A Facebook question led to Sarah Fisher Hartman Racing head engineer Nathan O’Rourke explaining the impact of temperature on the race car — in layman’s terms. We all thank him for that.
Ambient weather conditions, including temperature, have profound effects on both car and driver. This may seem like a simple question, but temperature has an influence on many different aspects of the car, and the ideal temperature for each generally isn’t the same and varies across the different types of tracks. Engineers are continuously monitoring and logging ambient temperature, track temperature, ambient pressure, humidity, and wind direction/speed and making adjustments as necessary to re-optimize the car’s setup. Instead of giving you our priority list for best adjusting the parameters affected by weather for a particular track, I’ll just go over a few of the primary temperature-dependent components of the car and the reader can decide which would be more important than others at a particular track.
Driver: I’m not a human performance specialist by any means, but in general it’s the cooler the better. Cooler temperatures reduce the chance of dehydration and help maintain consistent driver performance in the car during a race. Dehydration not only effects physical performance, but as we’ve often seen at the end of long, hot races when people start making mistakes, it can tend to effect cognitive/judgment skills and tempers! Even in the coolest of temperatures in which we run, the driver is still sweaty when he/she gets out of the car, and I’ve never heard anyone complain of being cold in the car, so other parameters would prevent us from running in temperatures that would ever be too cold for the driver (i.e. tire temps).
Engine: Engines run better with a cooler air inlet charge because it increases the air density (more oxygen available for a given volume of air). The role of the turbocharger is to increase the density of air entering the engine through increased pressure. Since the rules provide a maximum boost level, the density of the air entering the engine is very consistent regardless of ambient temperature or pressure because the turbo is maintaining this consistent “boost” level. From a temperature standpoint, the inlet air temperature is normalized to some extent by the heating of the inlet air that occurs while passing through the turbocharger. Some turbocharged engines use devices called intercoolers to cool the compressed air charge between the turbo and engine, but this isn’t the case with the current IZOD IndyCar Sereis specification. Turbocharged engines are considerably less influenced by ambient conditions than the normally-aspirated engines we had prior to 2012, as in the past a decrease in temperature resulted in a very noticeable increase in power and fuel consumption.
Aerodynamics: This is somewhat track dependent, but in general with the Dallara chassis and the current regulations and trends of setting a maximum downforce level for a particular track, this is another area where it’s generally the cooler, the better. Since cooler temperatures result in higher air density, the cars produce more downforce and drag with cooler temperature by increasing the amount of air passing over the car. Less angle of attack or smaller wickers are required on the front and rear wings to produce a given level of downforce in cooler temperatures, which in turn makes them more efficient (less drag for same downforce). If you take the 2012 Fontana race as an example, it seemed in the very hot daytime practices that the race was going to be extraordinarily difficult because of the low downforce level, but the 10 degree cooler temperatures during the night race added enough downforce that it wasn’t that bad. One thing to note in terms of the ambient temperature is that because IndyCars ride so close to the ground, the temperature of the air feeding the car’s aerodynamics is usually considerably warmer than the air temperature as measured at a weather station. On a 90 degree day, the track temperature might be 130 degrees, which is probably more representative of the temperature of the air passing over the car’s various aerodynamic devices.
Tires: Tires in general are very temperature sensitive, not only in their ultimate operating temperature, but also in the process by which they are brought to operating temperature. Firestone does a fantastic job of building tires that are very consistent and durable, that aren’t overly sensitive to abuse or using the incorrect processes of generating temperature (i.e. not to underscore the importance of this in IndyCar racing, but this process seems to be much more sensitive with European formulas and tires). Cooler track temperatures are generally better for tire wear and maintaining a higher grip level over a longer period of time, but there is a disadvantage in that it takes longer to generate tire temperature and get to the optimum running temperature. Warmer track temperatures require less time to reach ideal operating temperatures, but are generally worse for tire wear and require the driver to be extra careful about not abusing the tire and generating excessive temperature which will quickly accelerate tire degradation. The ideal situation would be to have pitlane tire warmers and cool track temperatures, but this is a bit of a fantasy situation for us as tire warmers are not permitted in IndyCar.
Final Answer: So what does all this mean? I’d like to just answer the question simply and say that the optimum temperature for an IndyCar is 78 degrees, and this applies to all tracks, but that’s not the case. With the diversity of tracks and aero packages in the IZOD IndyCar Series, the “optimal” weather condition for a given track may be decidedly different than another. As you could expect, the sensitivity of the car to weather and the emphasis we place on all of the parameters effected by it are going to be much different at a bumpy, high downforce street circuit like Detroit than it is at a smooth, high-speed low-downforce oval like Indianapolis. It’s important for engineers to closely monitor not just temperature, but all aspects of the weather to ensure that the proper adjustments are made to best optimize performance at a particular track. Thankfully, all of our manufacturers, including Honda, Dallara, and Firestone, have given us excellent products that have been developed to provide a wide range of adjustability which allows the teams to optimize all aspects of the car in a variety of different weather conditions. As it is with many things, the extremes are always bad (way too hot or way too cold), but we know from experience that the current IndyCar is capable of putting on a great show regardless of the outside temperature.