Coefficient of friction vs Drag factor

20 Oct by Zarir.Hafiz

Coefficient of friction vs Drag factor

Drag factor and coefficient are expression commonly used in crash investigation and reconstruction. They are usually used to calculate vehicle speed calculations based upon tyre mark geometry.

The coefficient-of-friction μ is defined as the ratio of the tangential force F(tan) (parallel to the surface) applied to an object sliding across a surface to the normal force N (perpendicular to the surface) of an object.1,2 Figure 1 shows the C is proportional to that of the horizontal force F and weight w.

Graphical definition of μ
Figure 1: Graphical definition of μ (George M. ,2005)1.

Drag Factor (f) is a non-dimensional (no units) number used to represent the   acceleration or deceleration of a vehicle.   It is defined as the total force F(tot) required for a vehicle’s acceleration (or deceleration) in the direction of acceleration, divided by the vehicle’s weight.   Figure 2, shows the relationship between total force F(tot) and vehicle weight w.

Vector diagram of forces
Figure 4: Vector diagram of forces associated with drag factor and coefficient of friction (George M. ,2005)1.

Full comprehensive articles by about the difference and comparison between coefficient of friction and drag factor with an example can be found here.


  1. FRICKE L.B. (2010). The Traffic Accident Investigation Manual, Vol 2 – Traffic Accident Reconstruction, Northwestern University Traffic Institute, 2nd Edition
  2. George M. (2005). The Difference Between Coefficient-Of-Friction and Drag Factor. Accident Investigation Services Pty Ltd, Sydney, Australia.


Crash Investigator & Reconstruction Specialist. Over 10 years’ experience in crash investigation and reconstruction.

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