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Are CO2 or other portable cartridge style devices feasible for use on fat bikes with tires 26x4.5" or greater? I know there are large 20g or 40g cylinders available? If so, what sizes of cartridges would be appropriate?

Though, I'm sure it might be good to have a pump as a backup as well in case you run out of cartridges.

  • The cartridges I used to use (these would have been 16g) were advertised as sufficient to pump up a 700x23 road tyre to 100psi or so, and a 26x2" to 60psi ish. So basically, one cartridge would give you a near-ideal amount of inflation. For a fat bike I'm guessing that you'd need multiple cartridges. Oh, and from experience, you're right to keep the pump ;-) I'm going by what I read in the blurb, I haven't done the calculations. – PeteH Nov 11 '15 at 22:26
  • To pump up a 4" tire you need 4 times as much gas as you would use for a 2" tire to achieve the same pressure. – Daniel R Hicks Nov 12 '15 at 2:01
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    Time's moved on - you can get on-bike compressed air tanks like Rideair or Zefal Tubeless Tank or Airshot which is aimed at tubeless and seating. No idea which one is better or best, but the idea is to have a much larger compressed air tank with you than a CO2 cart holds. – Criggie Nov 7 '17 at 1:27
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    @Criggie - Normally, I would say WTF on carrying a big canister like this, but I can actually see some use for fat bike tubeless, in case of beat coming unseated. I'm still partial to carrying an HV (High Volume) pump and a tube just in case. – Benzo Nov 8 '17 at 21:00
  • @Benzo fair enough - the "big inrush" of air is mostly to initially seat tubeless tyres on the rim for the first time. Given your tyre came off that same rim, its probably fairly well shaped already and should be easier to seat. – Criggie Nov 9 '17 at 7:54
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The short version: Yes, they are feasible.

The long version: You can do a minor adjustment of my answer in this question by changing the molar masses involved.

The molar mass of carbon dioxide is 44.01 g / mol. The ideal gas law says PV=nRT, where P is pressure, V is volume, R is the ideal gas constant and T is temperature (in an absolute scale, e.g. Kelvin). Rearranging terms and converting from mols to grams, we see (44.01 grams/mol)*PV/(RT) is the grams of carbon dioxide you need to achieve pressure P for a tire of volume V at temperature T.

Lets be crude about estimating the volume of a tire using a torus. The volume of a torus is V=(pi*r^2)(2*pi*R) where R is the major radius and r is the minor radius. Google will calculate it for you (and has a picture of what major and minor radius is). We'll use an estimate of 15.25 inches for the major radius and 2.25 inches for the minor radius giving 1524 cubic inches, or V=25 liters. Assume a temperature of T=293 K (room temperature) and asking for a pressure of 1 bar (14.5 PSI) gives 44.01*1.02 g which is about 45 grams. We'll also assume the volume of the tire is independent of the pressure (which is reasonable).

You can use this wolfram alpha link to play around with the numbers. Note that this calculation is linear in the pressure for fixed volume, so asking for half the pressure requires half the CO2.

Alternatively, rearranging the terms, if your cartridge has M grams of CO2, you can rearrange (44.01g / mol) *PV/(RT) = M to see P = MRT/(44.01 g / mol * V). Thus, the pressure you can achieve changes linearly with the mass of CO2 you use provided temperature and volume remain fix.

So, if you want to go to up to around 14.5 PSI, carry a 45 g cartridge. A 20g cartridge will give you 14.5*(20/45) PSI = 6.4 PSI, and a 25 g cartridge will give you 14.5 * (25/45) = 8 PSI. 16g gives you 5.2 PSI, 14g gives you 4.5 PSI and two 16 g cartridges (i.e. 32 g) would give you 10 PSI.

Note that these numbers are a bit rough -- you might lose some pressure connecting the CO2 inflator, and we used an estimate of the tire volume. With more accurate numbers you can improve the estimates (and they should be quite good with a good estimate of the temperature, volume of tire). Overall though, I believe this calculation gives a bit overestimated amounts of CO2 required -- the volume is likely a bit smaller than the volume we used as an estimate.

But they should be good enough for determining the right size of CO2 cartridge to use.

Note that increasing temperature means you need less CO2 (while holding pressure and volume fixed). Increasing volume (i.e. using a bigger tire than what you calculated for) means more CO2 (while holding pressure and temperature fixed). Increasing pressure means you need more CO2 (while holding volume and temperature fixed).

  • Cool. May need to consider temps for this as well. As a lot of people choose to ride fat bikes in the snow, ride through deserts, or do crazy things like race the arrowhead 135 in -30f temps. – Benzo Nov 12 '15 at 15:05
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    Yeah. Still, its not hard to adjust the math. 55.6 g CO2 is needed for 1 bar at -30 F (238 K). If you want a more reasonable 10 psi, thats 38.2 g at that temperature, so if you carry 45 g CO2 cartridges, you'd be good for that temperature too (and a bit below). – Batman Nov 12 '15 at 15:22
  • Yeah, appreciate the formulas. Gives the tools to do estimates for a varying amount of tire sizes. For practical purposes, it's probably more practical to carry a few 25g cartridges versus a single 45g cartridge due to cost concerns as 45g cartridges are $30+ each. – Benzo Nov 12 '15 at 15:53
  • And if you're running tubeless setups, as a lot of people like to do on mountain bikes and fat bikes, it's important to be sure that you inflate with the valve facing up so that the C02 filling the tire doesn't freeze the latex sealant. – Benzo Nov 12 '15 at 15:54
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    As an addendum, Genuine Innovations has a chart for how much pressure you can get here. The numbers are 3 PSI from 16g, 5 psi from 20g, and 7.5 psi from 25g. They may have empriically measured it which would take into account that theres some residual CO2 in the cartridge after you empty it and some loss from connecting the inflator (you can measure this stuff by weighing inflated tires and cartridges and what not). – Batman Nov 18 '15 at 17:04
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enter image description here

A quick way is to get the closest then then scale
p is presure
d is diameter

pScale = pKnown * dKnown * dKnown / dScale / dScale

here a close is 26 x 2.4 16 gram
so

pScale = 27 * 2.4 * 2.4 / 4.5 / 4.5
= 6.68

p is linear with mass so 25gram = 6.68 * 25 / 16 = 12

but on a fat bike I would just carry a high volume pump

  • 2
    You should cite the source of your diagram. – RoboKaren Nov 12 '15 at 17:21

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