Yaw, radius & curve
Critical Speed Yaw Calculator
Calculate speed from a vehicle's curved tire mark after establishing that the mark represents a critical-speed yaw.
Level-road formula
speed (mph) = √(15 × radius in feet × drag factor)
Radius can be derived from a measured chord and middle ordinate. Roadway crown, superelevation, grade, and changing radius can materially affect the analysis.
Measurements and validation
- Document why the tire mark is classified as a yaw mark.
- Use several chord and middle-ordinate measurements where the geometry permits.
- Measure the surface drag factor under comparable conditions.
- Record cross-slope and confirm the vehicle's path and mark progression.
The equation applies when lateral tire force is at the limiting condition. A curved mark is not enough by itself to establish critical-speed yaw.
Related tools
Radius from chord and middle ordinate · Lambourn multi-point method · Skid mark speed