OTT Thai tire pressures...

[papa al]

RE post 12, etc.

Tire pressure will closely approximate proportionality to absolute temperature filled with N2, or wet or dry air.

Simplicate, don't complify.

PV/T=k.

[12DrinkMore]

RE post 12, etc.

Tire pressure will closely approximate proportionality to absolute temperature filled with N2, or wet or dry air.

Simplicate, don't complify.

PV/T=k.

We are not discussing that level, no issues there.

We are moving on to higher levels.......

[Shurup]

Never looked on bike tire but my car/truck tires had a load (weight) rating on them. Didn't mention changing air pressure with different loads.

I too never looked at the bike tires in detail, but the car tire has the max load, or the max tire pressure stamped on it. The recommended pressure would be on the sticker inside the doorframe. Manual on my pickup truck also says to increase the pressure if you're hauling a heavy load.

[MartinL]

I like physics/thermodynamics so I'm going to continue this subject even if it bores the pants off others.

But whatever we say, the reality is far more complex than there's space for discussion here. For example, pumping the same mass of two different gases into two identical tyres under the same conditions will result in different pressures.

PV/T = constant© is true but only for a specific gas and, more specifically, a homogeneous (100% pure gas, including "pure" air) so wet air doesn't obey this "Law" at all.

If that is transposed & expressed as PV = cT, then that "c" is made-up the mass of gas m, the Universal Gas Constant Ro (the same for all gases) and the Molar Mass, M (different for all gases) so that the equation becomes PV = m*(Ro/M)*T.

For a tyre, we can say that V is constant, mass m is constant, Ro is constant and, for a warmed-up tyre, T is constant.

Therefore the pressure in the tyre is inversely proportional to the Molar Mass, M. For any particular condition, a high Molar Mass ---> low tyre pressure.

M for nitrogen is 28. For air it's accepted to be about 29.

Therefore if you run two tyres under exactly the same conditions, one filled with air & the other with nitrogen, the one filled with air will have a lower pressure than the one with nitrogen, but it's marginal and probably not noticeable. If the air-filled tyre is at 100%, the nitrogen-filled tyre would be about 103.5% or 32psi v 33psi.

Jeeze, this is going back a few decades.

PV = nRT

n = mass in grams/Molar mass = number of moles which is a constant for a constant volume of gas.

I believe that you are looking at the Specific Gas Constant (Rs) which does change with the gas, although for most purposes the Universal Gas Constant ( R ) can be used.

I believe Rs = R / M.

So although the conclusion is true, the argument is not correct.

(However I could be wrong, this is pulling in stuff from a few decades ago....)

Wotcha fink?

​ Same here - I last studied this sort of thing about 40 years ago as an engineering undergrad. and, having graduated, promptly never needed to use it again since I went into building services design where that sort of thing isn't really relevant. But I enjoy digging into the grey matter and seeing what lurks in there after so long being unused.

You use R where I use Ro, but otherwise we're talking about exactly the same thing, just different terminology.

[Badrabbit]

I'm not a great sleeper but if this keeps up I'll use the last few posts to drop off quickly, mega boring, yawn factor 10!

[anto]

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I'm not a great sleeper but if this keeps up I'll use the last few posts to drop off quickly, mega boring, yawn factor 10!

Yes ,when they start mentioning Nitrogen ,i go into instant sleep mode .