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Thread: Stupid Question

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    Car free since 1995 pm124's Avatar
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    Stupid Question

    This is probably showing my ignorance of basic physics and general stupidity, but why do folks use CO2 cannisters rather than helium? Would a tire filled with a lighter element lighten the bike? Certainly, the more pressure in the tire, the heavier the bike.

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    Neat - w/ ice on the side dalmore's Avatar
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    While you are correct that helium is lighter than CO2, both are so light and bike tires hold such little volume that it's inconsequental. I suspect someone here will have the actual figures at hand or the inclination to look them up. I'd bet we're taking about fractions of a fraction of gram.

    I may actually have to look that up now to see the numbers.
    Current favorite bumper sticker: Wag more. Bark less.

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    Bromptonaut 14R's Avatar
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    To save about 10 times that value I just wash my face before a competition to reduce the weight of bacteria pulling me down...

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    Just lose a minimum couple of ounces of body fat to lighten the bike load.

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    Man About Town eff-J's Avatar
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    Quote Originally Posted by pm124
    Would a tire filled with a lighter element lighten the bike?
    Yes, but too much, as it turns out. It's actually so light that you start to reduce available traction and you can't accelerate or brake worth a damn.

    For aluminum-frame bikes and/or with lighter riders, you might actually lose contact with the ground altogether and wind up just spinning your wheels in what they call a "track float".

    - Jeff

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    Bromptonaut 14R's Avatar
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    It works though, with some practice, the possibilities are infinite


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    Senior Member Speedo's Avatar
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    This question has been up for six hours and no one has taken the opportunity to use the Ideal Gas Law??!!

    PV=nRT

    Let gauge pressure be 90 PSI, then absolute pressure is 105 PSI, which is 723 kPa. The volume of a 1.5 inch tire on a 406 rim is about 1.58e-3 m^3. The ideal gas constant is 8.3145 J/K at temperuture of 297 K (about 75 deg F). Solving for n, number of moles of gas we get n=0.46 moles. The atomic weight of CO2 is about 44 vs atomic weight of Helium of 4. So, the mass of the gas would be:

    Helium: 1.85 grams
    CO2: 20.4 grams

    So, you can shave 0.65 ounces by using Helium instead of CO2

    If you use start with good old air it might not save as much becuase CO2 has a lot of oxygen and air has a lot of Nitrogen which is slightly less massive.

    My high school chemistry teacher would be proud...

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    jur
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    A good fart will save you more...
    My folding bike photo essays www.dekter.net/

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    Car free since 1995 pm124's Avatar
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    Ahh, PV=nRT. I do remember that now, and should have simply looked it up. You rock, Speedo. Especially if that's from high school chemistry; I've been through 2 years of chemistry and physics, and have forgotten it all.

    Nice, 14R. But that's nitrous oxide, not helium. (Yummy.)

    So, it actually works out to not entirely negligible--40 grams across both tires in the most critical part of the bike. Schwalbe charges a lot per gram at the end of the day.

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    Quote Originally Posted by jur
    A good fart will save you more...
    Light it and go even faster

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    Senior Member caotropheus's Avatar
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    Hhhuuummm, let me see... First point is that Helium is much more expensive than CO2 probably in the scale of 1:10, secound I always used the good old frame pump and the tires were always inflated, so what is the relation between Helium, CO2 and folding bicycles if you are not in competition?

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    Senior Member Speedo's Avatar
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    Quote Originally Posted by caotropheus
    Hhhuuummm, let me see... First point is that Helium is much more expensive than CO2 probably in the scale of 1:10, secound I always used the good old frame pump and the tires were always inflated, so what is the relation between Helium, CO2 and folding bicycles if you are not in competition?
    Not much. I was surprised that the effect was as large as it is, but for the standard recreational rider it's not much.

    On the other hand, for a racer who has no beer belly to loose, and who has purchased, at hideously high prices, the lightest widgets there are, and who has already drilled out their chainrings and other metal items, it might be worth carrying the Helium tank in the team car.

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    The helium molecules themselves are much smaller and you would be losing pressure much faster.

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    Senior Member law4jba's Avatar
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    Quote Originally Posted by MisterJ
    The helium molecules themselves are much smaller and you would be losing pressure much faster.
    Helium has a very high gas permeability constant for rubbers including butadiene/styrene rubbers commonly found in tires.

    Carbon dioxide in contrast has a low gas permeability constant.

    Consequently, the helium pumped into the tire leaches out several orders of magnitude faster than carbon dioxide.

  15. #15
    Senior Member Speedo's Avatar
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    Quote Originally Posted by law4jba
    Helium has a very high gas permeability constant for rubbers including butadiene/styrene rubbers commonly found in tires.

    Carbon dioxide in contrast has a low gas permeability constant.

    Consequently, the helium pumped into the tire leaches out several orders of magnitude faster than carbon dioxide.
    You learn somthing new every day!

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    Bromptonaut 14R's Avatar
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    [QUOTE=pm124] Nice, 14R. But that's nitrous oxide, not helium. (Yummy.) [QUOTE]

    All you need is a good looking female friend that is also a dentist. The possibilities are infinite...

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    use ex-lax

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    Some national track teams (US in early 80s) used to have helium tanks for the World Championships occasionally. Don't know if anybody does it now.

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