Coefficient of Kinetic Friction (Mu)
Definition
The dimensionless ratio of the sliding friction force to the normal force between a tyre and the road; typically 0.55-0.70 for dry asphalt, 0.30-0.45 for wet asphalt, and 0.10-0.25 for icy surfaces.
- Symbol
- mu
- Dry asphalt
- 0.55 to 0.70
- Wet asphalt
- 0.30 to 0.45
- Icy surfaces
- 0.10 to 0.25
Common questions
Why does kinetic friction differ from static friction in a skid analysis?+
Once a tyre locks and slides, kinetic friction applies, which is generally lower than the static or peak friction available just before the tyre breaks loose, so a reconstruction using kinetic mu on a fully locked skid can understate the vehicle's true deceleration capability at the start of braking.
How does surface contamination change which mu value applies?+
Loose gravel, oil films, or standing water lower effective mu well below the dry-asphalt range, so investigators note surface condition at the scene and, where possible, measure it directly rather than assuming a textbook dry or wet figure.
Related terms
- Coefficient of Restitution (E)
- The ratio of the relative velocity of separation to the relative velocity of approach in a collision; e = 0 for perfectly...
- Daubert Standard
- The US federal evidentiary standard (Daubert v. Merrell Dow Pharmaceuticals, 1993) requiring that expert testimony be based on scientifically valid methods with...
- Energy-Equivalent Speed (EES / Delta-V)
- The SAE J2080 metric quantifying the kinetic energy absorbed by vehicle crush deformation, expressed as the speed change equivalent; used in the...
- Event Data Recorder (EDR)
- An onboard module that records pre-crash vehicle kinematics (speed, throttle, brake, steering) in the seconds before airbag deployment; governed by SAE J1698...
- Head-Throw Distance (HTD)
- The horizontal distance from the vehicle's front bumper contact zone to the final rest position of the pedestrian's head; used in the...
- IRTE
- Institute of Road Traffic Engineers (UK); publishes the Forensic Collision Investigation manual that governs police and expert-witness reconstruction methodology in England and...
- Momentum Conservation
- In the absence of external horizontal impulses, the total linear momentum of a two-vehicle collision system (sum of mass times velocity for...
- PC-Crash
- Accident reconstruction simulation software (Steffan and Moser, Austria) that models vehicle dynamics, pedestrian multibody kinematics, and collision geometry; validated against crash-test data...
- SAE J1739
- Society of Automotive Engineers standard for traffic-accident reconstruction covering momentum, skid, and crush-energy methods; the primary scientific-community reference for admissibility arguments in...
- Skid-Mark Formula
- v = sqrt(2 mu g d), where v is the pre-skid speed (m/s), mu is the coefficient of kinetic friction between tyre...
- Yaw Mark
- A curved, striated tyre impression produced when a vehicle slides at an angle to its direction of travel during a steered or...