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
Radius can be derived from a measured chord and middle ordinate. Roadway crown, superelevation, grade, and changing radius can materially affect the analysis.
Florida / FPSI convention
Florida training commonly presents the basic U.S.-customary relationship with the 3.86 coefficient. It is effectively the same physics as √(15Rf), with a small difference from the conventional rounded coefficient. The live worksheet shows both values so the convention is explicit.
When a measured yaw radius follows the applicable tire path rather than the vehicle center-of-mass path, the live worksheet can apply an optional track-width correction before calculating speed. Use the correction only when the measured mark and vehicle geometry support it.
For Florida-specific sources, roadway references, and the USF/FHP momentum case study, see the Florida / FDOT reference.
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