Under pressure - Bridgestone, tyres, and the importance of air 13 May 2008
Air, its all around us and its a vital ingredient for our everyday life, but it also fulfils a vital aspect of a racing tyre - keeping the tyres shape when it is fitted on a rim.
The correct tyre pressure is vital for any tyre to function correctly and for the high stresses of a racing tyre this is even more important. The difficult aspect, in particular with racing tyres, is the amount of heat produced when race cars are using the tyres to their maximum potential.
Air is great, but increase the temperature of it and its density changes. Change the density of the air and one result is that the pressure changes. More density makes for greater pressure and a harder tyre. Less density makes for less pressure and a softer tyre. For racing drivers and tyre engineers trying to get the optimum pressure for the tyre to work at its best with the car and track, this is a real challenge.
The reason air density changes with temperature is the amount of moisture in the air, or its humidity. The pressure of any gas is affected by temperature, but air with moisture is far more susceptible and the pressure change is more dramatic. For this reason, Bridgestone fills their Formula One tyres with dried air - normal air which has been processed to remove the moisture content.
Moisture in the air makes setting tyre pressures very difficult, explains Tetsuro Kobayashi, Bridgestone Motorsports technical manager. If you set the pressure in the pits with a cold tyre, or even one thats been warmed by a tyre blanket, the pressure will be different when the tyre has been brought right up to full operating temperature.
This happens even when we use dried air, but this happens in a more progressive and predictable manner than when there is moisture present. If there was moisture present then there would be different amounts in different tyres depending on when and how they were filled so it would be impossible to predict the pressure change and it would be difficult to engineer the tyres to deliver their maximum performance.
Whilst Bridgestone uses dried air, there are other gases available and teams will often try or make use of these in their pursuit of an advantage.
In many race series, not just in Formula One, we see use of gases such as nitrogen, carbon dioxide and other weird and wonderful combinations of gases, explains Kobayashi. The most important consideration for whatever gas is used is stability or a predictable change in density relative to temperature change to enable the optimum tyre pressure to be both attained and maintained.
So now we know what to look for in a gas, but how should we get it into the tyres?
Of course, when a tyre is fitted to a rim there is not a vacuum inside the tyre, there is already air inside it from the outside atmosphere, explains Kobayashi. This is why tyres are purged of this gas which is trapped inside during the fitting process and why you see rims with two valves to enable this process to take place.
As tyre pressure is crucial to tyre performance Bridgestone give teams a pressure range between which teams must operate for safety reasons and it is up to the teams to find the best solution within this range.
We give the teams a safe pressure range in which they must operate, explains Kobayashi. An underinflated tyre can cause structural problems with the tyre. It may tuck under or roll when cornering, or create rim slip and accelerated tyre wear. An overinflated tyre means a reduced contact patch with the road and therefore less grip and increased heat and wear at the point of contact.
These are not problems which will be seen as long as teams stay within our safe pressure range, although the exact pressure at which individual cars will work best is left for the teams and their tyre engineers to work out. One car and driver combination may work best towards the higher end of the designated range, others towards the lower end.
One area where tyre pressure has an interesting effect is with wet tyres. Tyre pressure affects the shape of the tyre, and where there is a tread this effect is magnified.
With wet tyres a higher pressure opens up the tread whilst a lower pressure closes up the tread, explains Kobayashi. So, just as pressure is important on a dry track, it is also important on a wet track.
Maintaining tyre pressures are clearly important for race cars, but for road cars too they are crucial.
Checking tyre pressures regularly allows the tyre to do its job correctly, explains Kobayashi. The car will be safer as it will handle more predictably whilst the life of the tyre is far longer when the tyre is correctly inflated.
Rolling resistance also increases dramatically with an underinflated tyre which means fuel consumption increases and a drivers motoring costs increase per kilometre. It doesnt matter if you are on the track or on the highway, you should always regularly monitor your tyre pressures.