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Track evolution - the change from green to mean

In 2010, the Korea International Circuit registered the greatest decrease in lap times between first practice and qualifying as the circuit rubbered-in, with the pole time 5.302 seconds faster than the quickest time in practice one, compared to an average dry weather improvement of around 2.5 seconds. That’s typical for a new circuit on which the cars are running for the first time, and also strongly characteristic of little-used street circuits. But what exactly happens during the processing of ‘rubbering-in’? Mercedes explain more…

Q: In what condition does the circuit usually begin the race weekend?

A: Almost all circuits begin the weekend in a condition that is termed ‘green’ - even if rubber has been laid down on the racing line, any previous running will not have been with the same sticky tyres as used in Formula One racing.

Q: What happens when a circuit rubbers in?

A: ‘Rubbering-in’ describes the process by which tyre rubber is deposited on the racing line as a result of the 24 cars running during practice. This forms a layer of rubber on the track, which is then compressed on the racing line and increases grip levels. As the cars run around, they also clean the circuit: dust and grit on the track surface are either picked up by the tyres, or blown away by the aerodynamics on the underside of the cars.

Q: How does temperature affect this process?

A: The tyre choice made by Pirelli for each event takes into account the historical air and track temperatures, to ensure that the selection is appropriate. A historically hotter track may therefore require a harder compound, because softer rubber might wear too fast. The effects of temperature on the process of rubbering-in are secondary to compound softness.

Q: Has the process of rubbering-in changed significantly with the Pirelli tyres used in 2011?

A: There haven’t been any significant changes compared to previous years, as the compounds do not appear to be particularly softer or stickier than last season. However, the tyres do wear more, so while the circuits do not rubber in to any greater extent, the marbles produced by the tyres are more significant than last year.

Q: What’s the difference between rubbering-in and the formation of marbles?

A: Marbles are small chunks of rubber thrown off the tyres, which collect off the racing line, and are a function of the tyres’ wear rate. They are therefore not flattened down, and give the same effect as driving on marbles if the driver runs off line. Equally, the dust and grit that is cleared from the racing line does not clear from other parts of the circuit, which means the penalty for running off line at a dusty circuit generally increases as the weekend goes on.

Q: Does the circuit configuration make a difference to how a circuit rubbers in?

A: Yes: the more cornering involved over the lap, the more rubber is laid down. In Korea last year, the best sector time in the first sector improved by 3.2 percent between first practice and qualifying, and 45 percent of this sector is spent cornering. In comparison, the times in sector two improved by 6.2 percent (79 percent of sector spent cornering) and in sector three by 6.7 percent (77 percent spent cornering).

Q: Which circuits rubber in most significantly?

A: Generally, street circuits - Monaco in particular, and Melbourne, Singapore and Valencia to a lesser extent. At circuits like these, times will generally come down by 5.5 percent (up to five seconds) from the beginning of practice to qualifying. The other factor to remember is overnight rain: a reasonable amount of precipitation will result in a return to almost completely green track conditions, with a loss of up to two seconds of lap time.

Credit: Formula One Administration Ltd (www.formula1.com)