How does tire pressure effect tire temperature?

[Chris Wehrheim]

OK School has started and it is time for a science fair project. This year I am doing the effects of Nitrogen, air, and a mix on tire temperature. Now I know it depends on track conditions and I know the answer to this question, but my teacher requires info from experts. So if all you experts out there can help me I would greatly appreciate it. all i need to know is What gas(nitrogen, air, and a mix) produces the most and least heat.
Thanks

[ September 12, 2001: Message edited by: Chris Wehrheim ]

[Sean Robbins]

What makes this fun is that you have two opposing effects.

1. Lowering pressure increases the tire's footprint on the track, which you would expect to increase traction.

2. Conversely, a tire with higher pressure heats up more, and hot rubber sticks better than cold rubber.

However, if I understand your question correctly, you are talking about a combination of tire dynamics and Boyle's law (temperature increases with pressure). However, since the "envelope" (tire) deforms, holds heat, and depending on its interaction with the track surface, greatly contributes to the heat, you would be opening a real can of worms here...

[Jeff Braun]

quote:

---

Originally posted by Chris Wehrheim:
So if all you experts out there can help me I would greatly appreciate it. all i need to know is when you increase air pressure what will happen and when you decrease what will happen.
Thanks

---

Chris:
I am no kart expert, you and your Dad know allot more about karting than I do, but I can tell you about what we found on the IRL cars I engineer.
First the temp of the tire is usually recorded by a pyrometer that is inserted into the tire tread to measure the temperature, or an IR heat gun that records the temp of the tire surface.
The tire temp measured this way is mostly affected by the amount of sliding taking place between the tire and the road. Other factors do affect this reading, but to a much less extent. I will talk about them later. So, when we change the tire pressure we affect the tires "grip" with the road. If the surface temp goes up it probably had less grip and was sliding more. We see this all the time with the big cars. When we have high tire temps and the tires do not last due to this, we actually look for a SOFTER tire. This softer tire makes more grip, slides less, and lowered the tire temps. Now a soft tire is much more affected by sliding, so if you still slide the soft tire it really goes away fast.

More grip, less sliding, lower surface temps
Less grip, more sliding, higher surface temps.

Part 2 - As I said the surface temp is the most often recorded and is most affected by sliding, but there are other factors. The tire surface is heated from the inside out, it just takes longer for these factors to change the surface temp, and they tend to be masked by the sliding effect.
These include:
Tire pressure. The lower the pressure the more flex in the tire carcass. This generates heat in the carcass that is transmitted to the surface. Ask Ford and Firestone about this effect.
Normal load on the tire. More vertical load deflects the tire more and heats the tire. We see this effect when we change the downforce on the car.
Brake and wheel heating. The brakes on the big cars have a large heating effect on the tire temp. We drag the brakes on the IRL cars leaving the pits at Indy to help heat the air in the tire to get the tire to pressure quicker. Just grab the axle of your shifter when it comes off the track and see how hot it is. This heats the wheel and then the tire.
All of these complementary and counter acting effects result in changes to the tire pressure, grip and stability (spring rate) of the tire. That is why cars, and karts change handling from the start of the race to the end.
With the IRL cars I am lucky to have tire pressure sensors that give me the pressure in the tire real time as the car runs. We have also installed IR tire temp sensors on the car to give us surface temps as the car runs real time. This has all helped me to understand the tires better, and confuse me at the same time.
Long answer to a short question.
And as Dennis Miller says, that is just my opinion, I could be wrong.
Jeff Braun

[ron brolley]

Well i wont make it long but in my experience the lower the pressure the lower the heat in the tire whichs causes a loose condition. And the more air in the tire the hotter it gets which creates more grip. But i have also noticed you can only go so high with pressure before you overheat tires as well. Just my pair of pennies.
Ron

[Marc Miller]

Listen to what Jeff said-

His explanation was a good one and one good point he made referring to tire compounds was especially true.

Many times drivers will feel they are wearing out tires or overheating them so they need to go to a harder tire. A harder tire, given the same set-up and condition will slide more and surface temps get even hotter at times and certainly not give you any more grip. Going to a softer tire may be the key, not allow the kart/car to slide as much as it works better with the set-up and does not build as much heat, therefore lasting longer and giving you better mechanical grip.

The hard part is knowing if you generally have the least desired compound on, or if your set-up is so whacked that you need to compensate with a different compound

MM

[Chris Wehrheim]

Thank you everyone for your responses.
Jeff thank you so much for info and hopefully we will see you at the first round of the Manufacyures cup.

There is one problem now. My teacher has informed that my topic is a little to easy.
So my new topic is How does Nitrogen vs. Air vs. Half and half produce heat differently. Like what will produce the most heat in the tires and a constant starting pressure.
Thanks.

[lynn haddock]

All gases (pure) will expand at the same rate when heated. (nitrogen-oxygen-argon--etc..etc)
The main difference between nitrogen(pure) VS air-compressor air is the fact that the air compressor air normally will have a water content. This changes everything.
The main reason most people in racing use nitrogen is because it is cheap.

[Tony Gaylord]

quote:

---

Originally posted by lynn haddock:
All gases (pure) will expand at the same rate when heated. (nitrogen-oxygen-argon--etc..etc)
The main difference between nitrogen(pure) VS air-compressor air is the fact that the air compressor air normally will have a water content. This changes everything.
The main reason most people in racing use nitrogen is because it is cheap.

---

----------------------------

Not sure if that's entirely true. Nitrogen is a more 'consistant' form of gas in a tire compared to air. The pressure will be far greater in a tire that contains air then nitrogen.

IMHO, the water that is contained in 'compressor' air could simply be eliminated by a in-line water trap. So by this nitro and 'compressor' air would be equal as by your example, which they aren't.

PLEASE dont take this in a negative way, but since yesterday I have a negative feeling riding on my shoulders, it showed in my work today



Anyrate, this is why all CART,NASCAR etc inflate with nitro, it doesn't increase in presure nearly at the rate as air and nitro is simply more consistant when it comes to tires.

God Bless
*TG*

[aamjohns]

Good papers site multiple sources for information. It increases validity. Here's some links where you can learn more for your project
how things work - nitrogen in racing tires
"Oxygen and Moisture - the Killer of Tires"
atmospheric changes and air composition
Cartalk Columns

I got this by doing a search in google

Cool project.

I have a question. I always thought a higher pressure tires radiated heat more efficently than a lower pressure tire. As I've understood it, in racing, if you want to increase the heat in a tire, you drop the pressure in that tire.

I'm new to karting and the whole tire pressure thing goes against what I've learned. In karting it seems you want a certain amount of heat to increase the grip of the tire. So, lower pressure doesn't mean more heat. Increased scrubbing increases heat which increases grip. Too much air decreases the contact patch reducing grip. It's not like it hasn't been nicely explained above, but why does there seem to be a difference between auto racing tires and karting tires. Is there a fundamental difference here?

Aaron.

[Marshall Martin]

Chris,

When you have finished your paper I would like to read it and I am sure more people would be interested also. We are always trying to learn and your research will provide some valuable information.

[david_reynoldson]

I doubt if the air trap removes anything but excess water(saturated). From experience, the compressed air can be extremely humid and inconsistent. It's not a simple task to get air that is 100% dry. Nitrogen is dry, consistent and inert (cheap too). This is why serious racers use it.

Maybe there is a rocket scientist out there who can give us the details about the inconsistencies of compressed air.

[Jeff Braun]

Chris:
Your teacher made it easier now.
What gas you use is not that important, air or nitrogen. There are small differences in the thermal expansion coefficients of the 2, but they are small. What matters is the humidity of the gas in the tire.
Again from my IRL experience, we have a tire gauge that measures the humidity of the air in the tire. When we use regular air from a normal compressor the humidity can be as high as 90%. When we use nitrogen it is around 20%. When we use air from the compressors at Indy that Firestone use we can get as low as 13%. With our on board tire pressure sensors we can see the lower rate of pressure build up from the drier air.
For your project, try taking a beaker with water in it. Put a balloon over it and boil the water - very dramatic example of what happens to the water in the tire and you tire pressure.
A normal water trap will NOT get much of the humidity. You need a dryer made for lowering the humidity. Longacre sells them as do many racecar supply shops.
The Cup teams have purging systems that pump in dry air into the tire as they bleed the "wet" air out. I have talked to some Cup engineers that have told me that they used to see steam come out the valve stems after a long run due to the water boiling in the tire. Most every Cup team uses dry air now and not nitrogen because they can get it drier.
We started doing it with the IRL cars and saw similar results, but our tires run cooler than the Cup guys. The Kart tires run WAY cooler, but the effect is the same, just the magnitude is less.

Good luck Chris. We may run the Mfg Cup next year; depending on how many races Colin is going to do in Europe.

Jeff Braun

[Marc Miller]

Also-


Keep in mind that when using nitrogen or dry air, you will need to adjust your starting pressures accordingly. Many top guys know what the pressures rise to in the tires when they are hot. Just going to nitrogen will not make things any better unless you adjust the tire pressures for desired pressure when they become hot and grow. I will run anywhere from 9-11 pounds with regular air and they will grow 1.5-2.5 pounds. With nitrogen they may only grow 1 pound, ultimately affecting my set-up. ALSO - keep in mind, as Jeff points out that nitrogen contains less moisture, but any time you apply a liquid or lubricant to your tire to bead them, you have now wasted time with nitrogen due to the introduction of moisture. Most teams that use dry air or nitrogen bead tires dry as to keep as much moisture out as possible.

(I like this Jeff Braun guy- he keeps saying stuff that I am about to say!)

Marc Miller

[Chuck Bunnell]

Rocket Scientist alert. Not really, but I'm an engineer that plays a rocket scientist on TV. Not the TV part. Anyways, don't forget that common household air is about 79% nitrogen, 20% oxygen and 1% lots of other stuff. Compressor air will have the same gas composition but can have a varying water vapor content dependent on ambient temperature, time in the tank since last running, ambient humdity in the incoming air, length and temperature of the steel pipe, if any, before the regulator. Chances are it will be near saturation point with regard to humidity. There may also be some oil mist from blowby on the rings on the intake stroke. Assume the worst. It will be wet.

Marc makes an excellent point about introducing moisture during tire mounting. It can make using dry gas completely silly. If you do need to use a lubricate to seat your tires, which most of us will, you can do a "pulse purge" to eliminate nearly all of the moisture. Key here is that the moisture is a vapor and not a liquid. Removing a liquid will take a lot longer. Simply pressure the tire with dry gas and then relieve the pressure. Do this five times to 15 psi and you will have removed 97% of the moisture assuming perfect mixing. Higher pressures and more cycles will increase the purging.

If you want an air drier, simple go to McMaster Carr or similar on the web. In the long run it will be cheaper than buying bottles of nitrogen. Though it won't be as portable.

time to think more...

[Chuck Bunnell]

OK, I thought some more. Now looking back to the original question, this really doesn't directly answer it, but we'll get back to that soon enough. The water in the air will be in two forms, liquid and gas. The gas may be also be called a vapor. If you boil a liquid, it changes state from a liquid to a gas and expands rather dramatically. The bubbles you see in a boiling liquid are pockets of water vapor, or gaseous water, that naturally rise to the surface due to bouyancy. And yes any other dissolved gases will begin to come out of solution because a hotter liquid has less capacity for a dissolved gas. Now getting back to the air in your tires, any liquid water dissolved in the air has the potential to expand about 1700 times its volume when changing from a liquid to a gas. And since we have a relatively constant volume, the pressure inside the tire will rise.