Air volume at pressure.

[Nigelw]

Was puzzling this today at work, but if I have my air tanks fitted up in advance of the OBA and fill them to the max pressuure of 10BAR how much air do I actually have in them?

Tanks being 2X 6L

I was wondering if it would be enough to let me air down the 235/85s to say 15psi then back up to 30?

Any ideas?

[Blanco]

You have roughly 120L (2 tanks x 6each x 10) beware a little of stated volume though, often it is rounded up.To put 1 bar back in a tyre you need to work out the volume of the tyre and wheel combo. The residual pressure will be roughly 2 bar so you have to deduct 24L (2 x 6 x 2). That leaves 96L for four tyres ....... I am guessing but that might do two, I bet it runs out somewhere in the middle of the third?? Can you test it out somehow?

[Blanco]

Just a thought, if you carry a spare, every extra bar you put in it (just for the duration) will deal with one tyre, so if the tanks will cope with two (say) then put an extra 30 psi in the spare to cope with the other two. You need an adaptor hose with guage from one tyre to the other.

[sean f]

Calculation as Blanco has used is Pressure X volume = Pressure X volume.

After that it get more complicated, working out the free air volume (atmosperic pressure) is easy and done above, working out the volume of the tyres is more difficult particularly as you would need to make anllowance for the change in volume of the inside of the tyre as the pressure increases.

Volume of the tyre at 30psi (approx 2 bar) X 2 bar - Volume of the tyre at 15psi (approx 1 Bar) x 1Bar would equal the free air volume required. Air transfer would in theory be possible until the pressure in the tank equalled the pressure in the tyre, in practice there is friction loss as well (pressure to push air down line and open tyre valve).

How if you can work out the volume in the tyre it easy........

[Ex Member]

One 235/85/16 tyre is around 80 liters. So no. You will probably be able to fill one tyre.

[TheRecklessEngineer]

One 235/85/16 tyre is around 80 liters. So no. You will probably be able to fill one tyre.

Think you're out by a factor of 10 there. I make it approximately 8.5 litres/tyre.

8.5 x 2 bar = 17 litres free air/tyre

You have 12 x 10 = 120 litres free air. Although you also need to consider the volume of the bottles - so you have 120-12 = 108 litres.

Therefore you have enough for 108/17 = 6 tyres. Very approximately, neglecting loses.

You might just have enough - but it is likely to take an age to fill the last two tyres as you'll be down on pressure in the bottles.

[Tanuki]

You need to take temperature into account too: when you compress air it gets hot so if you fill your tank with air straight out of the compressor then as it cools the pressure will drop and you won't have as much air as you thought!

[this is the same reason why you always check tyre pressures cold].

[Ex Member]

Think you're out by a factor of 10 there. I make it approximately 8.5 litres/tyre.

Here is my math. Let's see yours.

235/85/16.... 0.81 m OD x 0.41 ID x ~0.2 wide.

3.14159 * (0.81^2/4 - 0.41^2/4) * 0.2 = 0.077 ---- 77 liters.

[TheRecklessEngineer]

Here is my math. Let's see yours.

235/85/16.... 0.81 m OD x 0.41 ID x ~0.2 wide.

3.14159 * (0.81^2/4 - 0.41^2/4) * 0.2 = 0.077 ---- 77 liters.

Same method, different order. (I misread slightly, used .28 wide above)

235/85/16 - 0.40m ID, 0.74 OD, .25 wide.

Copied and pasted from the Python command line:

0.25*3.14*(((0.74**2)/4)-((0.4**2)/4))

And the answer....

0.0760665

Bugger. Seems an awful lot, 7.6 litres seems much more reasonable. Anyone spot a mistake?

[Ex Member]

No, it is correct. A 235/85/16 is not 0.74 OD. That is 29 inches.

Maximum width is 0.235 m.... Less at the rim and the tread.

ID is actually a bit less. 16" is the bead inside diameter. The rim will sit in more.

[TheRecklessEngineer]

No, it is correct. A 235/85/16 is not 0.74 OD. That is 29 inches.

Maximum width is 0.235 m.... Less at the rim and the tread.

ID is actually a bit less. 16" is the bead inside diameter. The rim will sit in more.

Calculated using 1.85*0.40 = 0.74. I know it's not the actual case, but without going out and measuring one it's what I used.

Either way, it's obvious that the OP won't fill more than 1 tyre with his tanks.

[Ex Member]

Calculated using 1.85*0.40 = 0.74. I know it's not the actual case, but without going out and measuring one it's what I used.

Not sure what your math is trying to determine there.

0.4 + 2 * 0.85 * 0.235 = 0.8

Sidewall height is aspect ratio times section width. Rim diameter plus two times sidewall height.

[Nigelw]

Ok lets not miss a vital piece of info, the pressure drop is only by half so say it is 80liters per tyre I only need 40 as I have only dropped half the volume, so by those calculations which are way above my farmers type brain it can only do 2 tyres max or raise them all upto around 1.5BAR which would be adequate to get back to civilisation and a garage with compressor.

Unless I resort to one of the electric compressors in the back next the tanks until the OBA with A/C compressor is installed.

Answered my query though

[Ex Member]

If you go from 15 psi to 30 psi (which is what you asked), that is 1 atmosphere of pressure and equivalent volume of 80 liters per tyre. I doubt you are getting all 4 tyres up to a safe pressure to drive at normal speeds.

Just get a cheap compressor and learn to be patient.....

I don't have patience, so I have a compressor that can air up all four tyres is less than 2 minutes.

[TheRecklessEngineer]

At this point I'm going to concede that I've made a complete cock up of the maths and state that everything I've written above is complete bull, and perhaps I should reconsider doing maths after a boozy lunch.

Having an air compressor is the better solution. You only need a small air leak to empty your tanks.

[Nigelw]

I have learned and acquired an abundance of patience these last years as I have been busy rebuilding the Disco, but want to drive it more than work on it now.

I suppose I could gather all the bits together and replace the crank pulley for the A/C type with the extra "V" in it when I do the timing case next week and the rest should go on easy enough, just need a tap to open up the exhaust port on the compressor to take 1/2" fittings and could even put it on a PLC fitting and a switch for the beginning but never been one to leave things half done hence wanting to make a propper build of it.

We shal see.

[jad]

Erm cant you just try it an see??? Then you would like know....

Jad

[redneck]

If your tyre has a volume of approx 80 litres at atmospheric pressure (reading 0bar on your presure gauge) to get to 3barg, for example, from 0barg will take 240 litres per tyre.

In the more likely situation from 1 barg (aired down) to road pressures @ 3barg will take 160 litres.

Your 12 litre tanks at 10 barg have 120 litres total unfortunately you can only use the tank down to the pressure of your tyre so the tank will end up at 3bar i.e 36 litres of air will remain in the tank, therefore you have 84 litres of air to use.

Obviously 84 litres of air isnt enough to reach 3 barg in your tyre, so the tank pressure will go lower than 3barg until it equalizes with your tyre pressure.

So unless i've also bu**ered my maths up as well, you might get one reasonable pressure tyre out of it.

bored yet

[Nigelw]

So much so I have now opened the Whiskey bottle.

OK OBA to be brought forwad of locking diff. The comparable cost of installing OBA is the same as a cheap leccy compressor that might/might not fully inflate 4X 235/85 16 tyres.

Glad I asked though.

[Snagger]

A couple of problems with the pre-compressed air idea - firstly, it is potentially dangerous because the valves shoot out like bullets from the tank if hit hard; secondly, they're a one-shot deal, so once you've refilled a tyre, you're knackered, while a compressor will fill as many tyres as you need; thirdly, the compressor takes up less space and weight than just one tank, and; fourthly, when you hook up 120l of air at 10 bar, roughly 140psi, you will find it difficult to control the pressure of the charge to the tyre, but once the tank pressure drops to the same as the tyre's pressure, even if that's below the target, you're knackered again, so 120l of air will not be the result.

If you want 30 psi in your tyres, and they have a volume of 80l, then you need a little over 160l of air at atmospheric pressure (2bar is roughly 30 psi), which is less than you have in your tanks) - you need that 160l (or more for higher tyre pressures) plus the 12l of the tanks at the same 30psi (24l), plus the internal volume of the hoses and valves (lets assume 1/2L equating to another litre), so a theoretical air volume of 185l in those 12l tanks. You'll need to pressurise them to 15bar just for one 30 psi tyre.