Table of Contents
What Is Rolling Resistance?
Rolling resistance is the force opposing the motion of a wheel rolling on a surface. Unlike sliding friction, it arises from the deformation of the wheel and/or the surface at their contact patch. As a tire rolls, it continuously deforms and recovers, and this hysteresis in the rubber dissipates energy as heat. Rolling resistance is typically much lower than sliding friction, which is why wheeled transport is so energy-efficient.
At highway speeds, rolling resistance and aerodynamic drag are the two primary forces a vehicle engine must overcome. For heavy vehicles at lower speeds, rolling resistance dominates. For lighter vehicles at higher speeds, aerodynamic drag becomes more significant.
Rolling Resistance Formula
Where Crr is the coefficient of rolling resistance (dimensionless), N is the normal force, m is mass, g is gravitational acceleration (9.81 m/s²), and P is the power needed to overcome rolling resistance at velocity v.
Crr Values Table
| Surface Combination | Crr |
|---|---|
| Railroad steel on steel | 0.001 - 0.002 |
| Bicycle tire on road | 0.002 - 0.005 |
| Low rolling resistance tire | 0.006 - 0.008 |
| Car tire on asphalt | 0.010 - 0.015 |
| Car tire on gravel | 0.020 - 0.040 |
| Car tire on sand | 0.200 - 0.400 |
Factors Affecting Rolling Resistance
- Tire pressure: Lower pressure increases contact patch and deformation, raising Crr significantly.
- Tire temperature: Cold tires have higher rolling resistance. As tires warm up, rubber becomes more elastic.
- Speed: Crr increases slightly at higher speeds due to standing wave formation in the tire.
- Load: Rolling resistance is proportional to normal force, so heavier vehicles have higher resistance.
Frequently Asked Questions
How much fuel does rolling resistance consume?
Rolling resistance accounts for approximately 20-30% of fuel consumption for passenger cars at highway speeds and up to 40% for heavy trucks. Low rolling resistance tires can improve fuel economy by 3-5% compared to standard tires.
Does tire size affect rolling resistance?
Wider tires have a shorter, wider contact patch while narrower tires have a longer, narrower patch. At the same pressure and load, the total rolling resistance is approximately the same. However, wider tires may have slightly higher resistance due to more tread squirm.
How does rolling resistance compare to air drag?
For a typical car, rolling resistance force stays roughly constant with speed, while aerodynamic drag increases with the square of speed. They are approximately equal around 70-90 km/h. Above that, air drag dominates.