Tire Pressure 101

I have a good understanding of most chassis tuning adjustments and the effect they are supposed to have on the handling but something I still struggle with is the fundamentals of tire pressure. What is the main effect of starting with higher vs lower pressures? Is the effect different in the front vs the rear? How do the track conditions and weather play a role? What are some examples of symptoms that would have someone say “let’s raise the pressures” or “let’s lower the pressures” (front and/or rear)?

Largest objective is ensure tires are maintained in the temperature range that they provide the most grip. Secondary is balance tuning.

High pressures can reduce the contact patch creating ideal tire temperatures sooner in the session. Good for short qualifying sessions. For long runs, youre shooting for a hot tire scenario where the temps are within the range recommended by the tire manufacturer and the hot pressure is giving you the contact patch you need.

Once you are in the right ballpark mentioned above, Small pressure adjustments front to rear can be made to improve balance as long as long as you are still in the ideal temperature range of the rubber. Typically these adjustments are small, 0.5 or 1 psi. Lower pressures typically provide more grip but can feel slower to react on initial loading set as the tire is more damped.

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First, where can I find that range the manufacturer recommends? I bet its on the white sticker on the tires that I never read. I run LeCont Reds. Can’t seem to find any info online about recommended range.

Follow up question: I’ve read some threads talking about kart hopping under load and one of the adjustments that can be tried is raising tire pressure. So, if raising the tire pressure decreases contact patch, they will effectively have less grip which should should reduce some of the hopping. However, with the smaller contact patch, they will get hotter faster and potentially you’ll be out of the recommended range. What happens later in the session (in regard to handling and grip) if you started with a higher pressure? Pressure will go up with temperature so the contact patch is even smaller, generating even more heat. Is this where you just have to play around and find that ideal setting?

Any detail in the dynamics of tire pressure/temperature/grip for the duration of the session is much appreciated.

Lot of variables. Most my knowledge is with soft tires and your reds are hard so im not the best reference. Maybe temp isnt as large of a factor with hards. Watch this some good info.

He echoes the raise rear psi to mitigate hopping.

Good luck!

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Aren’t Lecont reds “FZ” tires? That’s all I can really find online. I haven’t used them, but the FZ was MG yellows, which isnt a hard tire by any means. If it is the same as the MG FZ, the recommended hot pressure is 11.6psi (see here).

Here is another good article for you to read

I don’t think so. Based on the homologation documents, the Lecont Reds are quite a bit harder than the FZ.

Thanks for sharing, I’ll check this out.

Iirc the lecont reds weren’t that much harder than the whites. In fact, iirc the juniors on reds were as fast as seniors in whites. I think. It’s been a while. But lecont reds aren’t hard tires in my book, kinda medium.

Thanks for the correction? Maybe contact this kart shop and tell them their description is wrong lol

Anyways back to OP’s topic

“Is the effect different front and rear”

From my experience the rears heat faster and get hotter than the fronts. In practical terms I’ll typically see rears 2-3 lbs higher when hot than fronts. So if I start at 6lbs on all 4 corners I might end up with 11 front, 13 rears hot. It’s track dependent on how hard the tires get worked and which tires in particular. You can have pretty big differences between sides too, depending on the track. You could see something like hot temps where the left side overall is getting worked more than right and rears getting worked more than fronts. So you can end up with something like 11 lf, 10 rf, 14lr, 13 rr.

In real life I never tuned with tire pressure at all, I’d just set em all at approx 10-12 lbs generally, and they’d come in where they kinda should and that was good enough, if it was cold out you’d start higher and vice versa. If it was wet, you’d start higher because the tire would never get truly heated up. I wasn’t good enough to mess much beyond “correct hot pressure”.

In sim I do often make sure my tire temps are specific to the front/back/side inconsistencies. I will pay attention to how the tires heat up and try to balance it out. I want the kart behaving consistently.

My logic is that if the tire and kart feels “good” and grips the way I want and results in good laptime at 12lbs, I want to make that consistent. I don’t want one side hotter or colder and thus gripping slightly differently. So, I adjust each tire to get a consistent hot temp across the sides.

I am not sure that it makes much of a difference in practical terms but since it’s easy to do for me in sim, I do it.

I don’t know the LeCont specifically, but tire pressure’s main job is to control the tire temperature, with its secondary purpose to control the “spring rate” since we don’t have suspension.

A higher pressure reduces the contact patch and will allow the tire to slide more and heat up the surface. But it also stiffens the tire, increasing “spring rate” and risks overheating the tire.

A lower pressure will allow the tire to physically flex more which can help build core temperature within the tire. It also will allow the tire shoulder to fold a bit more and keep the tire from snapping into traction loss as aggressively when you go over it’s limit. It adds some give to the tire. The risk with lower pressure is that you don’t get the tire into the ideal temp range for the rubber to work properly and provide maximum traction.

As noted above, most tires have a recommended range of pressure from the manufacturer and you work in that range depending on ambient temp, track grip, and handling needs.

You can stagger pressures to equalize them after they heat up as well. Set them all the same, go out, come back in and check them hot and see which tire is generating more temp and therefore more pressure. You can detect and infer over or understeer this way. Hotter fronts means you are overworking the fronts and likely understeering. Hotter rears means you are sliding the rear and likely oversteering. Assuming you’re not just overdriving the kart yourself and it’s actually a chassis issue.

If the kart starts to fall off throughout the run, you can drop pressure to keep the tire from overheating and keep it in the right temp window. If the kart is taking too long to come in, you can raise pressure to try and get the tires to heat up faster.

But always keep in mind there is the knock-on effect of slightly altering the “spring rate” of the kart too, so big changes in pressure can alter the fundamental balance of the kart.

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That’s a 5lb and 7lb increase from cold to hot. Everywhere I read says you don’t want more than a 2-3lb increase from cold to hot. Why is that?

I think you’re correct that the difference is small. And compared to other manufacturers “hard tires,” the LeCont Reds generally are not quite as hard. With that said, maybe I should find a comparable hardness tire and use their recommended range of optimal operating temperature as a starting point. On the other hand, I just realized every single class at my club uses lecont reds (I thought it was a mix of reds and whites). So I guess there are plenty of people to ask about tire pressure ranges.

What are the consequences of overheating the tire?

Here’s an interesting generalized plot on tire grip vs tire temperature.

I believe that above the 210 F range the oils and other liquid like material start coming to the surface of the tire and reducing traction. The tires start feeling greasy.

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I was just making an example. Also using numbers from sim which is it’s own thing.

I’m not sure what the range is irl, but yeah it’s a lot narrower. Also we’d start at 10-11 IRL, in sim I run low pressures that you wouldn’t irl, so ignore that part.

As the graph shows overheating makes em sloppier, greasier.

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Exactly as Larry said; the tire has the most grip when it is in its optimum temp range. It loses grip outside of that range.

Regarding pressure increases from cold to hot; as I said the tire acts as a damping tool and a spring rate for the kart, so if you start getting a big change in pressure, it can affect the fundamental balance of the kart over the course of a race. You want the tire consistent so the kart stays consistent.

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Interesting topic as I’ve been seeing “the fast guys” experienting with higher air pressures on Vega Reds. I used to only see a standard psi deviation beween 3 psi (depending on the track), but I some are going north of 4psi trying to find the sweet spot.

Nick Firestone is a very accomplished kart racer – and he has raced in the indy 500 a couple of times. Starting in 2010 he posted 24 monthly articles (each with multiple topics). As food for thought, in December 2010 he wrote the following on tires:

"To simplify the basic premise of tire tuning, once you are in the tire’s optimum psi range, think of tire pressures as follows:

  •      Higher psi = More Grip
    
  •      Lower psi = Less Grip
    

For those of you that are jumping up and down right now in disagreement because your tires overheated and lost grip in a race when your pressures got too high - we’ll get to that later. This is the basic tuning premise, a starting point. Think about it this way, if it is cold out or you are on a low grip track, you would start with higher tire pressures to gain grip. Conversely, if it is hot out or you are on a high grip track, you would start at lower pressures as to not overgrip the kart.

You can apply the same tire pressure premise when it comes to tire compounds. The softer the compound the lower your optimum pressure will be. Converely, the harder the compound the higher your optimum pressure will the same reason mentioned above, you are trying to give the harder compound tire more grip by increasing its pressure.

So, how do you find a tire’s optimum pressure? First, check the tire manufacturer’s website for starting cold and hot pressure ranges. Remember this is a starting point, not an absolute. As we’ve learned thus far, you need to take all of the above factors into consideration and add driving style and track likes/conditions into the equation.

For instance, the former Moran Raceway used to yield the best laptimes with the tires 2 psi higher than anywhere else we raced on the ProKart Challenge. The Moran hot pressures were also 2 psi higher than the manufacturer’s recommended hot pressures so if you had stuck to the manufacturer’s recommendations, you would have sacrificed laptimes. (By the way, the higher pressure did not adversely affect tire wear either, most of my fastest race laps at Moran were near the end of the 20 lap main [45+ laps on the tires on a 1 mile track]). That being said, there is one goal to try to adhere to - come in with the hot pressures even all the way around the kart."

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Keep in mind that tire construction and material has changed a lot since 2010. But if anyone knows about tires I would imagine a Firestone would.

Higher pressure can yield more grip to a point, but heats up faster and reduces the contact patch. Yes, you increase the “spring rate” by stiffening up the tire, reducing the force absorbed by the flex of the tire and driving it into the track harder, but you also increase the tire’s tendency to snap into traction loss more aggressively when pushed too much.

Again, we need to consider what “grip” means in this instance. Grip is good. You can never have enough grip, unless you have so much you flip the kart when you try to take a corner at speed.

When a tire pressure is too high, it will overheat and the kart will become “stuck”. This isn’t because the tire is physically grabbing the track too aggressively and generating too much grip, it’s because the tire got too hot, the rubber compound was pushed past it’s effective limit, and now it is LOSING traction at that higher temperature, causing slip on the outside tire, reducing the lateral force the kart is producing, and dropping the inside rear wheel back onto the track, producing scrub and binding the chassis up. The kart will act the same way if you are too low on pressure.

For anyone who has not chimed in but has knowledge/advice, what is the recommended hot pressure range for LeCont Reds?

This is something I have never done but am excited to experiment with.

I’ve been telling myself “I have too much grip and can’t take enough grip out of the kart.” My reasoning is whenever it’s a nice hot day, I struggle to keep pace but when it rains, I pull away from everyone. I always pray for it to rain. But if you can never have enough grip, that’s not my problem, its that my chassis is too flexible and I need to jack less weight. I am going to pay a lot more attention to tire pressure for next race weekend.

You have too much “grip” in the sense that your kart is not lifting the inside rear wheel properly when the track builds rubber, so both rear tires are on the ground at some point in the cornering phase. This could be down to a number of things, so don’t necessarily pin it down to the kart being specifically too flexible or that it’s jacking too much weight. It could be the opposite problem.

And of course yes, it could be a tire pressure issue.

Diagnosing it starts by breaking the corner down to its three phases: entry, apex, and exit, and figuring out what the kart is doing in response to your inputs at each phase. Is it oversteering, does it turn in well or is there understeer initially? How does it feel at apex and exit? And then you can figure out what’s actually happening and solve the problem.