Friction Coefficient, Rubber Tyre on Dry Asphalt
| Value | 0.8 — |
| Status | Measured: ± 0.15 — (0.19 relative) |
| Source | SAE / NHTSA accident reconstruction literature; AASHTO Green Book |
| Categories | Material PropertiesEngineering & Trade |
| part per trillion | 800,000,000,000 ppt |
| part per billion | 800,000,000 ppb |
| part per million | 800,000 ppm |
| per cent mille | 80,000 pcm |
| basis point | 8,000 bp |
| per mille | 800 per mille |
| percent | 80 % |
| ratio | 0.8 — |
Learning zone
Tyre grip is not Coulomb friction. Rubber generates traction by molecular adhesion and by hysteretic deformation around the road's texture, so the coefficient can exceed 1.0 — racing slicks reach 1.5 and drag radials higher still — which is impossible for a true Amontons friction pair. It also falls with normal load (a tyre's grip is sublinear in weight, which is why weight transfer costs total grip in a corner) and depends on slip ratio, temperature, tread compound and pavement age.
Accident reconstruction uses 0.7–0.8 for dry asphalt and typical passenger tyres, and this figure sets stopping distance directly: d = v²/(2 µ g), so 100 km/h to rest at µ = 0.8 is about 49 m of braking, plus reaction distance. Every safety margin in road design descends from this and from the wet value. Note that peak friction occurs at 10–20 % slip, not at lock-up — sliding a locked tyre loses roughly 20–30 % of available grip, which is the entire justification for ABS.