So I was thinking about filling my tires with nitrogen so I don't have to air down at events. Is this one of those procedures where you just let the tire go completely flat, and then fill it with nitrogen? And then do you repeat that several more times to get more of the oxygen out of the tire?
I prefer a bespoke 78% nitrogen mix I bottle at home.
It will only save you from having to air down at events if the difference between your street and track pressures is close to the difference between your hot and cold pressures. When race tires are filled with nitrogen, the wheels usually have two valves to allow the nitrogen to be bled through rather than having to do pressurize/depressurize cycles. I don't think nitrogen is worth it to anyone who isn't racing for big money though.
The only benefit of running nitrogen in your tires is that bottled nitrogen has no moisture content and therefore pressure increases at a predictable rate with temperature. You could fill you tires with bottled air and get the and get the same benefit but because bottled air is more expensive than bottled nitrogen it would cost more.
When tires are filled from an air compressor the amount of moisture that ends up in the tire is dependent primarily on the ambient humidity on the day that they're filled and the effectiveness of the drying system installed on the compressor. The amount of moisture in the air effects the rate of pressure increase as the tires come up to temperature.
To answer the original question it's best to mount using nitrogen in the first place and in humid weather it may still be necessary to purge using the second valve stem to get the moisture out of the tire.
There is an alternative to dry air or pure nitrogen. The FIA and ACO teams in Europe are not allowed to run nitrogen so they use air dryers between the compressor and air tank. This is a timely process, but does work in the absence of other options. Same basic result from a more complicated setup.
As a note, we were not allowed tire warmers on the sports cars either, so all the major teams setup ez-ups to act as tire ovens with 10+ setups of tires slowly being ramped up to temp. The blue haze coming out of the tents always made me laugh.
stafford1500 said:There is an alternative to dry air or pure nitrogen. The FIA and ACO teams in Europe are not allowed to run nitrogen so they use air dryers between the compressor and air tank. This is a timely process, but does work in the absence of other options. Same basic result from a more complicated setup.
As a note, we were not allowed tire warmers on the sports cars either, so all the major teams setup ez-ups to act as tire ovens with 10+ setups of tires slowly being ramped up to temp. The blue haze coming out of the tents always made me laugh.
One of the stock car teams I worked with in Upstate NY back in the day was sponsored by a body shop. We used their expensive air drying system for tires.
I've had a couple of hard sells on nitrogen fills for a premium, and it's by people that really believe in the idea that nitrogen is better.
Boyle's ideal gas law states that: PV = nRT and all you have to care about is that P (pressure) goes up proportionally to T (temperature). Doesn't matter if its nitrogen, air, helium, whatever. Does matter if there's enough moisture in the air to potentially condense, hence the good advice to get dry air. So while there might be some marginal advantage to using nitrogen which is less reactive than oxygen and better for the rubber inside the tire, it really doesn't matter what you fill the tire with, as long as the water content is low. (wonder how much weight advantage there would be to using helium?)
Edit: Apparently somebody on the intertubes figured out weight savings at about 81G for four tires. Didn't say what size. And apparently some bike racers do use helium fills.
I'm thinking about nitro since I live in an area where the temps fluctuate highly. I have autocrossed in the morning fog of 40°, only to have afternoon runs in the 70-80's.
A ten second Google search will tell you that the main benefit is that "pure" (I think it's usually something like 95%) nitrogen doesn't seep through the rubber as quickly as air so there's less long term refilling and less chance of unknowingly driving with under inflated tires. Short term and with temperature fluctuations there's almost no difference unless your air source is dripping with water. Certainly not enough that you can ignore a 40* temperature change in a day.
Consumer Reports did a leak test of nitrogen vs. air. Over the course of a year, nitrogen leaked down 2.2psi vs 3.5psi for air. Not a dramatic difference over a year.
jwagner (Forum Supporter) said:wonder how much weight advantage there would be to using helium?)
Pfff, hydrogen is significantly lighter and can easily be made at home. 
In reply to dps214 :
Not uncommon to see everyone at the event airing down after run one or two in the afternoon. I've measured a 5psi difference after just one run. And this is dry desert air in these tires.
In reply to RX Reven' :
Ah, the old Hindenburg Flambè.
RX Reven' said:jwagner (Forum Supporter) said:wonder how much weight advantage there would be to using helium?)
Pfff, hydrogen is significantly lighter and can easily be made at home.
I keep a vacuum in my tires, that's even lighter!
The best reasons to not run hydrogen in your tires would be that it can escape through solids and embrittle metal on the way out. Helium has the same escapes-through-solids issue.
Defined motorsports said:In reply to dps214 :
Not uncommon to see everyone at the event airing down after run one or two in the afternoon. I've measured a 5psi difference after just one run. And this is dry desert air in these tires.
Yes, and with nitrogen it'll still be 5psi, just with a bit more consistency due to the complete lack of moisture.
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