# Rolling Resistance

## Rolling friction and rolling resistance

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The force resisting the motion when a body rolls on a surface is called the rolling resistance or rolling friction.

The rolling resistance can be expressed as

*F _{r} = c W (1)*

*where *

*F _{r }= rolling friction (N, lb_{f})*

*c = rolling resistance coefficient - dimensionless (coefficient of rolling friction - CRF)*

*W = m g = normal force or weight of body (N, lb _{f})*

*m = mass of body (kg, lb)*

*g = accelaration of gravity (**9.81 m/s ^{2}, *

*32.174 ft/s*

^{2})The rolling resistance can alternatively be expressed as

*F _{r} = c_{l} W / r (2)*

*where *

*c _{l} = rolling resistance coefficient with dimension length (coefficient of rolling friction) (mm, in)*

*r = radius of wheel (mm, in)*

### Rolling Friction Coefficients

Some typical rolling coefficients:

Rolling Resistance Coefficient | ||
---|---|---|

c | c _{l}(mm) | |

0.001 - 0.002 | 0.5 | railroad steel wheels on steel rails |

0.001 | bicycle tire on wooden track | |

0.002 - 0.005 | low resistance tubeless tires | |

0.002 | bicycle tire on concrete | |

0.004 | bicycle tire on asphalt road | |

0.005 | dirty tram rails | |

0.006 - 0.01 | truck tire on asphalt | |

0.008 | bicycle tire on rough paved road | |

0.01 - 0.015 | ordinary car tires on concrete | |

0.03 | car tires on tar or asphalt | |

0.04 - 0.08 | car tire on solid sand | |

0.2 - 0.4 | car tire on loose sand |

### Rolling Coefficients Cars

Rolling coefficients for a pneumatic tyres on dry roads can be calculated as

*c = 0.005 + 1/p (0.01 + 0.0095(v/100) ^{2})*

*where *

*c = rolling coefficient*

*p = tyre pressure (bar)*

*v = velocity (km/h)*

### Example - Normal car on asphalt

The rolling resistance of a normal car *1500 kg* on asphalt with rolling friction (rolling coefficient from the table above) can be estimated like

F_{r}= 0.03 (1500 kg) (9.81 m/s^{2})

= 441 N

- compare car rolling resistance with car air resistance - or drag

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