Choosing tyres in Formula One Practices

The practice sessions also give the teams valuable information on the respective behaviour of the two different compounds of tyres that they have to choose from before Saturday qualifying. The sessions give them information on the differing wear rates of each tyre and also the difference in their performance pattern.
Practice sessions allow teams to see how the performance of each type of tyre changes over a number of laps. A softer compound is usually quicker initially, and invariably quicker from new on a qualifying lap. But it also tends to wear out more quickly than a harder tyre. Softer compound tyres also have a greater tendency to grain, a phenomenon where small tears appear on the edges of the shoulder, spreading across the whole width of the tyre, giving less grip until they stabilise. With less grip, the cars can’t go as fast as they would otherwise. Graining can last for up to 10 laps and, although the softer tyres may then be quicker than the hard tyres, after that the speed difference may not be enough to overcome the time lost because of the graining. Practice gives the engineers and drivers a feel for how this behaviour is panning out. Even though the data may tell a clear story on tyre choice, a driver may still be influenced by how each compound “feels”. A softer compound gives more grip and, therefore (in theory at least) offers a better lap time, but it also tends to feel less stable because its tread moves around more under cornering load. Occasionally a driver may find he is actually quicker on the harder compound despite its lower grip levels simply because it instils him with the confidence to push harder than when he’s using the soft compound. Practice gives the driver the chance to get used to the more squirmish behaviour of the softer tyre, or it enables him to quickly discard it and concentrate on setting the car up around the harder tyre.