Why tyre-wear forecasts can win or lose you a race
It will be a critical factor at Hockenheim and is a term often used by Formula One engineers. But what exactly is tyre degradation, and why does it matter? Renaults Pat Symonds explains
In simple terms, there are two main factors affecting race strategy: the amount a car is slowed down by carrying a heavy fuel load, the 'fuel effect' (usually represented as a penalty of x seconds for 10kg of fuel), and the amount the car is slowed down as the tyres 'wear'. Tyre degradation is a complex subject because not only does it vary circuit to circuit, but also year to year at any given circuit, and even day to day. The reason for this is that there are three fundamental types of tyre degradation.
The first of these is linear degradation: this is similar for both front and rear tyres, and dependent on their wear. In other words, if tyre performance is such that the car loses 0.1 second of lap time between the second and third laps, it will also lose 0.1 second between laps 22 and 23. During a race, the level of linear degradation can vary from nothing up to 0.15s depending on the circuit and the conditions. Linear degradation does not cause any change in the car balance, provoking neither understeer nor oversteer.
The second type of degradation is graining. This is a phenomenon that has become much more prevalent since the introduction of grooved tyres, although even slicks can suffer from it. Graining occurs when the surface of the tyre grips sideways and rolls up due to the shear force on the rubber. It is very easy to spot as the rolled-up rubber remains on the tread. In most cases, graining is seen laterally on the front tyres although on some circuits, it can occur longitudinally on the rear tyres. This happens when there are a lot of traction events, such as in Canada. However it is much more common for it to be seen at the front at circuits such as Imola which are 'front-limited' (where the front tyres are under greater stress than the rears). Tyres are prone to graining when temperatures are low and the rubber is perhaps stiffer than is ideal for the conditions. The result is an increase in understeer, but as the tyre wears, the graining disappears (the 'loose' rubber is cleaned off the tread) and the tyre behaves normally. The performance curve created by graining is illustrated by the second line.
The final type of degradation encountered is the type we will be more concerned with at Hockenheim. Blistering is so-called because the tyre does literally form heat blisters which are then ripped off the surface. This is commonly seen on the rear tyres, and is generally due to a combination of high acceleration and hard cornering forces through long, slow and medium speed corners With this type of degradation, the tyre behaves normally at the beginning, suffering only from linear degradation, until it gets to the point when the rear tyres blister, which can be quite sudden. The car then starts to lose lap time through oversteer and a loss of traction caused by the loss of rear grip.
The propensity of the tyre to blister is very much a function of the core temperature of the tyre, in other words the temperature of the rubber below the surface. While the surface temperature of a tyre can build up quite quickly, particularly through the tyre sliding, the fact that rubber is a good insulator means the high surface temperatures do not penetrate quickly to the core. Core temperature increases occur more from the work the tyre does and these temperatures, even under arduous race tyre conditions, can take 8 to 10 laps to stabilise. A blistering tyre therefore will behave well until the core temperatures reach the critical point, and the tyre starts to lose its tread as the blisters occur. Blisters are generally of similar size, and 'bad' blistering is therefore the consequence of a higher number of blisters on the tyre.
Since the circuit revisions at Hockenheim, the demands the circuit places on the tyres have changed significantly. The new tarmac has significantly higher grip levels than the old surface, and the nature of the circuit is radically different: previously composed almost exclusively of heavy braking and traction events, it now includes a much higher number of medium speed corners. As such, the set-up compromise required has altered radically, and the circuit now requires much higher downforce levels. The result is much higher speeds through the stadium section (an average rise of 16%), and for the tyres, the numerous acceleration phases out of these corners have made Hockenheim a 'rear-limited' circuit.
Once these factors have been taken into account, our data analysed and the delicate choice of tyre compound and construction has been made, we then hope that the only unknowns left are the ambient factors. However, the weather is such that it can make a finely-balanced choice look like inspired genius, or over-ambitious miscalculation.