Features - Technical
JULY 14, 2000
Too Hot to handle
BY PETER WRIGHT
Racing cars are sensitive to temperature; something is always either too cold or too hot for it to operate at its optimum performance, and often too hot for it to survive at all. Test times during pre-season testing are often prefixed by whether they were set during the "optimum time of the day". However, it is Magny Cours, the venue of the French GP that epitomises the problem, with lap times varying by a second or more according to the conditions. Varying track performance does not matter during a race, but selecting the right moment to run in Qualifying at Magny Cours may make a difference of several grid positions and, on a circuit where overtaking is notoriously difficult, grid positions can be crucial to the results of the race. The issue is more than just choosing the optimum temperature for components and systems to work best, the temperatures of the track and the air are fundamental to the overall performance of the cars.
Apart form weight, which is unaffected by temperature, the main parameters controlling the performance potential of a car - power, downforce, drag and tyre grip - are all affected by temperature. The power any heat engine produces is proportional to the mass of oxygen it can pass through it in a given time, to combine with the fuel to create heat. If air is the only permitted source of oxygen, then, for a given volumetric efficiency (set by the configuration and details of the engine design), the mass of air passed through the engine is a function of the air density. Air density depends on temperature, atmospheric pressure and humidity, but of these, it is only temperature that changes at a rate that is significant to the cars performance during the crucial hour. Density is proportional to the temperature of the air when measured in degrees Kelvin, i.e. the temperature above Absolute Zero, which is -273¼C. If the temperature rises from say 20¼C (293¼K) to 22¼C (295¼K), the density will fall by 0.7%, and power will reduce by the same percentage - say 5.5 PS in an 800PS, Formula1 engine. A change of 5¼C and the driver will really notice the loss of power.
Air density also has an effect on the aerodynamic forces, most significantly downforce and drag. A 1% fall in air density will cause a 1% loss of downforce and a 1% fall in drag. Because both power and drag are governed by linear relationships, ultimate top speed is unaffected by temperature, but acceleration is reduced as temperature rises, and thus the achieved speed at the end of any straight may be reduced.
Losses in downforce are all loss. A 1% change in downforce is of the same order as that gained by a couple of months work in the wind tunnel. Running at a time when the air temperature is just 3¼C hotter than a competitor is the same as suddenly being 2 months behind him in the development race.
Tyre and track temperatures are somewhat more complex, and it is in this respect that Magny Cours differs from other circuits. Tyres generate grip via two main mechanisms: "mechanical grip" and "chemical grip". Mechanical grip, where the rubber tread compound conforms to the granular structure of the track surface and the shear strength and stiffness of the rubber determines the maximum contribution to the overall grip, requires that the rubber is hot enough to conform, but cool enough that its mechanical strength is not compromised. Chemical grip, where the tread achieves temporary molecular bonding to the track surface in a way that is analogous to a Post-it note sticking to a surface, requires that the tread compound is hot enough for maximum adhesion, but again, not so hot that it lacks strength. The right balance of compound and temperature to achieve maximum grip with adequate wear on any given track surface, is an extremely complex issue and is only understood by tyre compound chemists. Re-surfacing of a circuit instantly destroys that part of their database. The problem is further complicated by the changes in the track surface characteristics that take place as the cars run on it, laying down a layer of rubber onto the racing line. This layer of rubber tends to fill in the pores in the surface, which hurts mechanical grip, but puts a layer of adhesive onto the track. Try sticking a Post-it note to the sticky part of another Post-it note - they can still be peeled apart, but the adhesion strength in shear is much greater than when stuck to the non-adhesive part of the note.
Magny Cours had a particularly smooth, fine-grained surface that is perhaps more dependant and therefore sensitive to changes in chemical grip than mechanical grip. Though racing tyres have a definite optimum working temperature, it is quite wide at 110-130¼C. The compounds used at Magny Cours are probably optimised for chemical grip, and may well have a narrower optimum temperature range that requires just the right track temperature to work. Tyre temperature is a self-generating effect - the higher the grip, the more work the tyre does, the hotter it gets and the greater the grip, until the peak temperature is exceeded and the car slides, generating even more heat until the tread blisters.
When is the best time for a qualifying run? The simple answer is: When the track is at just the right temperature for the tyres, and the air is as cool as possible. The black surface of racetracks absorbs the suns radiated heat well, heating up quickly. Reflected and convected heat from the hotter track heats up the air in close proximity to it - the air the cars run in. Glider pilots have long been aware that road junctions are often good thermal sources - started by the bubble of hot air that forms on the surface of hot spots. I have seen a dozen or so competition gliders circling in a single thermal over Hockenheim on a hot race day. Their pilots also know that as soon as the sun goes behind a cloud, that thermal source, now in shadow, stops working as the source of heat is cut off. That is why team managers spend much of Qualifying staring at the sky, looking in the direction from which the wind is blowing the clouds. If he can get his man out onto the track as the sun disappears, he may just time his run right for that short period when the track is hot for the tyres, but the air is cooling to raise the density for maximum power and downforce.
Of course, the answer to: "When is the best time to run?" is somewhat more complicated and does not neatly respond to a few measurements and simple rules. The Tag-Heuer screens have a page that shows basic weather data - track and air temperature, wind direction and strength, and humidity - including data for a few hours previous so that trends can be monitored. However the most accurate way of determining what the track conditions are doing is to monitor the performance of other cars on the track. At the start of Qualifying, the early runners' times are compared with their times at the end of the previous practice session and deductions made as to their meaning. Once first runs are logged, they form a reference against which other competitors running at any given time can be made. Drivers stay in their cockpits towards the end of Qualifying and scrutinise their TV screen carefully in case a rival has lucked into a particularly good set of conditions. If they appear to have done so, they too will quickly get out onto the track in an attempt to benefit from that same combination of track and air temperatures.
Temperature is just one of the many parameters that must be got right to get the optimum out of a racing car when the time of being out on the track is open to choice. At the majority of tracks, the available choice of when to qualify does not make a great deal of difference, but at some tracks, such as Magny Cours under some weather conditions, it can have a large bearing on the grid order and hence on the race itself.