Air Pressure, more than just keeping your tires round

An extremely popular topic at every kart race is tire pressures. How much? How little? Track conditions and weather will affect what air pressure you run as much as anything. Personally I like to think of tire pressure as a timing device. If you don't have enough the kart won't come in and the tires won't have maximum grip for the conditions. If you have to much then the kart will get tight during the run or the tires will get to hot and the kart will start to slide which is commonly referred to as "greasy". Just remember, more air pressure is more heat, less air pressure is less heat.

One important factor to consider when setting the air pressure is the length of the run. If it's only a few laps, like qualifying, you might need more air. If it's a 20 lap race then you'll definitely need to set the air low enough that the kart won't just come in for the first 5 laps and suck for the next 15! Best way to determine your air pressure is during practice. If you go out for practice and you run 8 laps you need to evaluate how the run went. Was it fast early then fell off? Was your last lap your fastest? If your last lap was the quickest then you know that you are closer to a long run tire pressure which is great for a long final. If you only have 3-5 laps for qualifying then the higher air that was only fast for the first few laps looks to be the better number for that scenario.

My typical rule of thumb for air pressure gain is 2 pounds per tire during a run. If you set all the tires at 10 pounds and go out and run 6 laps check your pressures when you come off. Did they all go up 2 pounds? Did the outside rear tire gain more than the other three (which is typical)? If you come off the track and any of the tires gained more than 2 pounds then you probably need to drop air on that tire or tires for the next run. Running staggered tire pressures is very common, actually it's necessary. If the track is predominately a left hand turn track the right rear will get a lot of abuse and should gain the most air pressure. As an example you'll see a typical cold air pressure setup like this - RR 9, LR 9.5, RF 9.8, LF 10. The right rear is lowest because it is worked the hardest and gains the most air so to keep it around a 2 pound gain on track you have to start it lower than the others. In contrast, the left front is the tire used the least in this example so it has to be started higher to see a 2 pound gain. If the track is mainly a right hand turn track then you would swap these pressures so the LR would be the lowest set tire and the RF would be the highest.

Now one thing that can throw a wrench into the works is new tires. Personally I think in most situations, especially if it is warm out and the track has good grip, you need to run lower air in qualifying on new tires than you think. My rationale for that is you have a cold tire, cold air in the tire and a cold wheel. As soon as you hit the track all three are increasing in temperature very quickly and you will see much more than a 2 pound gain. Once again the length of the run is extremely important in this decision. If you only have a 3 lap qualifying run then you need to make sure the air is high enough that the tire comes in before you get the checkered flag. If it's a 5 minute session then you don't want the air so high that the first lap is your best (unless it's good enough for pole!). In my experience you shouldn't run the air any higher on a new set of tires then you did on the practice set. Typically bumping up air on new tires (if qualifying isn't super short) will make the kart tight way to soon to run an adequate lap time.

Now setting air before you go on track can be a little tricky. If you set the air under the tent then push to the grid the sun will be increasing your tire pressure as the tire heats up. I like to set my cold pressures under the tent then 5-10 minutes before I have to be on track I will put the kart out in the sun to let the tires heat up some and bleed the tires back down to my "cold" pressure settings. Unfortunately it's as much art as science in getting it right but it's very common to see a .5 pound increase in all the tires just putting the kart in the sun. Sure the tire was going to gain that when you hit the track but if the tire was only 60 degrees under the tent and 90 in the sun I think some of that pressure gain needs to be adjusted before hitting the track just due to the initial temperature difference.

So what about rain tires? More air if it's really wet and less air if it's really dry. If it's a monsoon 18-22 pounds of air is a good range, if the track is drying then you'll have to drop air. I've seen guys go as low as 10 pounds when they had to go out on a very dry track with rain tires.

A final rule of thumb. Soft tires and/or high grip tracks require less air pressure, hard tires and/or low grip tracks and conditions need more air pressure. I've been to tracks where 20 pounds of air makes you fast and I've been to tracks where 7 pounds of air was all you needed and sometimes it's the exact same track with only different conditions or tires! The only right answer on air pressure is whatever makes you fast for a given day. One question that comes up a lot is what's better, air or nitrogen. The reason racers like nitrogen is the lack of moisture in compressed nitrogen. Water vapor expands when it is heated up so running regular air will cause the tire to gain more pressure over a run than running moisture free nitrogen, at least in theory. Here's the main issue with running nitrogen, you have to adequately purge the air from your tires to get as much moisture laden air out of the tire as possible and replace it with the nitrogen. According to the racing professionals I have talked to (stock car and Indycar guys) it's extremely hard to get all the air out of the tire and replace it with pure nitrogen. Most think it doesn't make a huge difference because purging all the moisture from the tire is extremely difficult. Some professional race teams use a vacuum pump to suck the air out of the tire and another line that is fed from a nitrogen tank replaces the air once the tire is empty. Is nitrogen better than air? Will you see a difference over the course of a 20 lap run? I think the jury is still out.

That pretty much covers the basics of tire pressure. As with anything in racing experience is the best teacher and keep notes on what did and didn't work so you'll know what you did the last time you were racing in those conditions.

Mark Dismore Jr.