All suspension modifications and adjustments are made to increase tire performance. Outside of lap times, tire data is the most significant data you have available for determining a car's performance, and is the data by which most suspension adjustments are determined.
Tire data consists of the tire temperatures, pressures, and appearance of the wear pattern.
For all the emphasis you'll hear in the paddock about tire pressure, it is actually more important to collect tire temperature data than pressure data. Ultimately, all suspension adjustments including tire pressure are aimed at maximizing the tire contact patch, and temperature data is the most direct measurement we have to optimize that. All other adjustments can be made in "less" or "more" increments without knowing exactly what they are.
To avoid getting focused in the wrong place, it is important to understand the role tire pressure plays. Tire pressure is an adjustable element of the complete suspension system of the car. Wheel camber, caster, and toe, shock compression and rebound stiffness, spring stiffness, anti-roll bar stiffness, aerodynamic downforce, tire design, and tire pressure all play a role in maximizing the tire's contact patch for maximum grip and handling performance.
One of the things that makes tire pressure different than the other settings (except aerodynamics) is that tire pressure changes during a run session. When you start a session with an adjustable shock setting of "4," or a camber of -2 degrees, they'll still be the same at the end of the session. Not so with tire pressure. It rises due to the friction generated between the rubber and the track surface. Tire pressure has to be estimated at the beginning of the session to account for this rise so that it ends up being what you want after about three laps. This estimating part is what people get confused about. This leads to the paddock obsession with "correct tire pressure," but you must remember that tire pressure is not an end unto itself, it is a means. The real goal is not a "correct" tire pressure, but maximized handling grip. There is no "correct" tire pressure, anymore than there is a "correct" shock setting, camber angle, or sway bar stiffness. All of these items must be balanced.
While most cars will utilize a similar tire pressure range, the difference of one psi will make a noticeable diffence in handling to even a moderately experienced driver. Two pounds can make a major difference in a car's overall grip level that even a novice can recognize. Therefore, asking what someone else uses for a tire pressure is completely irrelevent information (unless it's the exact same car setup). You must do the testing on your car to determine the optimum pressure setting for a given track and ambient conditions.
How to test tire pressures and interpret the data is covered in the Tire Pressure article of the Handling section. The rest of this article is focused on what data should be collected, and how to do it.
In preceding articles, we've mentioned the need to collect ambient and track temperature information, and lap time data. In the next article, we'll talk about making notes about handling performance. All of this information is needed in combination with tire data ultimately to make judgements on adjusting the car's suspension system (alignment, shocks, springs, sway bars, tire pressures) for maximum tire grip and handling performance.
When it comes to your tires, you need to collect the following information:
The nature of weekend racing makes it difficult to take valid cold pressures before each session. However, take pressures in the morning before driving, or before the first session. If you drive to the track early enough, there should be enough time after inspection, driver's meetings, etc. to allow the tires to cool down (optimally about an hour and a half). If you don't drive the car during lunch you can re-take cold pressure data before the afternoon sessions. If there's about an hour and a half between sessions, this is enough time to record cold temps before each session.
The last three items must be taken "hot." You will have to come in to the pits to take these measurements after you've been driving at maximum lap speeds. If you wait until after the cool down lap, all the data will be useless. The tires cool from their peak temperatures and pressures very quickly. The flagman at the start/finish line should use either the white flag or hold up one finger to indicate the next lap by will be the checkered flag. Even if you have to keep track of laps yourself, come in before the checkered flag, and avoid being stuck in a cool down lap.
When taking tire data it's best, but not critical, to have two people, one to take the measurements, and one to write them down. The faster the data is taken, the better.
Each time you take measurements do it in the same pattern. Consistent measurement technique is important to not introducing additional variables into the data. Always go to each tire in sequence the same way (such as front left, rear left, front right, rear right). The sequence should start with what you expect to be the hottest tires first. If the pit entrance is after a series of right turns, then the left tires are likely to be hottest, and you should measure them first. This will give you a better idea of the peak pressures and temperatures the tires are experiencing on the track. If there is a close series of hard braking zones beforethe pits, you may need to measure both front tires first. Look at the last quarter of the track before the pit entrance, decide which tires are likely to be worked the hardest, and measure them first.
There's no secrets to taking tire pressure data. Use a high quality guage (preferable an oil-filled, dial guage with a maximum reading of 50-60 psig), and do it quickly and accurately without bleeding air while fiddling to find the right spot.
Do not make adjustments as you meaure each tire. Measure them all first, them go back to make adjustments.
The purpose in having hot running tire pressure data is to correlate the pressure with the tire temperature profiles, and the handling performance of the car. When you find the sweet spot of the car's handling at a certain temperature profile and temperature, this can then become a target for future set ups.
To take temperature measurements, you need to measure three points on each tire: the oustide, center, and inside of the tire. If you are racing on treaded tires, the probe should be positioned in the center of a tread block. In all cases be sure to insert the probe at least 1/8" into the rubber, and be consistent. The outside and inside probe spots should be about 1" in from the edge of the tire. On a treaded tire, use the second tread block from the edge. If you measure the very outside edge, you'll likely get low readings.
The sequence of the three probe spots doesn't matter, but do it the same way all the time. Also, don't wait too long between each sample. At times the temperature will fluctuate between two or even three numbers. Pick one and move on. The lowest or highest number doesn't matter, just be consistent in which way you pick. The differences of 1-2 degrees is not going to be important. You are looking for trends in 5-10 degree range.
See the Tire Pressure/Tire Temperatures article for more details.
Read Next Article (Handling Notes)