After obtaining your tire pressure gauge and going to the track, write down the ambient conditions on your paper with your pen. This should be done before each session. You should ask:
1.) Is it cloudy?
2.) Is it raining? How heavy if so? If heavy, a lot of this article isn't applicable, so stop reading now.
3.) What is the humidity level?
4.) What is the ambient temperature?
5.) What are your cold tire pressures?
Next, go to Google and search the interwebs for what people with your tires on similar cars like to set their hot pressures at for the track. It's important to get multiple opinions to get rid of the incorrect data, or noise, from your observations. Forums, Facebook, hobbyist websites, and manufacturer websites are some sources you can mull through.
Note: You can also just TALK TO PEOPLE at the track!
Once you know a base for your starting hot pressure, write that down, set your tire pressures at a reasonable cold pressure, and head on track. For most Civics, Integras, Miatas, and E30s with 15-inch tires, at most ambient conditions, I find 30 PSI cold to be a reasonable starting pressure.
Note: Your cold pressure must be lower than your target hot pressure. Air EXPANDS when hot!
Next, drive the car. Go out for a full session on the tires. If the car feels funny at all, try to drive around the car's character. If the car feels unsafe, bring it in. Not only will driving around your car's imperfections improve car control and driving skill, but it will allow us to reach our final goals which are:
1.) Observe what your tires feel like (very important)
2.) Get enough heat in your tires to get an accurate hot pressure (also very important)
Lawnmower life
If you've observed what your tires feel like, you should know to a certain degree what your car feels like under braking and turning. We will now use the following table to determine whether your car's tires are under-inflated, over-inflated, or just right:
Figure showing car "feel" relative to tire pressure
If, per the table, your car's tires feel under-inflated or over-inflated, adjust the pressures as necessary. Repeat the steps of adjusting tire pressure and driving the car as necessary until you find a happy spot.
For a front-wheel drive car, when a hot pressure works well for the front of the car, I've found that the same hot pressure should be set at the rear of the car. However, I'm close to certain that this is the right approached based on feel, but also based on data, which we will now cover in...
The Second Approach (Hard but Betterer)
For the second approach, we need one additional tool, which is:
1.) A tire pyrometer
A tire pyrometer uses a probe, inserted into the tire's tread, to measure the temperature of the tire's rubber.
The temperature of your tire's rubber is probably the most important measurement you can have for your tire, after your hot pressure. This is because not only does tire temperature tell us about whether our pressures are set right, but it also tells us things like:
1.) Where the majority of the weight lies (front or rear of the car)
2.) How the spring rates or sway bars we've chosen affect tire loading
3.) Are our camber settings correct for this track?
A tire pyrometer
Some will use infrared (IR) temperature guns to read tire temperature when the car has pulled into pit lane, but this is not as accurate.
IR Temperature gun reading engine bay temperature
Part of why infrared guns are not useful for measuring tire temperature once the car has stopped and is in hot pit lane is because they only measure the temperature of the surface of the tire. Tires cool extremely fast after a hot lap. As a result, the surface temperature will not be representative of the tire's actual temperature at pace.
Additionally, user and machine error (distance from measuring target and surrounding brightness) can also affect the measurement.
Tire pyrometers can be found used on eBay for ~$50 if you wait for a good deal to pop up. Mine was purchased two years ago (2017) for $50 in used condition. It sits in the glove box of my Civic during practice sessions. My friend Alex's was purchased this year (2019) for $60 with a carrying case. He will be using this at upcoming events.
Alex (owner of TrashTeg) and Brian going head to head in their Honda Challenge H4 cars.
In the second approach, we do most of the same as the first, initially.
1.) Record ambient conditions
2.) Find optimal hot pressure for tire and car
3.) Set cold pressures as described above
4.) Finally, go drive the car-- observing how the car feels
Where the second approach differs starts with when we pull the car off track. In the first approach, we may wait until the checkered flag is thrown. After this, we may have a cooldown lap and bring the car in to record our tire pressures. However, remember that I said that tires cool extremely fast.
So, we pull the car in a lap or two early after running it at full pace. We come to a full stop in hot pits, and we have a friend immediately record tire temperatures-- or we do this ourselves. If you're going to do this yourself, put your pyrometer and tire pressure gauge in your glovebox.
Recording tire pressure at Watkins Glen in a racing suit
There are three tire temperature measurements to record for each tire-- a total of 12 measurements. We want:
1.) Inner tire temperature
2.) Middle tire temperature
3.) Outer tire temperature
I typically record my tire temperatures first, in order of driver front tire, passenger front, driver rear, and passenger rear for a clockwise rotation track. Then, since tire pressure decreases less rapidly with time, I record tire pressures second.
There are printouts available online that make the recording process easier. Or, you can make your own printout. Digesting the data from a printout is much easier than a bunch of scribbles in a beat up book (which is what I do).
As mentioned above, tire temperatures can tell you a lot about the car beyond what your pressures should be. This is why I prefer this method. I'll briefly go over what temperature can tell us for camber and chassis loading and get back to specifics about hot tire pressures.
Every tire has an optimal average temperature.
Average temperature should be close to that optimal temperature and is mainly driven by weight-to-tire-width ratio, spring rate, and other variables.
Camber and other alignment/chassis setup factors can affect tire temperature distribution from the outside to the inside. Generally we want a 10-15 deg. F. distribution from inside to outside with the inside hotter than the outside. This tells us that the camber is adequately negative for a given track and chassis setup condition.
However, for the sake of this article, we're looking for two things:
1.) The middle of the tire is not hotter than the outside and inside
2.) The middle of the tire is not colder than the outside and inside
Insert Civic picture here
We want to set the hot tire pressure so that the middle lies in between the outer and inner temperature. The procedure for adjusting hot tire pressure based on a pyrometer reading is as follows:
1.) If the middle temperature is higher than inner and outer temps, the hot pressure is too high-- lower it by 1 PSI
2.) If the middle temperature is lower than inner and outer temps, the hot pressure is too low-- raise it by 1 PSI
Continue adjusting tire pressure per the procedure above until the middle temperature is in between the outer and inner tire temperatures.
Once the tire temperature distribution is satisfactory per the above requirements, we employ the feel technique which we covered in the first method. Based off of experience with a car of the following specs, I find a range of ~5-6 PSI where I can adjust hot pressures to influence car feel:
2500 lbs Civic
B18B1 Engine (stock)
Integra brakes
Toyo RR 15/50 R205 tires
Once the car feels good, feel free to rip. Take the data from your notes home, read up on suspension setup, make tweaks, and be faster than before (hopefully).
Peace
Thanks for reading all! Have fun racing and driving.
