Drag Factor (μ)
Definition
The effective coefficient of friction between the tyre and the road surface during a braking event. Used in the skid-mark speed formula. Typical values: dry asphalt 0.7-0.8, wet asphalt 0.4-0.5, gravel 0.5-0.6, snow 0.2-0.3.
- Dry asphalt
- 0.7 to 0.9
- Wet asphalt
- 0.4 to 0.6
- Gravel
- 0.5 to 0.6
- Ice
- 0.1
- Also called
- Coefficient of friction (μ)
Common questions
What is drag factor used for in accident investigation?+
Drag factor (μ) measures the friction between a tire and road surface during braking. It's plugged into the skid-mark speed formula to estimate how fast a vehicle was traveling when the brakes were locked and the tires left skid marks.
Why does drag factor change with different road surfaces?+
The grip between a tire and road depends on what the road is made of and its condition. Dry asphalt offers more friction than wet asphalt, and ice offers far less. Investigators must know the actual surface at the scene to use the right drag factor value.
Can you use the same drag factor for wet and dry roads?+
No. Dry asphalt ranges from 0.7 to 0.9, while wet asphalt drops to 0.4 to 0.6. Using the wrong value will give an incorrect speed estimate. Gravel sits around 0.5 to 0.6, and ice is very low at around 0.1.
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Explained in these topics
- Accident Investigation: Skid Marks, Collision and Hit-and-RunThe effective coefficient of friction between the tyre and the road surface during a braking event. Used in the skid-mark speed formula. Typical values: dry as...
- Track Marks: Foot, Shoe, Tire, Skid Marks and Gait PatternCoefficient of friction between tire and road surface, used in the skid-mark speed equation. Dry asphalt 0.7 to 0.9, wet asphalt 0.4 to 0.6, gravel 0.6, ice 0....