Tuesday, June 18, 2019

A Method to the Madness: Setting Tire Pressures for the Race Track

It was about two to three years ago when a friend at the track asked me the question, “what tire pressures do you run” when I first dove into the world of car setup.  The conversation went something like:
Me: I run whatever, I just like to drive FAST
My friend: Okay, but have you ever considered that you could be faster?
Me: What do you mean?  I am fast. I drive flat out.  I don’t care about my tire pressures.  Did you see that FWD drift in turn 3? I’m fast.  I don’t need to set tire pressures.
Drifting at New Jersey Motorsports Park Lightning Course
My friend: …Okay, Go out this next session and when you come into hot pits stop up at the wall and I’ll take your hot pressures for you.
(The “Ricky Bobby Effect” is slightly exaggerated here)

Note: Hot pressure = tire pressure after a full-pace session on track
Cold pressure = tire pressure in paddock or while cold, before on track
I was having issues with lockup going into turn 1 at New Jersey Motorsports Park (NJMP) Thunderbolt.  I felt as though I had more brake in the car, but the tires didn’t want to stop.  The tires felt sluggish and unresponsive to my inputs.

Turn 1: NJMP Thunderbolt

After my friend took my hot pressures once I came back into pit lane, we looked up the optimal hot pressure for the Nitto NT-01s I was running on some forums.  Based on our research, we decided to add one PSI to the tire while hot.  Miraculously, my braking ability increased, my lockups deceased, and my times decreased.

A man checking cold pressures at New York Safety Track

As outlined above, knowing one's hot and cold tire pressures can make a seasoned driver faster given different ambient conditions, but it can also improve safety for a novice track driver.

Cold pressures are typically set with a target hot pressure in mind. Because air expands when hot, we set our cold pressures low and the air will expand after a hot session, at which point we can record our hot pressure.

The hot pressure is the pressure the tire works best at. While knowing your cold pressure is important, it's your HOT pressure that will tell you if you're extracting the most grip out of your tire's contact patch.
As an instructor with the National Auto Sport Association (NASA) it's common to encounter first-time students that don't know whether they torqued their wheels, let alone set their tire pressures correctly. I have heard stories of students going on track with 60 PSI cold or 10 PSI cold and coming back with horrified, but safe, instructors.
A scared instructor
However with the right approach, you can be both fast and safe. For both approaches covered below, the assumed tools at disposal are:
1.) Car
2.) Race track
3.) A pen and paper
4.) Tire data (Google)
5.) A REPEATABLE tire pressure gauge
6.) A friend to help (optional)
Alex, TJ, and I going over tire temperature data at Watkins Glen

The First Approach (Easy but Less Optimal):
Anecdotally, tire pressure gauges are most accurate in the middle of their range. So for track days in a regular car, a 0-60 PSI gauge should do fine. This statement applies because for a street car, your hot pressures should be between 30 and 40 PSI on average.
You can purchase a tire pressure gauge from Joe's Racing, a reputable brand, on Amazon for a little over $20.

Joe's Racing tire pressure gauge

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!
A review of the NT01 from Race & Track Driving

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.

Taking a reading of tire temperature (photo from Turnology)

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).

Photo taken from Quickcar Racing Products

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.

Setting camber with a plumb bob, a stick, and some stuff and things.  Pic from LexiLaron.

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).


Thanks for reading all!  Have fun racing and driving.

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