offsets as small as 0.005° or fore-aft offsets of 0.02 mm can produce spindle force components misread as rolling resistance. Always implement a misalignment correction technique.
3. Frequently Asked Questions
Why is ambient temperature referenced to 24 °C?
Rolling resistance varies with ambient temperature, and precise control of room temperature is difficult. Referencing measurements to 24 °C (75 °F) allows consistent comparison of results obtained under different thermal conditions.
What is the difference between capped and regulated inflation pressure?
Capped pressure allows the tire inflation to rise naturally as the tire heats up, reflecting actual road service conditions. Regulated pressure maintains a constant pressure via a rotating union, which is useful for isolating the effects of pressure from other variables.
How does test surface texture affect rolling resistance?
Rolling resistance is influenced by road texture. The standard selects a medium-coarseness abrasive surface to simulate typical dry, well-maintained public roads. Contamination or wear of the surface adds variability and must be avoided.
Why are parasitic losses in the torque method of particular concern?
In the torque method, parasitic losses include rotational test wheel and spindle bearing losses, which can be of similar magnitude to the rolling resistance itself. Careful measurement and subtraction of these losses are necessary to obtain accurate rolling resistance values.
