Coefficient of Friction (Mu)
Definition
The dimensionless ratio of frictional force to normal force between tire and road. It determines how quickly a skidding vehicle decelerates. Typical dry asphalt values range from 0.65 to 0.80; wet surfaces fall to 0.40-0.60.
- Symbol
- mu
- Nature
- Dimensionless ratio of frictional to normal force
- Dry asphalt range
- 0.65 to 0.80
- Wet asphalt range
- 0.40 to 0.60
Common questions
How is the coefficient of friction used to estimate a vehicle's pre-crash speed?+
It is combined with the measured skid mark length in the standard skid-to-stop speed formula, so a reconstructionist needs an accurate mu for the actual road surface and weather conditions at the scene, not just a generic textbook value, to get a defensible speed estimate.
Why do investigators test the actual road surface rather than using a table value?+
Mu varies with pavement material, texture wear, contamination such as oil or gravel, and moisture, so a drag-sled or brake test at the scene under matching conditions gives a far more defensible figure than a generic published range in a contested reconstruction.
Related terms
- Conservation of Momentum
- In a collision, the vector sum of all momenta (mass × velocity) before impact equals the vector sum after, provided no significant...
- Critical Speed Yaw Mark
- Curved scuff marks deposited when a vehicle slides sideways. The radius of curvature and friction coefficient yield the vehicle's minimum speed at...
- Delta-V
- The change in vehicle velocity during the crash event, recorded by the EDR as a time-history. It is the integral of the...
- Perception-Reaction Time (PRT)
- The elapsed time from when a driver can first perceive a hazard to when the brakes begin to apply. Olson and Farber...
- Skid Mark
- A dark rubber deposit left when locked wheels slide along the road surface. Length combined with the friction coefficient allows calculation of...