Pro teams and CO2

rstabler11
rstabler11 Posts: 67
edited January 2013 in Road general
Hey,
I'm not sure about this but I'm assuming all Pro teams currently fill their tyres with CO2 cannisters instead of pumping the air in by hand.
Does anyone know why they don't use Helium?
I haven't done chemistry in 3 years but tried to work out the weight difference and this is what I got:

pV = nRT
p = 160 psi = 1.103 X10^6 Pa
V = approx 1 litre? = 1 x10^-3 cubic metres.
T = 25 celsius = 298.15 K
R = 8.314

so n = 0.445 moles of gas molecules in each tyre.

Molar mass of helium = 4.00 so weight of air in each wheel is 1.78g
Molar mass of CO2 = 44.01 so weight of air in each wheel is 176.04g EDIT: 17.6g

So with two wheels potential weight saving = 31.5 grams. EDIT (not 348)

If this is true the difference seems large enough to warrant using the more expensive gas, especially under a 'marginal gains' system and on hilly stages... Does anyone know why they don't use Helium?

Thanks,
Rob

EDIT: for the sake of people joining the thread, I have changed the numbers where I accidentally moved a decimal point, so my original calculation should have been 31.5g (not 348g). Left the rest of it as is, although many answers are supplied below (the most significant being He would simply leak out of a tyre far too quickly).
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Comments

  • Careca
    Careca Posts: 95
    Perhaps pro-teams should insist that all their riders shave their heads too. That must save some weight :)
  • A single 16g canister of CO2 is enough to fill a 700c x 23 to a reasonably good pressure >100psi.
  • Interesting, but it's been too long since my A-Level chemistry to be able to check the figures, but ballpark, they seem OK.

    One problem would be maintaining pressure. Helium atoms are very small and would leak out of the tyre much quicker than air or CO2. Probably last a 4 hour race though?

    Oh and recently saw an article about a potential ban on using helium to fill balloons as it's becoming much rarer.
  • Camus
    Camus Posts: 189
    Because instead of racing all the riders would just inhale the canisters themselves and spend the day doing high pitched voices. :shock:
  • Ah, hang on, think you are way off on volume, 1 litre per tube is too much, maybe more like 200cm3?
  • Don't forget to factor in the minute amount of lift the helium would provide, thereby reducing rolling resistance..... :D
  • ddraver
    ddraver Posts: 26,383
    I think the reality is that they use an air compressor back on the bus....
    We're in danger of confusing passion with incompetence
    - @ddraver
  • ride_whenever
    ride_whenever Posts: 13,279
    I think it would leak too fast at 100psi. They don't use CO2 because it leaks too fast due to the high solubility of CO2 in rubber.
  • jibberjim
    jibberjim Posts: 2,810
    And there's a minimum weight for the bike which makes it pretty pointless trying to save weight by such means.
    Jibbering Sports Stuff: http://jibbering.com/sports/
  • careful
    careful Posts: 720
    From your fugures I make the difference as approx 40 X 0.445 = 17.8 g for each wheel. Seems a bit more credible.
  • Ah, hang on, think you are way off on volume, 1 litre per tube is too much, maybe more like 200cm3?

    Ah OK, I just guestimated a litre based on this but you're right when I look closely (at the bottom line) and carry it on to 23mm 200cm3 looks a lot closer - I just saw all the lines ending at around 1L and went with that. If the mass saving is then only around 70g I can imagine it's not worth it.
    jezzpalmer wrote:
    A single 16g canister of CO2 is enough to fill a 700c x 23 to a reasonably good pressure >100psi.

    This is probably the most concerning reply for me, as it indicates my maths was just really wrong... I don't know why though, but if experience tells you this is true then I guess the numbers must be off...
    I think it would leak too fast at 100psi. They don't use CO2 because it leaks too fast due to the high solubility of CO2 in rubber.

    I guess this is probably the answer, although it seems strange as CO2 is actually a larger molecule than nitrogen or oxygen in normal air... I don't know anything about the specific properties of CO2 so I guess it could be very soluble in rubber but I wouldn't know why...
    Can definitely imagine He would leak quickly though.
    But why has CO2 become so popular then? Surely a fast leak would affect someone on a sportive just as much as a TdF rider, but lots of casual sportive riders seem to use CO2...


    Anyway thanks for answers guys
  • careful wrote:
    From your fugures I make the difference as approx 40 X 0.445 = 17.8 g for each wheel. Seems a bit more credible.

    Oh yes. my decimal point moved :( well that clears a lot up :P
  • Volume calculation:
    Assume 700mm wheels, 23mm cross section. Assume tube and tyre have perfect fit.

    Tube length = 3.142 x 0.7 = 2.199m

    Tube cross sectional area = 3.142 x 0.0115 x 0.0115 = 0.0004155m2

    Tube volume = 2.199 x 0.0004155 = 0.0009 m3 = 0.9 litres

    Not 100% accurate as tube cross section is less than tyre but close enough I think
  • I think it would leak too fast at 100psi. They don't use CO2 because it leaks too fast due to the high solubility of CO2 in rubber.

    Is this why when I got a puncture last Sunday and re inflated the new tube with a CO2 canister I found it very deflated by the Wednesday but could not find a leak in the new tube? :?:
    :D
  • Pross
    Pross Posts: 40,452
    Volume calculation:
    Assume 700mm wheels, 23mm cross section. Assume tube and tyre have perfect fit.

    Tube length = 3.142 x 0.7 = 2.199m

    Tube cross sectional area = 3.142 x 0.0115 x 0.0115 = 0.0004155m2

    Tube volume = 2.199 x 0.0004155 = 0.0009 m3 = 0.9 litres

    Not 100% accurate as tube cross section is less than tyre but close enough I think

    The diameter of the tube will also be smaller as a 700c wheel has a diameter of 622mm so the tube diameter will be around 645mm and drops the volume to about 0.8 litres. My calculation comes out at around 100g per wheel. But why not go the whole hog, all those hollow tubes, stems etc. are full of air so seal them and pump them full of helium and the whole bike will be lighter :lol:
  • You're forgetting the opportunity to lighten the rider too. If he swallowed helium filled condoms (a la drug mules) and did a few as suppositories he could race up those hills.

    We might be on to something here...... :lol:
  • antooony
    antooony Posts: 177
    Is this why when I got a puncture last Sunday and re inflated the new tube with a CO2 canister I found it very deflated by the Wednesday but could not find a leak in the new tube? :?:

    Indeed sir it is.

    I have seen a pro teams mechanic at work, he was using a track pump to inflate the tubs then used a digital pressure gauge. No secret gasses, helium, marsh gas or methane in sight :D
  • sungod
    sungod Posts: 16,517
    er, chaps, i'm afraid i have to give you a fail on the calculation

    by my calculations inflating a road tyre adds about 8g

    taking a wheel that is fitted with a 24c tyre...

    depress valve until tyre collapses, i.e. atmospheric pressure, weight 1228.6g

    inflate to 7 bar, c. 100psi, weight 1236.6

    /smug mode
    my bike - faster than god's and twice as shiny
  • Pross
    Pross Posts: 40,452
    That's because there is still air inside the tyre unless you created a vacuum somehow :wink:
  • jibberjim wrote:
    And there's a minimum weight for the bike which makes it pretty pointless trying to save weight by such means.

    Not true? Better to save weight on tyres/tubes/rims and add it elsewhere if you have to.
    Sorry for being pedantic :oops:
    "You really think you can burn off sugar with exercise?" downhill paul
  • Pross
    Pross Posts: 40,452
    Not if the tyres are more likely to lose pressure over a long day in the saddle it's not.
  • I think we've answered this one, but I'm almost vaguely curious; what happens to helium when heated (admittedly this is less of an issue on bicycles) or compressed?

    Does it behave the same as air?
  • Pross wrote:
    Not if the tyres are more likely to lose pressure over a long day in the saddle it's not.

    Indeed :)
    "You really think you can burn off sugar with exercise?" downhill paul
  • ride_whenever
    ride_whenever Posts: 13,279
    I think we've answered this one, but I'm almost vaguely curious; what happens to helium when heated (admittedly this is less of an issue on bicycles) or compressed?

    Does it behave the same as air?

    Helium obeys pv=nrt pretty much exactly. Air isn't quite as obedient, it's a good approximation but you need a little fudge factor.

    CO2 is soluble in butyl rubber because it is a polar molecule.
  • Is this why when I got a puncture last Sunday and re inflated the new tube with a CO2 canister I found it very deflated by the Wednesday but could not find a leak in the new tube? :?:
    I read an article about CO2 on DCRainmaker's site a while ago and he talked about this ( www.dcrainmaker.com/2008/08/how-to-use- ... nners.html ). CO2 molecules are smaller than air so if you do an emergency repair while out and fill your new tube with CO2, when you get home you should deflate the tyre and reinflate it with air as otherwise the tube won't stay inflated for more than a day or two. Not something I was aware of either.
  • ricky1980
    ricky1980 Posts: 891
    Air is a composition of gases and molecules you can't say co2 is smaller than air as air is not a molecule by itself.

    You can pass air through an atomic filter that only lets hydrogen through...on the other side of the filter you still get some "air" or parts of it.

    You won't get any helium through tho.

    Helium is an inert gas should be used not sure why they don't. Use argon! Wait that's radioactive! Nitrogen is fairly inert as well should use that!
    Road - Cannondale CAAD 8 - 7.8kg
    Road - Chinese Carbon Diablo - 6.4kg
  • ricky1980
    ricky1980 Posts: 891
    Also can't remember if co2 dissolves or not. But I know rubber isn't tight! It leaks gases due to the molecular the hydrocarbon chain and how they sort of just meshed together to form rubber. Maybe helium has a much smaller atomic footprint it will.leak faster. Sod it all should just just compressed air
    Road - Cannondale CAAD 8 - 7.8kg
    Road - Chinese Carbon Diablo - 6.4kg
  • UCI weight limit of 6.8kg so what's the point?
  • The OP's reasoning falls down a bit because CO2 is about as far from an ideal gas as you can get, and PV=nRT is true only for ideal gases.

    F1 teams use (or used to - I think it might be banned now) nitrogen because its much closer to an ideal gas than the unpredicatable mixture of gases & vapour called air. This means the way the tyre pressure changes as the tyre heats up is more predictable. I doubt bike tyres temperature changes are as large as 200mph F1 cars, so that benefit will be essentially pointless.

    Helium is no good because it leaks too easily and the leakage rate increases with pressure. Even kept in cylinders, the pressure in the cylinders reduces over time.
    CO2 is used for bike tyres because the non-ideality means it liquifies under pressure and normal temperatures, and that means its practical to keep enough of it to fill an inner tube in a small cylinder. Technically, CO2 isn't a gas. Its a vapour because it can be liquified by pressure alone. That makes it relatively cheap to liquify. The rate of absorbtion (or is it adsorption - I can never remember which is the physical and which is the chemical process) is low enough that it doesn't matter in the short term. N2 would require cooling (to below -150°C) and high pressure (~50atm) to be usable in the same manner and would be expensive. Not really too practical to have a cryo unit on a bike either. And you'd almost certainly get freeze burns using the things.
    I've not used a CO2 cylinder for tyres - I'd assume they'd get cold. Does the valve ice up at all if you use them on a cold, damp day?

    CO2 phase diagram
    co2%20phase%20diagram.GIF

    N2 Phase diagram
    gif&s=52&w=300&h=236
  • graeme_s-2
    graeme_s-2 Posts: 3,382
    Guanajuato wrote:
    I've not used a CO2 cylinder for tyres - I'd assume they'd get cold. Does the valve ice up at all if you use them on a cold, damp day?
    They go ice cold when you use them, yes. Cold enough to stick to a bare hand and cause discomfort if you're not careful. You do generally get frost around the valve, and I've heard stories of people freezing the tube opposite the valve leading to a puncture, although I've never had this happen.