OK, all you big time racers out there, what is the correct procedure to purge air out of a tire before filling it with nitrogen?

The reason I ask is that any moisture mixed with nitrogen will create nitric acid, or some such vile stuff, if I remember correctly.

What about using air tools? Looks like a job for rubber gloves.

i hear the benefits, if any, are neglectable (is that a real word?) ...

i'd say find something else to do on your car that will yield better results.
Andy

> any moisture mixed with nitrogen will create nitric acid,

OK, I'll be nice and simply say that this is wrong. BTW, air is 80% nitrogen. Why would replacing the 20% oxygen with another 20% nitrogen help? Maybe you should have the air cyrogenically treated instead.

Jonathan

the nitrogen benefit (IIRC) is that it won't gain as much pressure as regular ole atmosphere air, so therefore, tire pressures can be maintainted easier.

The other benefit is that I can run air tools. Nice to have an impact wrench to change tires.

the word is that that is a big pile of bologna ...

Andy

andy,

I've always heard this is legit...
actually I've already met with a gas outfitter and put him on the hunt for the remaining parts I need to finish out my conversion to nitrogen...

the benefit is that in a lapping session, your tire pressures don't change as radically as they do with air... ie: if I start the session at 26lbs, I can finish the 30minute session with less than (hopefully) the 35lbs I have experienced in the past....

Randal,

from my research (and not actual experience), I understand that just removing the core and letting the air out is inadequate... you have to mount the tire from the beginning with nitrogen...
I hear this is a pain in the #$%, to haul your bottle down to your favorite tire shop with you....

I've been told by multiple sources that if you take a pre-mounted or used tire and try to purge it that there is still condensation and that it doesn't work...

your mileage may vary..
my new car will be set up this way though...

brant

So gang... why does Airgas bring over 1000 bottles of Nitrogen to EVERY single pro racing event (ALMS/GrandAM)... it works. Air expands when heated... Nitrogen expands but expands a lot less. It has to do with the moisture in the tire. Randal they sell a small purging device. Every Indy car team uses Nitrogen...

It is much easier to maintain consistan tire pressures when utilizing Nitrogen.


B

well from a beginning level of chemistry I can tell you;

PV=nRT

that is the Pressure times Volume equals number of moles (or molecules) in the gas times a constant times the temperature.

It would make logical sense that, assuming n, R, and V are going to stay the same (because no air escapes and volume doesnt change), as temperature increases, pressure increases. To combat that, you would have to decrease the number of molecules in the gas (a property of the gas) to make the pressure less sensitive to temperature. I dont have any values on me, and this problem would be very complex because n of ambient air varies a lot, but I can say the atomic mass of oxygen is 16 and nitrogen is 14. Now if you were to go to hydrogen or something (atomic mass of 1) there might be a noticable difference, but I think 2 molecules would be considered neglegable.

But what do I know...

I can tell you from experiance this is something we do on race bikes religiuosly as the heat expansion of the specifically rear tire on a bike can actually "grow" the tire and change the steering geomitery (sp).

The first reason nitrogen is used is that it is the cheapest gas you can get in large quantities to take to the track.

Next, once processed, it contains little to no moisture. Moisture is a bad thing in extreme temp - freezes when cold (bad in airplane tires at altitude) and creates vapor when hot (raises the tire pressure much more than the ideal gas PV=nRT).

Also, if a tire explodes, regular air has a higher oxygen content that will support combustion if ignited.

There is nothing magic or particularly special about using nitrogen in tires - it just has a couple nice benefits at relatively low cost.

ok, here are some numbers ...

At a temperature of 0°C and a pressure of 1 atmosphere, the densities of air, nitrogen and oxygen are:

Air: 1.293kg/m3
Nitrogen: 1.250kg/m3
Oxygen: 1.429 kg/m3

Notice the very small difference between good ole Air and Nitrogen?
less than 0.043 kg, that's a whooping 43 gram per m3 !

now, let's look at some more numbers, comparing Nitrogen and Oxygen:

	O2	N2
Normal boilingpoint at atmospheric pressure, oC	-183	-196
Ration of volume of gas tovolume of liquid, measured at  15oC and absolute pressure of 101.3 kPa	842	682
Relative density of gas at101.3 kPa and 25oC (Air = 1)	1.105	0.967
Liquid density at absolutepressure of 101.3 kPa, kg m-3	1141	807
Latent heat ofevaporation, kJ kg-1	213	199

mind you, we're actually interested in how AIR compares to Nitrogen, not Oxygen to Nitrogen, so we need to take the above numbers and put them into a 80/20 split in favor of the Nitrogen ...

bottomline, the expansion rate of pure Nitrogen gas compared to normal Air is about the same ...

now, that doesn't mean that someone can't make a shitload of money selling that stuff to racers who would sell their firstborn to get 1/100th of a second ahead.

flames on Andy

hmmm...

Brad elaborate further on the small purging device
(I did not know this)

is it pricey?

brant

So back to the original question; just inflate with nitrogen and let down 3 times one after the other and reinflate with nitrogen. You can use a purge device if you like but you don't really need one.

As Andy said, the comparison here is between Nitrogen and AIR. It is my understanding that the reason "dry" Nitrogen is used in racing tires is because the rate of expansion and contraction is more consistend when compared to that of AIR. This is a results of the composition of AIR which is 78% N2, 21% O2, and 1% of lots of other stuff, including varying amounts of water vapor that changes as a function of the relative humidiy of the environment. It is this water vapor that tends to be inconsistent in its rate of expansion and contraction and therefore gives varying amounts of tire pressures. By using "dry" air, it eliminates the variability of the tire pressures from one venue to another and allows the teams to set the tire pressures and forget about them.

-Britain

The problem is that you can't buy "air" that has been processed with the moisture removed, but you can buy nitrogen cheaper than any other gas.

Now, lets see how smart you guys are: why not use helium (or hydrogen!) because it would be lighter? (I'm not saying you should, just seeing what the response will be ).

Check this product out...it allows you to purge all four tires at once...

Tire Squid

The above data was collected using industrial grade nitrogen, inflated to 20 psi on non-inner liner 15” Goodyear tires.
Ambient temperature 73.5f, ambient humidity 44%.



Should I purge after the tires have been hot lapped, qualified or practiced?
It would be a good thing to do. Moisture and other tire manufacturing elements may have been released from the tire during that first heat cycle. Also if the tires have been sitting in the sun after purging (usually the impounded qualifiers) it’s a good idea to purge them again before putting them on the car

Why purge?
Humidity inside your race tires causes the pressure to increase when heated. If the amount of humidity is decreased the tire pressure buildup is decreased. After each purge the humidity held in the air is replaced with a dry inert gas, such as nitrogen.


Why purge more than once?
Each successive cycle dilutes the remaining air until most, if not all, humidity laden air is replaced. A vacuum pump could be used but they are expensive, time consuming and you risk breaking the bead.

Why worry about pressure increase?
An increase in tire pressure changes the dynamics of the tire. The circumference of the tire may change and therefore change the stagger. The pressure change also changes the effective spring rate. If the pressure change can be minimized the tire will perform more consistently allowing the crew to concentrate on other areas of the cars’ performance.

Andy,

Your a smart guy Andy, but I just wonder if racing teams would really be going to all that trouble if it wasn't worthwhile.

Yea, everyone will buy stuff to go faster (OH MY GOD is that me!) but the big racing teams have so much experience they must see a difference otherwise I can't believe they would be messing around.

Well I do know what my tires typically heat up to in a 60 second (15-20 turn) autox, so I'll check and report my findings.

I'll do the test at WWC.

I just need to find a purge device (that I can afford!) to get the air out of my existing tires.

Randal

don't underestimate "the power of suggestion" !

but you're right. what do i know.
i don't even have race tires, nor have i ever driven a real race car on the track. i'm sure those guys know what they're doing.

don't listen to me, i'm just babeling because it's sooo boring here at work,
Andy

Carrol Smith recommends using a shop vac and some duct tape as a purging device. Remove the valve core and tape the hose to the valve stem and suck.....

Thanks groot.

Who is Carol Smith?

But I like the idea.

Erm..... i reckon if you asked it would go down like the Hindenburg

Unless I missed the boat, all you guys missed the boat....

The nitrogen tanks run the jacks in the cars. The jacks require something like 300 psi to operate so compressed air is not an option.

My buddy had a cup car with built in jacks and he had all the crap with nitrogen tanks, regulators, etc.

he used the nitrogen in the tires just because he had it around to work the jacks.

if you think having nitrogen in your tires is going to get you by your next toughest competitor keep dreaming. whatever makes you happy.

dr

dr,

I'm willing to acknowledge that people have a right to their own opinions...

but there are a lot of racers that would disagree with you...

yes the tanks can and do run equiptment...
but try running a double stint or an enduro and dealing with the handling difference caused by tire pressures...

A track tire (non slick) will change as much as 10psi from the beginning of a session to the end...
thats a lot.. the car handles EXTREMELY different..

have you ever noticed that a race crew might make a 1lb change in tire pressure? They are not making a 10lb change, just a 1lb change.....

I really don't think its all bunk..
I don't think SCCA runnoffs competitors are uneducated, or using gimmicks in their cars...
ie: I haven't seen a lot of "turbo tornado" (registered trademark) installs on true race cars...

I'm not expressing myself very eloquently.. but this is what I do know... I have often run a 2driver car. Its not fun starting a race with your tires squirly and off by 10psi.. I'll try nitrogen (and I strongly believe what I've been told by credible racers who are friends) that there is some benefit to it..... and I actually do believe that I can pick up a couple of tenths per a lap by having my tire pressures dialed in by more than a 10psi window...


brant

Andy, the word is negligible
Except that the difference is not negligible. It is small, but noticeable. The pressure change will be smaller as the tire heats up, compared to moisture laden compressed air.
There is no point for most guys to use nitrogen in their tires. If you are planning to have it around anyway for running air tools etc, then you might as well use it in the tires too. An air compressor requires electricity, a tank of compressed gas does not.

Oh, the biggest reason for not using helium or hydrogen is the molecules are small enough to fit through the rubber. The stuff is too expensive to constantly refill the tires.

Chris what do you run, and what do your competitors in the run offs use?

brant

So far I have not budgeted for nitrogen, even though I already rent 6 tanks of gas for my business. Most SCCA club racers, even at the runoffs level don't use N2.
My slicks gain about 5-7 psi from cold to hot, depending on which corner of the car is working hardest at a particular track. Most of the pressure change is within the first 3 laps. After that I can't feel any change in handling. Anyway, most of the handling change is due to the rubber softening as it warms up. I get very good, consistent tire wear because my cambers are dialed in to my driving style, ie. smooth and fast. Also I use a tire pyrometer to check for even temps, then adjust my cold pressures accordingly. I do my best to come in from a practice/qualifying session as fast as the traffic will allow to get accurate readings on the tires.
I don't need no stinking cool down lap.

Oh, and I set my pressures with less than 1/4 psi increments sometimes.

Oh gosh I'm an idiot.. I forgot you were on slicks..
b

I don't know about anyone else, but thinking about and doing stuff to your race car, trailer or tow vehicle is just plain fun... Also a learning experience.

Like most of us if I wasn't doing this then I'd be totally involved in something else.

BTW being competitive is important - and it makes me really happy when my car and more importantly I drive effectively at an event. But I do know the difference between using nitrogen in my tires and feeling uncomfortable using my new Renn shifter and hitting the rev limiter twice in a course. One might make a tenth of a second difference; the other seconds.

And by the way Brad, thanks for the advice (“finesse Randy!”) on using the shifter. Can’t wait until WCC.

I wouldn't say that.

Slicks or DOT tires - it makes no big difference.

I used to race on DOT tires, and I don't think I ever saw 10 psi increases. The pressure settings were higher though.
I set pressures between 18 and 19 psi cold with the Hoosier slicks, and I used to start out with 28-30 cold using both Yoko or Hoosier DOT tires. The pressure increase was about the same.

The people who get such large pressure changes have cars with front engines, either fwd or rwd. The lack of balance of those cars causes the tires at one end of the car to work much harder. Anybody driving a 914 who sees a 10psi increase is probably abusing their tires IMO.

Chris,
I was always told the DOT tires were a bit more dramatic in change.. Sounds like that was not your experience though....

I think I'm experiencing extreme temps due to abuse for 2 reasons really...

1) its a 2 driver car and the sessions are often back to back with no time for any cool down.. the car is literally pulled onto the grid and then back on the track as soon as the drive change can be completed..
2) its a locked diff car. So there is some pretty extreme rear wear going on, with a lot of tire sliding..

brant