2 valves per wheel, what don’t I understand?

[rwest]

Hi,

Not a Porsche, but car wheels are on all cars!

Parked next to some rodden out Mitsubishi today and the glint of an anodized valve stem cap caught my eye and then I noticed a second one on the same wheel and the other wheel I looked at had two valve stems as well...

For the life of me, I cannot come up with a logical reason to have two valves on each wheel.

So you can make sure all the old stale air leaves when you put fresh air in?

Please educate me, thanks

[Mixed76]

Originally Posted by rwest

So you can make sure all the old stale air leaves when you put fresh air in?

When you're filling with dry nitrogen so you can purge air out. Can fill and empty a few times to achieve the same thing

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[bkreigsr]

My guess is that ,since the car was clapped out, they used a space-saver spare.
I always thought the extra valve was for a quicker evacuation of the contents on the road.
YMMV

[rwest]

Originally Posted by Mixed76

When you're filling with dry nitrogen so you can purge air out. Can fill and empty a few times to achieve the same thing

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So maybe an actual reason, but if it was that good of a method, you would think more vehicles would use it.

[Mixed76]

I think it was an import marketing innovation- looks racy, rationale sounds plausible, costs the wheel manufacturer almost nothing, 10-20% extra profit.

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[EC900]

Nitrogen doesn’t fluctuate due to temperature change, the PSI stays stable and doesn’t leak like ordinary air can over time, that is what I was told when nitrogen was first offered years ago particularly on motorcycle tires where inflation is very critical factor.

One can mix ordinary compressed air to a nitrogen filled tire, so it seems the two-valve approach is a bit unnecessary. Faster deflation maybe, to air-down tires on a rock climber. Not sure if adding some nitrogen to regular air would be of any benefit and again wouldn’t require a separate valve to do so. But filling a tire(s) with nitrogen is costly, so any purging of nitrogen makes no sense.

On the subject of changing out air….
There were a couple of slackers at work. They would head down to the motor pool to hang out for hours while everyone else was stuck doing their runs. We came up with the excuse that they were changing out all the summer air in the tires over to the winter blend air.

[MikeD930]

I've seen an article about using two valves. One valve is use for filling tire with air/nitrogen while another valve is used for tire pressure setting at the same time.

[SpeedracerIndy]

My track wheels for my Miata have two valves. I never thought much about it but now I’m wondering why. The wheel tire package I have for my Miata came from Tirerack and I don’t remember what brand they are, but obviously not factory.

[rwest]

Thanks for all the insight.

It does seem like there can be uses for it, although my opinion would be that the benefits are very minor and that the second stem doesn’t add much value, except maybe on an off road vehicle were you might need to adjust pressures and a second valve could help speed it up.

[Dr J]

Originally Posted by EC900

Nitrogen doesn’t fluctuate due to temperature change, the PSI stays stable

They lied to you in that part.

PV=nRT applies to nitrogen just as much as oxygen or other gasses. Assuming the volume in a tire is fairly constant (not a bad assumption) temperature changes would cause a pressure change of P2=P1*T2 / T1 (where the Ps and Ts are absolute)

[Quasimoto]

I would imagine a side benefit would be that they would have better initial static balance ?

[Keithg]

manufacturing error? First set of Minilite wheels in 1979 had 1 wheel with 2 valve stem holes,

[bkreigsr]

This is what i was referring to above - old-timey spare...