Why don't you weigh one tyre fully inflated on some scales, then deflate the tyre and weigh again . If there is no significant difference then take the tyre to the garage and refill ( most cost 20p and should be long enough to fill it back up 40p at most).I'm sure there are some boffins out there who can tell us exactly how much air weighs (under pressure).
Treacle
Edited to add scooby 96 is obviously a boffin!

[Edited by Treacle - 11/6/2003 11:04:28 AM]

I am selling my alloys at present and was wondering if anyone could shed some light on the following....

If I were to let all of the air out of the tyres, would it decrease the overall weight of the wheel/tyre by a substancial amount to reduce postage costs?

Any thoughts would be appreciated....

you could replace the air with helium............;)

How long before this gets transfered to the muppet forum?

Air weighs 0.0765 pounds per cubic foot. Therefore approx 13 cubic feet of air = 454g (1lb).

Although I have no idea if pressurized air weighs much more?
]

[Edited by Scooby96 - 11/6/2003 11:07:07 AM]

Talizman - just use parcels2go.co.uk - I sent 4 inflated tyres & wheels (+ 20 wheel nuts) to Oldham for £21.23 inc VAT.

Cable tie 2 together then bubble wrap them.

I've not had them asking me for anymore cash as I think they weighed more than the 25kg parcel limit

Just grabbed a handfull of air and put it on my bathroom scales.............bizarrely enough it weighed exactly bugger all!

Makes ya think though, douwn it? :D

Mikey :cool:

Very rough calculation says about 71g of air per 205/50 r16 at 30 psi.

Assuming weight of air as given in post above, and taking atmospheric pressure as 15psi, therefore at 30 psi we have twice the amount of air in there.

Ah but Judge did you scoop that air from near the floor or waist height or even on tippytoe.

You have to vector in the coefficient of density per metre above sea level.And if the bathroom was steamy then water density & humidity will play an important part in the calculation. :D

Mark son of Albert E.

Yes - of course it does - you're putting more air into the same volume.

There's three times as much air at 2 bar (30psi) as at atmospheric pressure, so it weighs three times as much.

If there's about 1 cu ft of air per tyre, then according to the calculation above it'll weigh about 2/3 (actually 8/13ths) of a pound - about 11 ounces - more for four wheels if they're pumped up than if they're left at atmospheric pressure.

Interestingly (or not) - if you squeezed all the air out of the tyres, the mass would go down by almost a full pound, whereas the weight would only go down by the same 11 ounces.

One of us is wrong. Pressure guages start at 0, not 15, so presumably 30 psi is 30 ABOVE atmospheric pressure.

Anyone still awake?


:D

30 psi is roughly 2 bar but as said this is 2 bar(g) (called 2 bar gauge) so 2 bar above atmospheric pressure.

If you measure absolute pressure, bar(a), bar absolute, then it's a bar higher, your tyres are at 3 bar(a). So three times is right.

Still negligible so academic but interesting.

http://www.stopstart.freeserve.co.uk.../moresleep.gif

Talizman....... One thing matey if you wrap your wheels in bubble wrap perhaps you should pop all those little bubbles first :p
Think of all that extra weight in the pakaging :eek: LOL

Simon :D

This thread deserves top billing in the general gallery not the muppet forum......... :p one of the best i have ever read !!

Danbo's idea is a killer! ;)

Where can I get someone to fill my tyres with helium?

By the time I'm finished, Royal Mail will be paying me for them to send them! :D

Get the helium from one of those gift shops that do the big party ballons that have a toy inside them.

Strap to a homing pigeon & Bob's your close relative. :D

If i fill my whole car with helium would it go faster?

yes but the extra weight of the oxygen bottle would slow you down

Doh! yes, of course.

As for the helium idea, I've seen it tried with bicycle tyres, but it leaks out surprising quicly because the helium molecules are small enough to pass through the rubber ! Also, remember Helium does still weigh something, so more inflating your tyres with heium won't make them lighter and lighter the more you put in...

The lightest option would be a vacuum - and I don't mean a Dyson ;)

A good approximation can be given by plugging the right values into the Univeral Gas Law:-

PV=nRT

if you know
P - the volume of air in the tyre (in Pascals)
V - the pressure in the tyre (in m^3)
T - temperature (in Kelvin)
R - Universal Gas Constant

This would give you "n", the number of mol of gas particles. If you then multiply by the relative molecular mass for air, you have the total mass of air in your tyre.

But it will probably cost you more in paper, ink, calculator batteries and lost hair than you will save in postage ;)