Tyres are an essential ingredient for a Formula One car, as they are the only part of the car that touches the road, but choosing the correct tyre for a circuit is not a simple task.

Bridgestone considers many factors when looking to determine its tyre allocations for grands prix. Track surface and layout and the likely ambient and track temperatures are just some of the aspects which need to be considered.

“There are many variables involved,” explains Hirohide Hamashima, Director of Bridgestone Motorsport Tyre Development. “These are variables such as the weather and the track surface which we can only predict, but not be certain what is going to happen.”

As a contrast to a competitive tyre supply environment, Bridgestone does not make different tyres for each circuit or team.

“In times of no competition we have worked closely with the FIA and we now only produce four different specifications of dry weather tyres to cover the whole season,” explains Hamashima.

“This means that each tyre has to have a wider working range than before, so it can be used by a variety of different cars and at a variety of different circuits.”

An additional challenge is provided by the regulations necessitating the use of two different tyres during the course of each grand prix.

“This provides a challenge for Bridgestone as well as the competitors,” explains Hamashima. “When we started with this concept at the beginning of the 2007 season we could see that the lap time difference between the two tyres was quite big, and it was interesting to watch the teams and drivers develop their knowledge and ability of how to best use the two tyres.”

The weather, as everyone has seen so far in the 2009 season, can be a large influencing factor, and it’s not just if the weather is wet or dry, hot or cold makes a big difference too.

“The temperature can certainly change the tyre focus for teams over the course of a race weekend,” explains Hamashima. “As an extreme example, an unexpectedly hot track could mean that the harder tyre for a race weekend is actually better for qualifying than the softer one, which would usually be the better one.

“In contrast, an unexpectedly cold track could mean that the softer tyre becomes the better race tyre, so there is a lot to consider for tyre selection over the course of the race meeting.”

The track surface also provides an interesting variable, and aspects of this are related to the weather too.

“The general trend for race weekends is for the track surface grip to improve as rubber from our tyres is left on the track, providing a good high grip surface,” explains Hamashima. “However, how dirty or clean the track is on the first day makes a difference, and that can depend on how many recent race meetings there have been at the track as well as how much rain there has been before the race meeting, or even how hot and sunny it has been.”

Bridgestone did not make any significant change to the tyres for the 2008 season, except for a modification to the super soft compound, meaning that the teams had a good knowledge base to work from.

“Last season we saw that the use of both compounds at a race by the teams was much better than before, and this is because they understood the tyres better than they had previously,” explains Hamashima.

As the understanding improved, so the differences on track between each tyre diminished, so the rationale for the regulations to have a difference between each tyre was diminished. So, for 2009 Bridgestone has worked to present more of a challenge for the teams.

“The move back to slick tyres has allowed us more scope for a greater difference between the tyres,” explains Hamashima. “This year’s tyre has around 20% more contact patch and 15% more grip than last year’s, as well as the inherent greater consistency of a slick which doesn’t have the raised tread areas of a grooved tyre.

“This means we are able to develop tyres to the allocation philosophy of having one at a race which is softer, and one which is harder – which is the same as we did for the previous two seasons, but also giving the softer tyre a lower temperature working range, and the harder one a higher temperature working range.”

As both tyres have to be used during a grand prix, the best compromise set-up from the cars to harness the qualities of both tyres in the most competitive manner must be found.

“It is one of the biggest challenges for competitors over the season,” says Hamashima.