Why temperature changes can vex even top engineers
The super-smooth track surface at Magny-Cours possesses some very unusual characteristics. Ambient conditions at the French circuit have an unusually strong effect on lap times, as well as on the handling balance of a car. As Renaults Pat Symonds explains, it is a phenomenon that is still not fully understood, but one that experienced race engineers have learned to cope with.
At Magny-Cours, the challenge is one of anticipating and adjusting to large fluctuations in temperature. Last year, the circuit temperature rose from 18 degrees when we first ran on Friday to 34 degrees by qualifying - over the same period at Silverstone two weeks later, the temperature only increased three degrees in the same period. Furthermore, track temperature is not just a function of the ambient temperature, but as dark tarmac retains heat so well, it is also proportional to the amount of sunshine on a given day. On a cloudy day, the track temperature is similar to the ambient. On a sunny day though, it can easily be 20°C higher than ambient. Even two types of tarmac can respond quite differently and tarmac that is light in colour is less susceptible to heating from direct sunlight than one that is dark in colour. One often sees members of the teams and tyre companies checking the temperature of the track surface throughout the day, and this is even done on the day before the cars run to get information on the profile of temperatures throughout the day.
In general terms, the hotter the circuit gets, the less grip the car has. When we select tyres for a given circuit, we must take into account the nature of the track surface, and the work the tyre must do around the lap, in order to generate an average operating temperature between 120 and 140°C. For a circuit such as Barcelona, we will select a durable hard compound, while a circuit such as Monaco will require a softer tyre with greater grip, but less durability. Physically, the difference between these tyres is the degree to which the rubber molecules interact with the track surface: in a hard compound, there are a greater number of cross-links between the rubber molecules, which restricts the length of the rubber molecule that can interact with the track surface. Less interaction between rubber molecules and track surface means less grip. The reverse is true for the soft compound; fewer cross-links between molecules means that longer molecules can interact with the track surface.
In order to get a tyre performing to its maximum, we must generate operating temperatures that fall within the window mentioned above, and track temperature is a critical parameter for doing so. Throughout Friday practice, our tyre assessments are focused on resolving a simple equation that might be thought of as the Optimum Tyre Temperature is a function of both the Car Set Up and the Track Temperature. From this, it becomes clear that whatever we do with the set-up of the car, we must also take account of the track temperature in order to get the tyres working correctly. Above or below the tyre's optimum operating temperature, the grip provided by the tyre is lessened, and the further the tyre's operating temperature moves from this optimum, the less grip it provides. If track conditions alter significantly, or the tyre is inappropriate to the circuit's demands, and blistering can occur in extreme cases. As such, the extreme and rapid variations in circuit temperature at Magny-Cours that we have already mentioned, can seriously perturb the behaviour of the car and tyres if not managed correctly.
Regardless of the ultimate effect on lap time, the car balance - the degree to which the car understeers or oversteers - generally changes in a consistent manner with variations in track temperature. As a general rule, the hotter the track the more the car will oversteer, and as a general rule, because the race runs longer into the afternoon than qualifying, we can expect track temperatures to be higher - and this might lead people to suppose that the car balance we find for qualifying must take account of the first stint of the race, and potentially increasing amounts of oversteer, However, an unsatisfactory balance in qualifying can cost the driver a significant number of grid positions, and with the ever shorter first stints under the current format, any handling imbalance is something that can be rectified at the first pit-stop - rather than anticipated the day before.
"During the race, we are very limited as to what we can do to alter the balance of the car, but we can still make small adjustments to the front wing angle - and hence aero balance between front and rear of the car - and also tyre pressures. Lower pressures at the rear will give more grip, and reduce the tendency to oversteer; lower at the front, will increase front-end grip and neutralise oversteer. Changing tyre pressures alters the mechanical grip of the car, in order to try and alter the handling characteristics for different temperatures.