Graining

Graining happens when a tyre is running cooler or harder than its ideal window and the rubber surface shears rather than melts smoothly, tearing off tiny pellets of rubber that roll along the tread instead of gripping the track. It shows up as a sudden, temporary loss of grip early in a stint, often on out-laps or after a safety car has cooled everything down.

It's easy to confuse with degradation, but the mechanism runs the opposite way: degradation is a tyre getting too hot over time, graining is a tyre that's too cold. Graining usually fades once the tyre works back up to temperature and the torn rubber wears away, so a car that looked slow for the first few laps of a stint can recover on its own.

Because graining is a temperature problem rather than a wear problem, it doesn't map cleanly onto the stint-length and stop-count signals a strategy model reads from long-run practice data: a genuinely graining-prone stint can look worse early than it will actually run.