Riders on the storm - wet-weather tyres explained 30 Sep 2008
Either by luck, or shrewd placement on the calendar, most Formula One races occur in dry conditions. However, as has been seen more than once in the past year, rain does fall during races and a wet event is a very different proposition from a dry one.
From hot, dry weather to heavy rain, Bridgestone has a motorsport tyre to suit, and as Formula One prepares for its annual visit to Japan - which last year saw one of the wettest races of the season - thoughts turn to the wet-weather tyres.
Just like at any Formula One race meeting, each driver will have four sets of wet and three sets of extreme wet race tyres for the weekend should the rain start falling. This contrasts with the 14 sets of dry tyres available for every driver, broken up into seven of each compound.
I cannot remember a race weekend when every session was wet, but even if we did face that situation we would have enough wet-weather tyres for a Grand Prix to take place, says Hirohide Hamashima, Bridgestones director of motorsport tyre development.
There is only half the amount of wet tyres available but this is due to how wet tyres work, and therefore not as many are required.
A dry compounds stickiness is similar to packing tape in the way the tyre sticks to the road, explains Hamashima, but the way a wet compound works is more like flour - when you add water it gets sticky. The way the wet tyre works means that it lasts longer in pure wet conditions than a dry tyre does in the dry. We also see less wear on a wet tyre. A dry tyre would also last a lot longer in wet conditions, but a dry tyre in the rain does not provide as much grip.
Of course, when it rains it is seldom wet on track in a consistent manner. When it rains the grip levels vary tremendously, says Hamashima. And that is one of the reasons why a wet race can be so entertaining for spectators, as drivers never know exactly how much grip there will be from the track on each corner.
Bridgestone makes two wet-weather tyres. Although known to some people by different names (most commonly intermediates or inters, and wets), the official FIA terminology is for a wet and extreme wet tyre, and these two varieties fulfil different purposes.
The extreme wet tyre is used when there are water puddles on the track, and this tyre is capable of displacing twice the amount of water of the wet tyre. The extreme wet has a deeper and more prominent tread pattern than the wet tyre, and it is the tyre which most closely resembles a passenger car tyre.
In contrast, the wet tyre is used when conditions are not so extreme and there are fewer puddles and standing water.
The most difficult part of the equation is choosing which tyre to use when the conditions are changing, explains Hamashima. Matching the tyre to the conditions is crucial and this was well illustrated this season in the British Grand Prix when the conditions changed rapidly and being on the correct wet tyre meant a much faster lap time than being on the wrong one.
The tread patterns of these tyres are a very sophisticated area of tyre development, and one which does not only have an impact in motorsport.
We use hydro-simulation for our development of wet tyre tread designs in motorsport and knowledge we have gained here is very useful for our passenger vehicle tyre tread developments as the concepts are very similar, says Hamashima. A successful wet tyre for motorsport can cope with a range of conditions, and a tyre for the road certainly needs to cope with a wide range of situations.
In motorsport we try to get a wet tyre that can work over a relatively wide range of conditions, ranging from damp to very wet. Also, the tyre should allow the same balance for the car as in dry conditions, which means the teams have a wide range of strategy options and a greater chance to win. For a road vehicle, drivers do not have the luxury of making a pit stop when it rains so a passenger vehicle tyre has to be able to face all possible weather conditions.
Of course, a wet or extreme wet motorsport tyre can be used in the dry, but not for long. Wet tyres are by definition designed to be used in wet conditions and they do not work well when it is dry, says Hamashima. They wear very quickly and also heat up rapidly, and that is why we see drivers using the wet parts of the track when they are on wet tyres on a drying track and their strategy is not to pit at that time.
And what of the future of wet tyres in Formula One? The Bridgestone-supported GP2 Series uses a very clever single wet tyre with a sloping block tread profile, meaning that as it wears when a track does dry, it changes characteristics from a tyre similar to an extreme wet tyre to one more like a wet tyre.
We have learnt a lot from our Formula One wet tyre development and our GP2 single wet tyre is an interesting area of development, says Hamashima. We do have a new single wet Formula One tyre which we have been developing. However, the current regulations call for two wet tyres, a wet and an extreme wet, so it would be a long time before we could see this tyre in use at a race weekend.