Kart Tire Pressure by Conditions: Hot, Cold, Grippy, Green and Wet

This is the guide for setting kart tire pressures, and for having a general understanding of what to do based on all the different conditions. And the conditions differ a lot. Track temperature, hot or cold. Grip level, whether the track has grip or gets slippery. Wet, dry, or drying up. And it depends a lot on the tire compound, hard tires or soft tires.

What I'm sharing is a guide, a rule of thumb for how to set your kart tire pressures as the conditions change. It will vary from time to time, and you should keep it as a rule of thumb. But it has a lot of value, it clears my decisions a lot, and I can say it works about 90% of the time. So I'm very confident in what I'm sharing here.

If you want the method for finding your own number, the stint-shape test and the logging discipline, that's in the tire pressure method. This is the cheat sheet for the day the conditions move.

The rules at a glance

Condition	Hot target moves	Example
Cold track (January, zero degrees)	Higher	0.90 becomes 0.95
Hot track (60 to 65°C)	Lower	0.90 becomes 0.82 to 0.87
Green, slippery track	Higher	Counterintuitive, but it holds
Grippy, rubbered-in track	Lower	Very low in the 2010 to 2012 grip era
Harder compound	Higher	0.95, 1.0, even 1.1 bar
Softer compound	Lower	0.25 to 0.30 bar with LeCont
Standing water	Higher
Very little water	Lower
Drying line appearing	Very low

Those are the rules. Here is where each one comes from.

Should tire pressure go up or down on a cold track?

Up. The first condition that changes your tire pressures is the track temperature. That's why you always have to measure the track temperature, because it plays a huge factor in the decision. If the track temperature is very low, you'll have to run higher pressures. If it's very high, 65°C like we've had for many consecutive days here in Italy between July and September, you'll have to play with very low pressures instead.

At the end of the day, the hot target pressure is what counts. But I'm a firm believer that it varies from track to track, and with the track temperature. Let's say, on average across the year and the different tracks, your hot target for an MG tire is supposedly 0.90 bar. I don't know exactly what that is in PSI, you can Google the conversion. Let's say 0.90.

What I've noticed over the years, and it has helped me win a lot of races, is that if the track is very cold, say January with the track at zero degrees, my target will not be the same as it was in the summer at 60 degrees. So even though the year's average is 0.90, in that case I'll try to come out at 0.95 and see whether that's quicker than 0.90. Then we stop, drop it to 0.90, and see how the grip and the lap times change. If it's worse, we lower it even more, to 0.85. If that's even worse, okay, then we see the direction is to be a little higher, so back around 0.95, maybe we increase it even more.

Tire pressures are always a moving target. They're not fixed in stone, and they're not the same for all tracks and all conditions. You always have to experiment.

So number one: if the track temp is low, I go for higher hot targets. Whether that means starting cold at 0.75 or 0.70 depends on many, many things, even on the rim temperature when you set the pressures.

Set pressures at a consistent rim temperature

That's really important, and something you need to always remember. You need a consistent place where you set the tire pressures, because you want consistent rim temperatures. Inside the tent the temperature changes a lot during the day. In the morning it's very cold, at midday it's very hot, especially if you have a heater in the tent in January or February. So be mindful of that and set the pressures at consistent rim temperatures.

If you can't, then you need a calculator that converts, based on the current rim temperature, what pressure you should actually set. That's extremely key. Otherwise your numbers from one session aren't comparable with the next, and the rule of thumb has nothing to work on.

Hot track: lower the hot target

The rule in winter, with really cold track temperatures, is to come out a little hotter than you would in summer. In summer, if it's 65°C track temperature, which is very usual now, it's crazy, with 41 or 42 degrees of air temperature, it's becoming the norm to have above 60 degrees on the track. So you need to be ready to have a little lower target. Instead of 0.90 you may have to come out at 0.82, 0.85, 0.87. You should experiment.

Tire pressure affects tire temperature, but there should be a whole separate guide on that relationship, because it's not that correlated. Sometimes dropping the pressure flexes the tire more and actually heats it up more. With a different compound it's the opposite, higher pressure gives higher temperature. It plays differently with different compounds.

But normally, if the track is really hot, the tires are running really hot and you're overheating in the last few laps, then you need to run lower pressure, even if that means resetting and lowering the hot target that was perfect in winter and isn't right for summer. That overheating drop-off is the same signature I described in when hard tires beat soft.

Readjust your hot targets based on the track temperature. So you should always track your track temperature.

Why does a green track want higher pressure?

The next very important variable is the actual grip condition of the track. If the track is really grippy, in most cases you can run lower pressure targets. When the track is very slippery, you should run higher targets.

Don't ask me exactly why. It just seems to be the rule of thumb over the years. It might sound counterintuitive, because when the track is slippery and kind of green you're sliding quite a bit more than on a grippy track, so you'd think it should be the opposite: lower pressures on green tracks, higher on grippy tracks, because you slide less. Somehow that's not the case.

What I believe is that when the grip is high, you carry more speed. The tires are stressed more because you have more speed, more effort on the tires. Imagine the track is three seconds faster than on a green day. You carry way more speed, and the tires heat up more, not because they're sliding but because they're doing more work. So they need lower pressure, otherwise they overheat. And it's the opposite on very green tracks. You can't carry a lot of speed into the corners anyway, because the track is green and sliding, so you run a little higher, which may be unusual, but it may be the only way to extract some performance.

Green tracks, higher pressures. Really grippy tracks, you can go low. Stamp it in your brain.

I remember when we had really soft tires and really high grip, in 2010, 2011, 2012, literally 15 years ago, when the compounds were super soft. Not only did the compounds need a low pressure, but when the grip level was so high, given the same compound, you'd go very low on the pressures. That was the only way to keep consistent performance through the whole session. If you didn't do it, you just weren't fast. Reading whether the track is green or rubbered in is its own skill, and track grip evolution shows you what to look for.

Harder compound up, softer compound down

The next condition is the compound of the tires. And it's not just the compound. It depends from tire to tire, brand to brand, because every tire has its own DNA: grip level, dimensions, literally the tread of the tire. So it's not an apples-to-apples comparison. It's more complicated than that, and I don't have all the answers, so I'm not going to pretend I do.

The rule of thumb is that with a harder compound, which is basically less grip in general, you need to run higher pressures. If the normal target is 0.90, with a harder compound maybe you need 0.95, or 1 bar, or 1.1 bar. It depends a lot, and you need to be flexible and adapt. Whereas with a really soft compound you may have to run very low pressures.

With LeCont I remember we were running 0.25 or 0.30, even. I'm not saying that's the right thing to do, but in some cases it worked really well.

I'm sure it wouldn't have worked with MG tires or with Maxxis, running that low. I've never actually driven Maxxis, and I'd like to try once. But over my whole career: softer compounds, lower pressures. Harder compounds with less grip, higher pressures.

Wet: how much water is on the track?

The final variable, if the track is wet, is the actual amount of rain on the track. This should have its own guide, but let's include it for a second. If there's a lot of standing water, run higher pressures. If there's very little water, run lower. If the track is on the edge between dry and wet and you can see a drying line, go super low.

It makes a big difference if you get it wrong. Too low when the track is very wet and you suffer. Too high when it's drying up and you suffer too. So you want to be really fast and clever at adapting to those conditions. You'll see it's not that important to change other stuff on the kart, like sprockets or setup. I covered the tire call itself in when to gamble on slicks, and what a wet session does to your data in wet session analysis.

What's going to play out the most is nailing the right tire pressures for all conditions. That's what extracts the most performance.

Art, science, and the question I ask every driver

Even so, it's more of an art than a science. It's not easy to outsource to somebody without enough experience, or to just have AI do it for you, because it still comes down massively to experience. You can look at data and take decisions based on it, but data is most of the time rearward-looking, so it won't tell you exactly what to do in the moment when you're sitting in the pit lane choosing pressures at the last minute.

So I like to ask myself, and I'm always asking the drivers, this question. Okay, let's say the pressure is not going to be perfect. What do you want? Would you rather be quicker in the first laps and drop off by the end, or struggle more in the beginning and be faster at the end? Would you prefer to be faster when the track is full wet, and struggle a bit if it dries up? Or the other way around?

The perfect pressure doesn't exist, just like the perfect setup and the perfect lap.

The real goal, as a team manager, a mechanic, a coach or a driver, is to get as close as possible to that sweet spot. You'll never really nail it. You may feel like you've nailed it, but you could always have been a little lower or a little higher. In hindsight it's always easy. In the moment you need the right state of mind, and ideally the right expertise behind you, to take the decision.

That comes down to experience, but data helps a lot. If you track your performance and the pressures you ran in past races and sessions, with the track temperatures and the compounds, that massively helps compared to doing everything blindly and leaving it to chance. I'm sure in F1 they're not doing things randomly. They do everything scientifically and precisely, and that's ideally what we'll be doing in the future, with the drivers I work with and with you driving on your own. Log the cold pressure, the hot pressure, the track temperature and the compound every session, and PURPL keeps them next to the lap times, so the rearward-looking data starts pointing forward.

The feeling of nailing the optimal pressure is gorgeous. The feeling when you get it wrong is quite awful. Sometimes you nail the conditions and then it starts to rain, and you're too low for that. Or you're perfect and then it dries up super quickly, you're too high, and you have to look for wet patches on the straight to cool the tires down. It's a moving target. You can't get the perfect pressure for every lap of every session. The goal is to get as close to perfection as possible, and if you apply these rules of thumb and combine them with a strategic, scientific approach, you'll get there.

I hope this was useful. Make sure you sign up to PURPL, there's a free trial to get started, and join the six-month training program if you want to elevate your karting and become a world champion, as I managed to in 2013. I'll see you.