Airing up - the technical side. How much air do you need?

[Apocalypse]

Another off forum conversation that might be more fun on a forum

I got very fed up of the incredibly tedious 160l/min compressor which really doesn't dent the 35x12,5R17s so I've moved onto a 12ldive tank filled to 200bar fitted with a 10 bar regulator.

It'a very nice. filling the 35s takes a minute per tyre.



So, the first question is - how much air does a tyre hold?

then, secondly, how much air do we need to fill it to our desired pressure?

and thirdly ,why does it take so flipping long for a 12V 160l/min compressor to fill the tyre anyway?

Well. without knowing the precise shape the easiest way to get close to the volume of the tyre is to average out the width and profile and use the formula for the volume of a torus.

A Torus is a geometric shape that looks like a donut for those of you who didn't do Geometry at school.

the formula is thus:

Volume of a Torus:

V=(πr2)(2πR)

where:

R = Major radius

r = Minor radius

and yes, I had to look it up because it's far easier to draw it in CAD and query the volume, and the last time I did it the hard way I was considerably lighter.

anyway.

because it will confuse Manny I made a quick spread sheet for you:

EDIT: the mods have made it possible to upload a spread sheet... so you can download this or use Nic's link

tyre volume excel.xlsx

(6.83 KiB) Downloaded 133 times

if you know your metric tyre size - put it in the top yellow blocks and it will calculate your imperial size in the green blocks below and do the rest - or just put your imperial size in the green blocks. the spread sheet will work out the volume of your tyre

I didn't lock the other cells because I'm tired - just don't mess with them.

the next section deals with the volume of air at atmospheric pressure that you need to inflate them the the deisred pressure. Again, put the figures in the yellow blocks and don't mess with the others.

If you are filling your tyres from a compressed cylinder, such as a dive tank or fire extinguisher , the third section will calculate the volume of air it carries at atmospheric.

this will try easily explain why a 6l fire extinguisher at 8 bar isn't going to fill your tyres at all, but a 12l dive cylinder at 200bar will inflate your 35s in a few minutes and take care of a couple of metrosexual 4x4's tyres as well.

we shall then explain why your 160l/min 12V compressor takes forever to fill a small tyre...

AND.... the forum doesn't like excel spread sheets. you'll have to do it manually/

HELP! Mods!!! why can't I load an excel?

[Disco Nic]

https://docs.google.com/spreadsheets/d/ ... sp=sharing

Created a Google Sheets document, with open access. Anyone can log in and go play around with Alex's formula

[Apocalypse]

Thanks Nic!

[AndreasR]

You two guys just did more work than i did the whole of last year. Thanks.

Quick question. How safe is it driving around with a dive cylinder in your car.

[BushWacker]

... well ... Manny could work out the displacement by immersing in his fishpond with a few wall thickness adjustments ...

...

one of my BFGs is quite new vs the others and I find at 2.5b all round it currently runs a lot warmer ... pushing it up 1/4 a bar definitely helps ...
I always feel tire temp to also ‘gauge’ how things are going ...

[Apocalypse]

... well ... Manny could work out the displacement by immersing in his fishpond with a few wall thickness adjustments ...

...

one of my BFGs is quite new vs the others and I find at 2.5b all round it currently runs a lot warmer ... pushing it up 1/4 a bar definitely helps ...
I always feel tire temp to also ‘gauge’ how things are going ...

Well, if you were going to do it properly, you'd take the mounted and inflated tyre on a rim, immerse it, measure displaced volume.... then dismount the tyre, immerses it and the rim, measure displaced volume, then your actual volume would be the difference.

but it seems incredibly tedious. Also it would offend the fish. Anyway, Manny is a Porra. his idea of a fish pond is a deep fryer....

[Rabbiddog RIP 7/6/21]

Nick

How do we fill in the second part is we use compressors for comparison.

[Apocalypse]

Nick

How do we fill in the second part is we use compressors for comparison.

In the original post I added a downloadable spreadsheet .

We’ll discuss free flow and flow through an orifice later in the week.

But in the meantime , consider this.

The average lung capacity of an adult male is 6litres. At rest you should be respirating at 12 to 20 times per minute . With mild exertion you are looking at 20 to 30 times per minure (let’s not mention respiration rate when looking at your latest coct rates account )
.
Anyway . At around 27 breaths per minute you are moving more ‘uncompressed ‘ air from one place to another than your very expensive 160l/min ‘compressor’ ...

Now. Try blowing up a 5 liter rugby ball to 1,2 bar in a minute...

[rudi.hinze]

Alex, the 160lpm compressors take so long because the rating is at free flow, not any back pressure. Come to the dark side (Gauteng) and I'll show you how an ARB twin laughs at filling not one but 2 35's at once with an indeflate. It is rated at 132lpm, at 2 bar back pressure and will go all the way to 10.3bar, while drawing up to 69Amps at max load (yet with a 100% duty cycle). Thus, your air tank but without a limit to available volume.

To give you context. I have inflated 4 fully deflated (as in no presure inside) 285/70R17's at once to over 2 bar in just under 3 minutes. A single 35 takes less than a minute. Add an air tank and you can run air tools.

Yes, this is essentially a punt of my stuff but once you've heard it sing and seen it in action, you will see what I mean and the near R10k price is not so steep.

Sent from my BV6800Pro using Tapatalk

[Wooky]

I have one of the EL-Cheapo Ardendorff 160L/min compressors.
Works quite well actually for inflating my 31's. But then I am only inflating form 0.4 to 1.2 bar on the SJ.
Each tyre is done in less than a minute.... I can live with that.

I like the idea of a SCUBA cylinder but was put off by the need to visit a dive shop each time for a refill and the inspection requirements.
Not much of an issue if you dive regularly but not having a valid dive card makes it a problem.

What pressure do paintball cylinders run at?

[Saxj]

I have one of the EL-Cheapo Ardendorff 160L/min compressors.
Works quite well actually for inflating my 31's. But then I am only inflating form 0.4 to 1.2 bar on the SJ.
Each tyre is done in less than a minute.... I can live with that.

I like the idea of a SCUBA cylinder but was put off by the need to visit a dive shop each time for a refill and the inspection requirements.
Not much of an issue if you dive regularly but not having a valid dive card makes it a problem.

What pressure do paintball cylinders run at?

I tried using CO2, as in Sodastream (Similar to paintball cylinders, just a different thread and proprietary refill mechanism)
1. It's super expensive (R80+ per refill)
2. 1 cylinder could reinflate 2 215/80R15 tyres from 0.8 to 1.5 bar
3. The valve freezes quite quickly using CO2 (limiting flow)

The only thing it was really good for was rebeading a tyre, which it did exceptionally well.

[Apocalypse]

Alex, the 160lpm compressors take so long because the rating is at free flow, not any back pressure. Come to the dark side (Gauteng) and I'll show you how an ARB twin laughs at filling not one but 2 35's at once with an indeflate. It is rated at 132lpm, at 2 bar back pressure and will go all the way to 10.3bar, while drawing up to 69Amps at max load (yet with a 100% duty cycle). Thus, your air tank but without a limit to available volume.

To give you context. I have inflated 4 fully deflated (as in no presure inside) 285/70R17's at once to over 2 bar in just under 3 minutes. A single 35 takes less than a minute. Add an air tank and you can run air tools.

Yes, this is essentially a punt of my stuff but once you've heard it sing and seen it in action, you will see what I mean and the near R10k price is not so steep.

Sent from my BV6800Pro using Tapatalk

Pretty much the point of the post - I was getting to that part!

The cheaper ‘compressors’ are not so much compressors as ‘air shufflers’ - they just move air around .

The ARB ones are way better .

The dive bottle is great , you have 10bar head behind the inflation, rebeading is a blast . I’m not sold on the indeflate octopus at all - I can reinflate from 0,5bar to 2,4bar all round while the indeflate guys are still untangling hoses...

[Wooky]

I tried using CO2, as in Sodastream (Similar to paintball cylinders, just a different thread and proprietary refill mechanism)
1. It's super expensive (R80+ per refill)
2. 1 cylinder could reinflate 2 215/80R15 tyres from 0.8 to 1.5 bar
3. The valve freezes quite quickly using CO2 (limiting flow)

The only thing it was really good for was rebeading a tyre, which it did exceptionally well.

Thought the paintball cylinders were somewhat larger capacity than the Sodastream cylinders.

[Ricof4e]

I had (and still use) the 160l/min compressor.

On the beach, it would usually take about 3min a tyre to get from 1,2bar to 2,5 bar. Back then it usually amounted to two cigarettes (when I smoked I measured all time in cigarette time).

[Oppies3800]

I don't mind using a slower compressor

Gives more time to finish my case of beers I started while offroading

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