Track and weather conditions play a significant role in a car's performance. The top pro teams keep track of every weather measurement imaginable. For your weekend romps, you'll need to keep track of only the few things that will have the most significant impact on the car. The things listed below apply to every car from street stock to retired formula racer.
Ambient Temperature -- colder air has a higher density than warmer air. This affects at least three aspects of car performance: starting tire pressures, engine power, and aerodynamic downforce. Record the ambient temperature at the beginning and end of every session.
The most significant affect of ambient temperature on a street car is the impact on tire pressures. When a fixed volume of air is cooled, it will contract to take up less space. When that air is in a fixed-size container, it will exert less pressure on the container walls. Air in a container which is heated will exert more force on the container walls. In tires (a fixed sized container), this is seen as changing air pressure. It turns out that every 10 degrees Farenheit causes approximately 1 psig change in tire pressure. You will need to compensate for this throughout the day as the ambient temperature changes. From morning through the peak temperature of the mid afternoon, you'll have to bleed a little air before each session. Then you'll probably have to add some back in for the last session. Knowing the ambient temperature will help you maintain the same tire pressure throughout the day. Refer to the Tire Notes article for more details.
To a small degree the air temperature also affects the power of the engine. The higher density of cooler air includes a higher quantity of oxygen for a given amount of space (volume). This has the same effect as using a larger intake system, and helps build more power which can help to improve acceleration or top speed. It can be quite noticeable that a car seems a little "torquier" on cooler days, and this might explain slightly higher speeds or rpms at certain markers than on a day of 100 degree F.
If your car has adjustable aerodynamic features, higher ambient air temperatures will also reduce downforce as the air density decreases. As a day warms up, you may have to tune in a little more aggressive wing settings to maintain the same downforce.
Track Temperature -- warmer track surfaces develop more heat in the tires, and will develop heat faster. Until the maximum effective temperature of the tire is exceeded, higher temperatures in the tires develops more grip. This will improve braking, cornering, and acceleration. On the flip, it is possible for a track to be too cold to build suffiecient heat in the tires. A cold overcast day, drizzly, and or windy days can keep a track surface very cold. Knowing that the track temperature was more favorable during certain days or sessions can help explain performance differences as you compare otherwise equal runs.
Record the track temperature at the beginning and end of every session (just meadure the paddock or pit lane durface if it the same material as the track).
Wind -- for a street car, or any car not designed to either utilize or adjust for aerodynamics, light wind is not a major consideration. However, if its downright windy on the day you're racing, you should make note of it as it may be hindering top speed on the straights, or cornering speed on long high-speed turns. A strong crosswind can affect the car's stability under braking, and it can affect the handling of the car in certain corners. You will want this information to correlate to your other notes regarding handling and lap times for the day.
Wind can also be a cause in keeping track temperatures low which will reduce overall grip. A certain track on day one at 90 deg F and no wind will be noticeably faster than on day two if it is 75 deg F and quite windy.
As you return to the same track,you'll want all the information possible to explain or correlate your laptime performances on different visits.
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