Track tire pressures: hot vs cold
Tire pressure rises as the air inside the tire heats up, often by several psi over a session. The pressure that matters for grip is the hot pressure, measured right after you come off track. You set cold pressures lower so the tires reach the hot target your tire maker recommends, then adjust from what you measure.
- Work to a hot target from your tire maker or a trusted expert, not the door-jamb sticker.
- Measure hot pressures immediately after pitting, before the tires cool, and in the same order every time.
- If you bleed hot tires, they’ll be low when cold. Note it and re-check before the next session.
- Same gauge, same order, same time: consistency matters more than any single number.
Why pressures rise
Air in a sealed tire behaves roughly like an ideal gas: when it gets hotter, the pressure goes up. As a rule of thumb, absolute pressure rises by roughly 1 psi for every 10°F (about 5.5°C) the air warms, at typical road-car pressures.
On track, the tire carcass flexes hard and the tread rubs against the road, so the air inside can warm by 40–80°F (20–45°C) or more. That’s why a tire set at 30 psi cold can read 36 psi or higher after a few hard laps. Brakes, sun, track temperature and how hard you drive all change the rise.
Moisture in the air makes the rise bigger and less predictable, because water vapour expands more as it heats. Filling with dry air or nitrogen makes pressures more consistent, but the bigger win is measuring carefully.
Setting cold pressures to hit a hot target
Front and rear, and left and right, often heat differently. On a clockwise track the left-side tires (the outside in most corners) usually run hotter, and the driven or heavier end usually gains more.
- 1Get a hot target for your tire. Track and competition tires often come with a recommended hot range from the manufacturer. For street tires, ask the tire maker, your instructor or a tire specialist.
- 2Before the first session, set all four tires a few psi below the target, with the car cold and out of direct sun. Write the numbers down with the air and track temperature.
- 3Drive a normal session at your normal pace.
- 4Measure hot pressures as soon as you stop (see below) and compare them with the target.
- 5Adjust the cold starting pressures for the next session by the difference, and keep notes.
Measuring right off the track
Hot pressures start falling as soon as you stop, so the first minute matters. Drive straight to your pit spot after the cool-down lap and check all four tires straight away, always in the same order.
Use one good-quality gauge for everything. Two gauges can disagree by a psi or more, which is as big as the adjustment you’re trying to make. A gauge with a bleed valve lets you measure and release in one step.
If you have a pyrometer, measure tire temperatures across the tread at the same time. A hot centre suggests too much pressure; hot edges suggest too little.
Bleeding: when and how
If your hot pressures are well over the target, you can bleed them down to the target right after the session. That fixes the next session’s hot number, but remember the tires will now be lower than your usual cold setting once they cool.
Never set hot tires to a cold number. Bleeding a hot tire to 30 psi leaves it badly under-inflated when it cools, and the next out-lap can feel vague and overheat the sidewalls.
The cleaner method is to adjust cold pressures between sessions, once the tires have cooled to air temperature, using the notes from the last hot reading.
FAQ
What hot pressure should I run?
It depends on the tire, car and conditions. Start with the tire maker’s recommended hot range and adjust using tire temperatures and how the car feels. Never go below the minimum safe pressure for the tire.
Should I use nitrogen?
Dry nitrogen or dry air gives a slightly smaller and more predictable rise than humid shop air. It helps consistency, but careful measuring makes a bigger difference.
Why does my car feel greasy after a few laps?
Pressures may have risen past the tire’s best range, shrinking the contact patch and overheating the tread. Check hot pressures and temperatures as soon as you come in.
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