When I started competing in autocross and time attack, one of the first lessons I learned was about the importance of tire pressure as it relates to tire performance and available grip. At first I was just guessing at what the right pressure might be for my car (a BB6 Honda Prelude) based on how much the tires appeared to be rolling over and wearing the outer shoulders.
This type of basic experimentation with tire pressures helped me go faster, but I had no idea how an increase or decrease in tire pressure was affecting tire performance across the entire contact patch. All I really knew was what the stopwatch was telling me. It was hardly a scientific approach, though, as lap times are also affected by changes in your driving, changes in driving conditions, and changes in your car's overall condition (brake wear and temps, engine temps, tire degradation, and so on). Eventually I started to take a closer look at what the fast guys were doing, and that's when I spotted a couple of the nerdier ones sticking a needlelike probe into the tread of their tires.
Turns out they were collecting tire temperature data with a probe-type tire pyrometer, a handy-dandy (and very affordable) device used to measure a tire's core temperature as a way of determining if it's within the temperature range at which it makes maximum grip. After this early piece of pit lane detective work, I formed a close working relationship with my own probe-type tire pyrometer, collecting temperature data every chance I get as a way of learning more about the relationship between tire temperature, tire pressure, wheel alignment, and other chassis setup variables like static weight distribution, cross weights, spring rates, shock rates, bar rates, and suspension geometry (all of which influence how each tire's contact patch is used).
Before getting into how to interpret tire temperature data, first let's briefly cover how to record tire temps accurately and consistently. First of all, avoid using an infrared pyrometer. These are great for measuring brake rotor temps or engine manifold temps, but because the surface of a tire cools quickly, you'll get a far better idea of what the tire is really experiencing out on the track by using a probe-type pyrometer that penetrates the rubber and gives you a core temperature.
Also keep in mind that your tires will cool down quite quickly when you're not pushing them to the limit on the racetrack, so to get the best representation of on-track temps, come into the pits hot, meaning you haven't allowed the tires to cool at all before pitting.
In terms of what the best technique is when using a probe-type pyrometer, you want to measure temps at three points across the contact patch, an inch or two in from the outer and inner edge and in the middle. I always start with the front driver side tire and work my way around the car in a clockwise direction, but the important thing here isn't the order in which you measure temps but to be consistent with whatever procedure you choose so you get repeatable and thus comparable data sets from session to session. It's also important to move quickly so the tires don't cool off while you're taking the temps, but it's just as important to leave the probe in the rubber long enough for its reading to stabilize. I leave the probe in the tire for a count of 3, as that lets the probe get up to temp and stabilize.
Once you've recorded all your tire temps, immediately record tire pressures in the same order. Again, it's important to move quickly so pressures don't drop as the tires cool. The objective here is to see the relationship between tire temps and tire pressure so you can quickly and easily get the tires into their optimum temperature range by starting them at the correct pressure. Higher tire pressure allows the tire's tread to heat up more quickly, while lower pressure slows this process. So if your car tends to overheat the tires quite quickly (experienced by the driver as a loss of traction, often described as the tires feeling greasy), for example, then lowering starting tire pressures is a useful way to combat this.
I personally use a good old-fashioned pen and paper data sheet when recording tire temps and pressures, but there are some very nice (if a little pricey) memory pyrometers that record the temps for you and will even average them for you afterward. The value of averaging tire temperatures (which you calculate by adding the three temp readings for each tire together and dividing the total by three) is that it can tell you whether you need to make adjustments to weight distribution or cross weights to better utilize the available grip at each contact patch.
When looking at average temps, the goal is to get the reading at each tire to be as close to identical as possible, which translates to each tire doing as much work as possible. If one tire is getting much hotter or staying much cooler than the others, it's an indication that you can adjust your chassis setup to better utilize that tire. Similarly, if one side of the car is seeing more temp than the other, some setup changes could help equalize the load on each side of the car. Same goes for front average temps compared with rear average temps.
Keep in mind that in the real world, it's very difficult, if not impossible, to completely equalize average tire temp at all four contact patches, especially when you consider that most racetracks have more corners going right than left (or left than right) and that FWD cars tend to have more front weight distribution and put more load into their front tires by virtue of using them to turn and put the power to the ground.
Rather than chasing perfect average tire temps, which often requires making time-consuming changes to my chassis setup (and if I'm happy with the handling balance of my car, I'm loath to make any big changes here), I tend to focus more on getting the tires used most at a given track into their optimum operating temperature range. For most high-performance street tires, DOT track tires, and racing slicks, this is in the 160-220-degree-F range, though many street tires and some DOT semislicks still work well at temps as cool as 140-160 degrees F. It's also important to aim for slightly higher temps on the inner shoulder of the tire, as this maximizes cornering grip. Most tire engineers I've spoken with aim for 10-20 degrees F higher on the inner shoulder, depending on the tire compound and chassis setup in use.
Keep in mind that these temperature ranges are ballpark figures, so you'll need to do your own testing to find out what temp range works best for your car and tire combo. On Project RX-8, for example, when running 265/35R18 Bridgestone RE-11 summer tires, I find the car starts to pick up some understeer, a sure sign of the front tires overheating, at around 180-190 degrees F, so I aim for hot temps in the 160-170-degree-F range (lap times have also confirmed that this temp range is generating maximum grip). To get all four tires into this temp range, my hot tire pressures are usually about 39 psi on the fronts and 37 psi on the rear, but this will change depending on ambient and track temps.
When I equipped the RX-8 with Hankook Ventus TD semislicks in the same size, I spent a full day testing and collecting tire temperature and pressure data before I discovered that this tire works best at temps in the 200-220-degree-F range. This meant running more pressure than I run in the Bridgestones, and I also had to make some wheel alignment and shock damping adjustments to work this tire up to temp and keep it there over a five-lap session. This just goes to show that every tire has a different temperature sweet spot, and every tire works a little differently with your chassis setup and driving style, so be prepared to make tire pressure and chassis setup adjustments whenever you change tire compound or tire width.
I've shared these two simple tire tuning scenarios in the hopes that it'll inspire you to begin recording tire temperature and pressure data at every opportunity and learn how to interpret and use this data to dial in your setup for maximum tire performance. No other type of chassis setup data is so easily and affordably collected, making a tire pyrometer the single most valuable tuning tool you can buy, assuming that maximizing grip and thus speed around an autocross course or racetrack is your objective.
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