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 pm124 10-11-06 12:21 PM

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.

 dalmore 10-11-06 01:55 PM

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. ;)

 14R 10-11-06 02:00 PM

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

 stargazer48 10-11-06 02:22 PM

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

 eff-J 10-11-06 05:30 PM

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

 14R 10-11-06 06:20 PM

It works though, with some practice, the possibilities are infinite

http://www.crystalhorizon.com/images...nshot_bike.jpg

 Speedo 10-11-06 08:53 PM

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...

 jur 10-11-06 09:06 PM

A good fart will save you more...

 pm124 10-11-06 10:15 PM

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.

 clonmult 10-12-06 06:21 AM

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

 caotropheus 10-12-06 06:47 AM

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?

 Speedo 10-12-06 07:03 AM

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.

 MisterJ 10-12-06 09:19 AM

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

 law4jba 10-12-06 10:51 AM

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.

 Speedo 10-13-06 06:45 AM

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!

 14R 10-13-06 08:07 AM

[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...

 pxboii 10-13-06 11:13 PM

use ex-lax

 LWaB 10-14-06 04:35 AM

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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