Why tyre size matters - Bridgestone explain 28 May 2009
As far as racing tyres are concerned, size does matter, particularly for this Formula One season, which has seen the return of slicks after an 11-seasons absence. The removal of the grooves which were previously on the tread area means that the proportional size of the rear tyres contact patch has changed relative to the fronts.
There are many differences between a slick and a grooved tyre, but the change of the proportional tread area in contact with the road is a very important aspect of the performance potential, explains Hirohide Hamashima, Bridgestones director of motorsport tyre development. This year we are hearing a lot of talk about weight distribution and the balance of the cars and this is related to the big changes to the regulations for the cars and the tyres for 2009.
The latest aerodynamic regulations mean a lower and wider front wing, but a taller and narrower rear wing. This means the proportion of aerodynamic grip - the grip provided by the downforce pushing the car down onto the road - has moved towards the front of the car.
In addition to this, there is more mechanical grip - grip provided by the tyres interacting with the road surface - than before at the front of the car, due to the proportionally bigger contact patch of the front tyre, so the latest cars have a lot more grip on the front than previously.
The additional grip at the front means that the latest cars work their rear tyres harder than before.
We can certainly say that the current generation car has an oversteer tendency, where the rear of the car doesnt have as much grip as the front, and this tendency is a focus for teams in their car set-ups and designs, explains Hamashima.
An oversteering car is generally one that is good at turning in to corners, but one where the rear of the car has trouble following the path of the front. In American terminology, the car is loose, meaning the rear slides around. This is often fun to watch, and can result in the cars being spun, but its not necessarily the fastest way around a race track. So, if theres more grip at the front than there is at the rear, surely its time for more grip at the rear?
Of course, we could make more grip for the rear tyres through different rubber compounds or construction or wider tyres. However, we cant make use of different rubber and wider tyres, because of the regulations. When we make use of different construction, however it is the front tyre which has too much grip rather than the rear tyres not having enough, explains Hamashima.
When we moved to grooved tyres for the 1998 season, the opposite was the problem, as the front tyres did not have enough grip. To counter this we made a taller and wider front tyre. When the rules were changed to allow slicks back we recommended that the tyre sizes were changed back to the sizes before grooves came in - with smaller front tyres - however the teams car designs were already very far advanced for the same size tyres as used with the grooved tyres.
The size of the front tyre has a particular impact on a Formula One cars aerodynamics. The front tyres present a large surface cross section to the airflow and cause a lot of drag, so a big factor in the design of the cars bodywork and wings is trying to compensate for this.
For the future, Bridgestone has already tested a narrower front tyre and is working with the FIA and the teams regarding any potential change for the future.
For Bridgestone we can make front tyres of the current size, or of a narrower width, so it is not a problem for us once a decision is made about the tyre size, explains Hamashima. The size is set in the regulations, so it is not a change we would make in isolation, it is something that if it happens it will be because there have been consultations between us, the FIA and also the teams.
Bridgestone tested a smaller front tyre in Jerez in Spain back in March.
The test in March, where eight teams were present, was useful for evaluating a potential new front tyre size by six teams, explains Hamashima. We went narrower on width, but based on what we learnt from running that tyre we would probably want to go a bit narrower still. For now, however, we have the excitement of the 2009 season.