Marbles

As tyres wear, small pieces of rubber shear off and get flung outward by a corner's forces, landing just off the line the fastest cars actually take. Lap after lap, that rubber builds into a layer of loose grit known as 'marbles'.

The racing line itself stays clean and grippy because cars constantly scrub over it, but step off it, even by a car's width, and the marbles cut available grip. That happens when defending a position, running around the outside to overtake, or picking a line to a grid slot at a standing restart.

The effect builds through a session, and it can catch out a driver who has to leave the line, sometimes badly enough that a slide looks like a mistake when it is really just where the rubber piled up.