I've got a question about tires:

When you buy them at Costco, they fill them with nitrogen, not air. Does anyone know why?

:shrug:

TIA!

compressed air does). The pressure in tires filled with air will increase when hot and decrease when cold because of the water vapor. At least that is what I have heard.

Since most of the gas stations I've seen only have air, it seems like it's almost more trouble than it's worth.

:)

at work for the same reason, in cold weather the water vapor in the lines would freeze and the valves that controlled hydraulic cylinders wouldn't operate.

that the nitrogen in the tires will increase the tire life and improve gas mileage. He explained the technical details to me, but I have forgotten what he said.

I've also heard that it's less likely to leak and loose pressure.

:D

Nitrogen is an inert gas, it wont expand with heat,so as the tire runs down the road the internal "Air" pressure stays constant even tho the rubber that makes up the compound of the tire heats up.

What this means is that your tire pressure stays the same weather you drive a few blocks or across country.

Tires filled ith air start at lets say 32 PSI but when you heat the tires up that pressure goes up,sometimes WAY up to as much as 50-60 PSI,by using nitrogen the internal pressure stays constant,the tire can do a better job kepping "Squrim" (Tread Deflection) under control,keeps the handling characteristics (Sp) more in line with what the engineers designed it for,helps combat "Funny" wear (Uneven tread wear,Cupping,sidewall blistering,tread separation from overheating ect) and improves tire wear and longevity.

NASCAR,NHRA,INDY,CART,F1 and most other pro level racing groups use nitrogen to fill their car tires. In NASCARS case its mandatory.

Edit for clarity

I went to Wiki to look this up,The Wiki read like a Ginormas Algebra Equation...

Let q = (qx, qy, qz) and p = (px, py, pz) denote the position vector and momentum vector of a particle of an ideal gas, respectively. Let F denote the net force on that particle. Then the time average momentum of the particle is:
\begin{align} \langle \mathbf{q} \cdot \mathbf{F} \rangle &= \Bigl\langle q_{x} \frac{dp_{x}}{dt} \Bigr\rangle + \Bigl\langle q_{y} \frac{dp_{y}}{dt} \Bigr\rangle + \Bigl\langle q_{z} \frac{dp_{z}}{dt} \Bigr\rangle\\ &=-\Bigl\langle q_{x} \frac{\partial H}{\partial q_x} \Bigr\rangle - \Bigl\langle q_{y} \frac{\partial H}{\partial q_y} \Bigr\rangle - \Bigl\langle q_{z} \frac{\partial H}{\partial q_z} \Bigr\rangle = -3k_{B} T, \end{align}
where the first equality is Newton's second law, and the second line uses Hamilton's equations and the equipartition theorem. Summing over a system of N particles yields

3Nk_{B} T = - \biggl\langle \sum_{k=1}^{N} \mathbf{q}_{k} \cdot \mathbf{F}_{k} \biggr\rangle.

By Newton's third law and the ideal gas assumption, the net force on the system is the force applied by the walls of their container, and this force is given by the pressure p of the gas. Hence

-\biggl\langle\sum_{k=1}^{N} \mathbf{q}_{k} \cdot \mathbf{F}_{k}\biggr\rangle = p \oint_{\mathrm{surface}} \mathbf{q} \cdot d\mathbf{S},

where dS is the infinitesimal area element along the walls of the container. Since the divergence of the position vector q is

\boldsymbol\nabla \cdot \mathbf{q} = \frac{\partial q_{x}}{\partial q_{x}} + \frac{\partial q_{y}}{\partial q_{y}} + \frac{\partial q_{z}}{\partial q_{z}} = 3,

the divergence theorem implies that

p \oint_{\mathrm{surface}} \mathbf{q} \cdot d\mathbf{S} = p \int_{\mathrm{volume}} \left( \boldsymbol\nabla \cdot \mathbf{q} \right) dV = 3pV,

where dV is an infinitesimal volume within the container and V is the total volume of the container.

Putting these equalities together yields

3Nk_{B} T = -\biggl\langle \sum_{k=1}^{N} \mathbf{q}_{k} \cdot \mathbf{F}_{k} \biggr\rangle = 3pV,

which immediately implies the ideal gas law for N particles:

pV = Nk_{B} T = nRT,\,

where n = N/NA is the number of moles of gas and R = NAkB is the gas constant.

The readers are referred to the comprehensive article Configuration integral (statistical mechanics) where an alternative statistical mechanics derivation of the ideal-gas law, using the relationship between the Helmholtz free energy and the partition function, but without using the equipartition theorem, is provided.


Please,for those of us who don't carry a Phd in Math,explain this in plain english!

the volume will increase; or, if volume is constricted by the container, the internal pressure will increase.

The opposite for temperature that goes down.

Applies to all gases, even the noble gases and inert ones.

p - pressure
V - volume
n - number of moles of the gas
R - the gas constant
T - temperature

let's consider a hypothetical tire filled with nitrogen: For all intents and purposes, we can consider the tire to be uniform in volume over a wide range of temperatures (it's not, but the variance is so small, that we can consider it a constant for this instance). The amount of gas will remain constant in this instance. R is always the same (because it's a constant), so in our equation we have V, R, and n all remaining constant, leaving only p and T to vary.

So, therefore, as T increases, so does p. As T decreases, so does p.

Even with nitrogen.

So your initial statement that nitrogen never changes in volume is quite erroneous, and if true, would make it impossible to fill tires with it, or to make liquid nitrogen.

that made a lot more sense,Thanks!

plain old compressed air is the culprit, nitrogen is dry.

Personally I have my own air compressor and I empty the tank after each use. That keeps the water vapor down. Those huge swings in air pressure you mentioned is a result of a LOT of water inside the tires. Filling up your tires with some convenience store compressors is bad news because the tanks rarely get purged. I don't have problems with uneven wear with my tires and I live in Texas.

78% nitrogen blend.