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Car Tire Pressure

Part 1 Part 2 Part 3 Part 4 Part 5

Starting "Somewhere"

For your first racing experience, or your first experiences with a new tire, you'll need to first determine the realm of the gross tire pressure setting. Assuming there's no one to tell you how your tire and car combination are best set up, you'll need to experiment to know whether the car handles better in the 30-34 psig range, the 33-36 psig range, or the 36-40 psig range. Once you have a feel for this, you can fine tune to within 1/2 to 1 psig for optimum handling balance and performance.

There are two points from which to set tire pressures. First, you have to adjust the cold (ambient) pressures before the car takes to the track for the first time, or after it has completely cooled off between sessions. After the tire has reached full racing temperature, it is possible to fine tune from the fully heated point, but it will also be important to know when the tire has cooled back down what it's cold temperature is.

We know the tires are going to heat up when they're driven on. The friction between the rubber and the road will generate heat. A lot of heat. This heat is going to transfer to the gaseous air in the tire, and cause it to expand which leads to an increase in the tire pressure. It happens to work out that an increase of about 10 degrees F causes about 1 psig increased tire pressure.

On the track, you're going to work the tires much more than is possible on the street. Repetitive, high cornering forces and aggressive braking are going to generate much more heat and therefore higher tire pressures. Because of this, the typical pressures you run on the street are going to be too high for the race track. During normal street driving, tire pressures will increase over the recommended factory "cold" settings about 2-4 psig. During racing, a cold tire pressure setting will increase anywhere from 6 to 10 psig. Tire pressures must be lowered from their usual street settings before going onto the track.

As with just about everything, the "correct" starting pressure "depends." There's a lot of variables. Still, you have to start somewhere, then you can test various pressure settings to find the best balance in performance. Remember, even the pros have to test several settings every race weekend to find the best performing point. There is no formula, no "right" answer that you can use every time.

So, if you're using street tires on the track, the generic, "start somewhere" point is 5 psig lower than the car manufacturer's recommended tire pressures. (Note: use the car manufacturer's recommended tire pressure as the baseline. Many service and oil change stations with poorly trained techs will pump tires up to pressure stated on the tire side wall. Hello! That's the maximum setting, not the recommended setting. Watch out for those guys!)

From a cold starting point of 5 psig less than street pressures, you can begin the trial and error cycle of making small adjustments and determining the effect. To do this, you will need to run several laps to get the tires hot to start with. Take it easy the first 3 laps or so until the tires come up to temperature. The car will be quite sloppy with the initial low pressures.

After 3 or 4 laps, the tires should feel consistent. Run several more laps to get a feel of the overall grip level and the handling balance. If you're held up by traffic, stay out until you get at least three maximum-effort laps. Then come in to take the pressures and temperatures, and report on the handling. Do not make a cool down lap before coming in as the tires will cool down significantly making the tire temperature data useless (When you park, be sure to not use the parking brake, or rest on the brake pedal as you can easily warp the brake rotors).

If you have a temperature probe, you can use the tire profile reading as the primary guide to tell you if the pressure should be increased, decreased, or left as is.

  • if the centers of tires are 5 or more degrees hotter than the edges, then the tire pressure should be lowered. Try about 1 psi for each 4 to 5 degrees the center is higher than the lowest edge temperature.
  • if the center temperature is more than a few degrees lower than the edge temperatures, then the tire pressure is too low. Try increasing it by 1 psi for each 4 to 5 degrees the center is lower than the highest edge temperature.
  • use the guide below for more detailed tire temperature interpretation tips.
Description Temperatures
Left Front Tire
Temperatures
Right Front Tire
Interpretation
Center is low by more than 5 degrees of the highest reading 175 168 173       Tire pressure is too low. Increase 1 to 2 psig per 5 degrees difference
Center is high by more than 5 degrees of the lowest reading 168 176 169       Tire pressure is too high. Decrease 1 to 2 psig per 5 degrees difference
Inside edges always hotter than center or outer edges 160 162 170 172 165 162 Too much negative camber
Outside edges always hotter than center or inner edges 172 166 162 163 165 170 Not enough negative camber, too much body roll during corners (causing positive camber). Try stiffer springs or anti-roll bars.
Left or right edges of tires hotter than center and opposite edge 172 166 162 170 165 163 Probably the last set of turns before exiting the track are long or hard rights. This could be normal. Otherwise look for alignment errors.

If you do not have a temperature probe (get one!), then use this as general guide.

  • Generally the tire pressures should be close to equal on a car with neutral handling. If the front or rear tire pressures are significantly different, you can adjust them to a mid point between the two readings. (If the fronts are 38 psig, and the rears are 32 psig, then try adjusting them while they're still hot to all be 35 psig). Try this setting for the next session.
  • If the tires felt very slippery, especially if they got worse on every lap, then the tire pressure is too high. Take each tire pressure and write it down. If only the front or rear tires felt slippery (the front was slippery if the car understeered and wouldn't respond to more steering input, the back was slippery if the car wanted to oversteer and spin), then lower the pressure of just that pair by 4 psig while they're still hot. Try this setting for the next session.
  • If the tires felt spongy, then the tire pressures are too low. When the pressures are too low, they will have low grip and still tend to slide like a tire that is too hard, but the feeling will be different. Instead of a smooth slippery slide, the tire will scrub and have more vibration in the slide. The difference is subtle, but you'll have feel for it in the steering wheel, and from the seat in your legs and back. In this case, increase the pressure while the tires are hot by 2 psig. Try this setting for the next session.

For the first few sessions you'll want to make large pressure adjustments of 2 to 4 psig. This way in about three sessions you can note whether the car felt better in the low 30's, mid 30's, or high 30's psig range, Once you feel the car behaves better in one of these ranges, you can move on to 1 psig incremental changes to close in on the optimum setting. If after 5 or 6 laps you know the car is just not right, don't force it. Come in and make an adjustment. There's no point to driving a really poor handling car and risking an off track excursion over it.

Once you have some experience with your car, you may find that the best starting point is only 2 psig less, or as much as 6 or 8 psig less than the street pressures you run at. There is a large difference in cars, tires, and driving style that affect this. However, starting at 5 psig lower than street pressures should prevent most tires from becoming excessively hard and slippery, and from exceeding their maximum safe pressure.

Fine Tuning

Once you know the general tire pressure range you're aiming for, you can make smaller increments to target the optimum setting.

If you're racing on a particular brand and model of tire for the first time, or you've never taken tire data before, you have some trial and error work to get through, and this will take some time. Depending on the track time available to you, it may take more than your first event to close in on the optimum pressure settings.

If you're running on a race track for the first time ever, this will lengthen how long it takes to determine the best tire pressures as your own inconsistent and incorrect driving will cloud the data you collect. However, take the right data, (use the charts we've provided, and the information in many of the recommended books), and you'll be able to quickly narrow in on the best performing pressures.

Based on the car's handling and tire temperature readings, make adjustments in small increments of 1/2 to 1 psig as needed, then run several more laps to feel the difference. Repeat this cycle as many times as needed until the best balance and maximum grip is achieved (as shown by your lap times). Use the charts we've provided (above and the Tunding Guide) to help interpret the feel and the tire temperature data into how to adjust the pressures.

Each time the car comes in, use your log sheet (see the Practice Sessions section for a downloadable log sheet) and immediately write down the tire pressures and temperatures. Make notes as to the specific handling responses in each corner of the track. Also, write down exactly the changes made on each tire.

After several sessions, you'll get to know the range within a pound or so for the tires that they perform best when hot. When they cool down, you can make note of the cold pressures, and use those pressures the next time as a starting point.

Once you have a feel for the target pressure when the tires are hot, there is some math you can use to calculate a starting point that should be within a pound or two of the best settings. How that works, requires some further understanding of the ambient air temperature and it's effects on the tire's pressure.

Read Next Article Section
(Part 4: Ambient Temperature, Optimizing)

Tags: Race Tires, Performance Tires, Car Tires, Portable Air Compressors, Tire Gage, Tire Temperature

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Article subsections:

Part 1

  • Introduction

Part 2

  • tools
  • taking temp measurements

Part 3

  • starting somewhere
  • fine tuning

Part 4

  • adjusting for ambient temperature
  • optimizing tire performance
  • optimizing tire temperature
  • optimizing contact patch

Part 5

  • road racing vs. auto-crossing
  • stock front-drive cars

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