Welcome to part eight of this racing concepts series. Remember: the most dangerous words in the English language are “I already know that.”
I got this idea from legendary motorsport coach Ross Bentley. This is how he keeps track of tire pressures while racing autocross.
I’ve added a few components because I like being overly thorough.
Monitoring your tire temperatures and tire pressures is the key to understanding what’s really happening with your tires.
To do this you will need three pieces of equipment.
- Tire pressure gauge. Ideally, you’ll want to find a digital gauge that can read in half pound increments.If you can find one that reads in tenths, that’s even better. You will probably spend $7 to $15 on a decent one.
- 12 volt air compressor. Unless you have deep pockets and a racing trailer, there’s really no need to get fancy here.A good quality compressor that you can plug into your car’s cigarette adapter will suffice quite nicely.I’m currently using one that cost around $25.
- Pyrometer. Pyrometers measure heat signatures and show you a numeric value in degrees, either Fahrenheit or Celsius.You’ll need one of these to note the temperatures across various points on the tire.There are two main types of pyrometers:
- Direct contact. This type uses a probe that you apply directly to the tread of the tire, which then creates a digital readout of the real-time heat signature.Direct contact pyrometers are cheap to buy, but can take up to 20 seconds for the heat of the tire to warm up the probe.By the time you make your way around the car, the tires may have slightly cooled off and you won’t get an accurate reading.
- Infrared. This type uses a laser to read temperatures.The results are instant, but the units can be relatively pricey.The infrareds are more versatile, and can also be used to measure brake temperatures, coolant, etc.
- I have used both, and I prefer the infrared type. The infrared unit I’m currently using is $40.
- It’s also helpful to have a method of reading ambient air temperature.But since most of us have phones that do that for us, it’s unlikely that you’ll have to spend money on this.
Collecting Tire Data
- Before you race the car, you’ll want to notate the cold tire temperatures (on the tread surface) for reference.If you’re starting from scratch, just begin with the manufacturers recommended pressures for your car.You can find these on a sticker in the driver’s door jam or in the vehicle maintenance manual.After you’ve warmed up the tires, whether it’s the first run of the day at an autocross event or a few warm up laps on the track, bring your vehicle back to the pit box.
- Measure the temperature of each tire in three locations across the width of the tread–the outer tread, center tread, and inner tread (Figure 20).Make sure that the tread is clean and free of sand or foreign debris or you may not get an accurate reading. A small brush works well to clear the surface.
Because the tires will begin cooling down rather quickly, this is a time sensitive operation.
Go back around to each tire and record the tire pressures.
- Measure the driving surface temperature.Since pavement is comprised of tar and small filler rocks (called aggregate), try to be consistent and measure the black tar.A light colored rock won’t show as high a temperature.
- Note the ambient air temperature.
- Depending on your equipment and level of enthusiasm, you should be able to collect 12 tire temperatures, 4 pressures, track temperature, and ambient air temperature in under one minute.
Analyzing Tire Data
Here’s where we put everything together.
As we saw in the Tire Management post, your tires will fall into one of three conditions– perfect pressure, underinflated, and overinflated. Let’s look at how tire temperatures correspond to tire conditions.
Perfect Tire Pressure (Figure 21)
We know that a perfect pressure allows maximum contact patch across the width of the tire–an even usage over the entirety of the tread.
We can see that a perfect pressure has been reached when the tire temperatures taken at the outer tread, center tread, and inner tread are all within 5 degrees F. This means that each section of the tread is gripping the track an equal amount, which is excellent.
Underinflated Tire (Figure 22)
We also saw in the Tire Management post that an underinflated tire tends to lift the center tread away from the track.
This is apparent when the center tread temperature is more than five degrees cooler than the outer and inner tread.
Overinflated Tire (Figure 23)
The overinflated tire bulges around the circumference and lifts the inner and outer treads away from the driving surface.
Taking tire temperatures will show us that when the outer and inner treads are more than 5 degrees cooler than the center tread, the tire is overinflated.
NOTE: Negative camber is a powerful asset for increasing lateral grip. If your car is camber adjustable, I highly recommend using that to your advantage.
Tires tend to produce the most efficient lateral grip when the top is tilted inward. Temperature readings should be progressively hotter from the inside to the outside in increments of 5 degrees F.
As a general rule, 5 degrees Fahrenheit is equal to 1 pound of air.
FOR EXAMPLE: If the temperatures of an overinflated read 115O/120C/115I, then the tire should be relieved of 1 PSI of air.
If an underinflated tire reads 120O/110C/120I, then 2 PSI should be added.
If the differences are less than 5 degrees I suppose you could experiment with half pounds of air to correct temperature differences of 2.5 degres F, but I haven’t found this to be necessary.
Are You Driving The Car Hard Enough?
It’s important to know your tires optimal temperature range. To obtain this information you can simply put in a call to your tire manufacturer.
If your temperature readings at the track fall within this range, you’re doing great.
If they’re low, the tires are still coming up to temperature or you’re not driving the car hard enough–it’s capable of producing more grip than you’re current driving style allows.
If your temperatures are too high, you’re over-driving the car (or not allowing the tires to cool between runs).
If you slow down, you’ll actually go faster.
If the temperature readings you take at the track fall into the tires optimal performance range, there’s a very high probability that you’re using the proper slip angle.
High slip angles will show higher temperatures as well. High slip angles will also overheat your tires. The driver is in complete control of this.
Is The Car Balanced?
If your tire temperature readings show that the front tires are consistently hotter than the rear, the car is telling you it has a natural tendency to understeer.
This means that the front tires are using larger slip angles than the rear.
This is common on front wheel drive cars where a large portion of the vehicle weight is over the front tires.
Likewise, if the rear tires are consistently hotter than the front, the car is exhibiting its naturally tendency to oversteer.
This condition shows that the rear tires are either spinning too much under power or the rear slip angles are larger than the front.
It’s very common for the left and right side tires of the car to show different temperatures.
On a left turn bias track, such as an oval, most of the cornering force is done by the outside tires.
This causes a higher overall right side temperature. In the same way, a right turn bias road course will produce more heat in the left side tires.
- If you aren’t using data to tune your car, you’re just guessing. Guessing is the same thing as hoping.
- The driver is responsible for staying within the optimal slip angles of the tire.
- All the data you need to reach peak tire performance can be collected and analyzed in just a couple minutes.
- Use your knowledge of tire temperatures to make logical changes to your car.If they work, great. If they don’t, go back to where you started and try something else.Trying new things is the key to progression.