Tire Pressure 101

My thought was that when the track builds rubber and gets grippier, my speed is scrubbed on exit when the hopping starts. In the rain, I’m able to corner faster than everyone because my high COG becomes advantageous.

Dry track - Entry: Good response from steering input. Apex: slight oversteer. Exit: hopping.

It’s hard to generalize the symptoms as every corner doesn’t act like this but this is my issue with high speed sweepers. I actually don’t feel like I have an issue in tight hairpins. That’s where I usually make up lost time in other parts of the track.

Anyone has the links to these articles? Would be an interesting read…
I can search the internet archive, but a start to the site / blog would be great.

Would like to know what grip means to you all. Tire grip? If so which tire. Or is grip referring to the overall “state” of the kart/

Ok I’ll take a stab at this. Grip in a kart: a force placed by the tire to act opposite inertia/momentum through friction with the surface it is contacting. i.e. in a corner, inertia/momentum wants the go kart to keep going in the same direction while the tire’s contact with the ground is counteracting those forces.

Btw I am not a physics expert so I could be using these terms totally wrong.

In terms of tires, a tire has a set amount of grip it can provide. If that amount is exceeded by the forces mentioned in my definition of grip, the kart slides. After input from others in this thread, I understand how tire pressure is important. If the temperature (Edit: largely managed by pressure) of the tire is not in the ideal range, the grip it can provide changes.

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Turns out Nick Firestone is the great grandson of Harvey, who founded the tire company. As to access to the articles, I’ve reached out to Nick to see how this might be done. At the time he charged a subscription fee. If the articles were in book form, the book would be substantial. He wrote very clearly and included color pictures to illustrate numerous points.
As I trust we all know, when a track rubbers-up and your chassis is still tuned for a clean, dry, green track, you will go slower. I do club racing and the best we ever get is a clean, dry, green track. Given that condition, I try for my best lap times when the “national” teams are at our Tucson track once a year. A day or two before their official Friday practice, I go out with the ones that are practicing early – and really scrubbing our track surface before rubber begins to build. Using Nick’s approach of having a higher psi (within the manufacturer’s operating range) and coming in hot with the same psi in all 4 tires, I had my best lap time ever driving my Rotax with MG Yellows when I came in right off the track with tire pressures at 14.7, 14.7, 14.8, and 14.7.


Hmm, interesting that you believe that too much grip (binding?) only occurs when the IR is set down. I’ve always thought that there was a case for increased rolling resistance on the OR due to the OR’s high slip angle, even though the IR is lifted.

You’re correct, you can have binding by overworking the outside tire to the point it can’t twist properly and produces more rolling resistance. I find that only really happens in pretty high-grip situations on softer tires. For the type of racing most people are doing, the track conditions won’t be so that you encounter that; usually the kart will overload the outside tire and just slip rather than bind and twist. I would say that the modern tire with a stiff sidewall is less likely to do that as well. The softer sidewall tires will probably suffer a bit more from that. I know we experienced it more on older spec MG Yellows or even Bridgestone YHCs if the track conditions were grippy and hot enough.

Pretty sure Col from K-Racer did a good video on this exact phenomenon.

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Need help!

Had a race yesterday and I played with tire pressure. I found the sweet spot and ended up qualifying 2nd by 0.1. I won the first heat and had the fastest lap by 0.1. (This was the hottest part of the day). The main was under the lights and the air temp cooled down a lot. I didn’t change any settings going into the main because I didn’t know if going higher or lower in pressure was the right move. My lap times dropped by half a second! My opponents’ lap times dropped by 0.1 and I couldn’t keep up… it was my first night race in a while so it was a terrible feeling having the fastest kart that day but not knowing what to do when conditions changed and it costing me so much time.

The kart felt very tight in the final compared to the earlier sessions.

So I guess my question is, when it goes from hot air and sun during qualifying and heat, to cold air at night during the final, what do I need to change in terms of tire pressure?

The website itself has been archived. Firestone Kart Info
Scrolling trough it even now and it seems like there’s some valuable content on there.

You’re right though, it would make a good book that could be self-published on Amazon etc.

I spoke to Nick about content at the Supernats in 2017 (ish), I can’t remember what we concluded other than he was a super nice guy that was open to doing something.

Could be an opportunity for someone to put such a book together for a fee.

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Next time you run in the heat of the day see how much pressure the tire gained after the race is over. It should be around 2 pounds more. when the temp drops it is hard for the tire to heat up. So increase your starting pressure.

I was keeping track and each tire would increase about 2 pounds. After the night race I did not check. I will next time.

I started at 13 in the front and 16 in the rear. Coming off the track, I would have about 15 in the front and 18 in the rear.

Not that I know much but generally the cooler air in the evening would likely require some additional pressure the be equivalent. From what I’m told the night air makes the engines breathe better but tires are gonna be cooler.

Am i correct to assume the track will get stickier/grippier as it cools down later into the day? I’m trying to figure out why i felt so tight and was so much slower compared to the heats. I thought there would be less grip if anything but maybe with all the added rubber it just gets stickier as it cools?

It’s tricky. It depends. The track will gain grip as it warms up, but then it will get to a point where it’s so hot that it overheats the tire. So there is a window. Your goal is to match the tire temp (and setup in general) to that window.

If the track cools down significantly and ambient temp goes down, you’re generally going to need to increase pressure to maintain your hot temps and pressures you had earlier in the day. If you’re going from a daytime to nighttime situation, there is usually a big decrease in track temp, and depending on your climate, could even be some moisture from condensation or dew on the track. I know in the muggy Midwest summer, we’ve done night races where the track gets dewy for a few laps.

TCKC is pretty dry air. I wish I had checked the pressures after the final. But it sounds like I should have gone up 1-2 lbs in pressure. I’m guessing my opponents did that and were able to get hot tires while I was struggling to get heat in the tires.

You’d think that but the golden hour for hot laps is after the sun has dipped below horizon.

The track retains heat for a bit from the sun and remains well gripped but the engines open up in the cooler air.

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The first race after the dew sets is always my favorite one to watch. I pit right beside turn 1 and can always count on 3-4 karts out in the grass. Adults, kids, doesnt matter. Keep an eye on that dew. Wipe your hands across your bodwywork right before you get in your kart. You will know whether the dew has set or not :slight_smile: