# Turn a rubber mark into a speed band
A motorcycle often leaves one clear rear mark, a lighter front mark, or an interrupted ABS print. Measure the locked-wheel portion, not the later gouge of a sliding bike. Then let friction, grade, and brake use push the estimate into a range instead of a fake precise speed.# How the estimate is built
The working model is the slide-to-stop energy balance: speed is the square root of twice gravity times drag factor times mark length. Drag factor is friction times brake efficiency, plus grade as a decimal. An uphill grade helps the motorcycle stop. A downhill grade reduces drag and can make the equation invalid if gravity exceeds friction.- Length is converted to metres internally so metric and imperial stay the same physical mark.
- Surface chips load a typical locked-wheel friction and a low to high span.
- Front-only and rear-only efficiencies are educational fractions of a fully used tire-road couple, because FMVSS 122 tests those controls separately.
- Unknown brake use stretches the band from a rear-only assumption to a both-brake assumption.
Measure the locked tire, not the crash
Start at the first visible striation or shadow of a locked tire and stop where rotation resumes, the mark fades, or an impact begins. Do not add the distance the motorcycle slid on bodywork after a capsize. That is a different drag problem.# Typical friction used as a starting point
| Surface | Mid friction | Low to high span |
|---|---|---|
| Dry asphalt | 0.70 | 0.55 to 0.85 |
| Wet asphalt | 0.50 | 0.35 to 0.65 |
| Dry concrete | 0.75 | 0.60 to 0.90 |
| Gravel | 0.50 | 0.35 to 0.70 |
| Grass or dirt | 0.40 | 0.25 to 0.55 |
| Ice | 0.10 | 0.05 to 0.18 |
# Motorcycle brakes change the story
Most motorcycles have separate front and rear controls. A single rear skid does not mean the rider used all available friction. The estimator therefore lets you choose both, front, rear, or unknown. Unknown is the honest setting when you only have a photo of one stripe.Educational estimate, not evidence
ABS can leave dashed or missing marks. Contaminants, tire wear, load, ABS pulse, and an impact at the end of the mark all change the true speed. If the downhill drag factor reaches zero, the page refuses a number instead of inventing one.