Undoubtedly helium would be lighter than air but I wonder how much lighter the bike would become considering what a tiny volume of air would be replaced, even considering it would be under pressure.

At normal ground-level temperatures and pressures, helium doesn't readily form molecules, and each helium atom is itself smaller than the inter-molecular gap in many compounds. Toy helium balloons spontaneously deflate because the gas literally passes through the rubber.

Successfully maintaining pressure in tires inflated with helium would require a significant redesign of competition bike tires. My guess is that a helium-filled racing tire would actually be heavier than the current design.

What [Macartisan] says. Helium displaces 1 gram per litre. Do the math, and you'll see that you're going to need huge, helium grade tires to make any difference.

You could develop a new sport with this kind of bikes though. Something like "bounce 'n ride".

The question is how much does even a few grams matter in rotational mass?

I found the text below on the web at http://priuschat.com/forums/archive/o_t/t_3808/free-nitrogen-tire-fill-up-at-costco.html

Back before bicycle tech went all titanium and carbon fiber there was a lot of effort to lighten up by drilling holes in the components, using short pitch chains and so forth. One of the tricks for track racing (short duration events) was to use helium in the tires. Not only did it save weight but rotational weight, the best of all. And made for some high pitched fun onthe infield between races. Unfortunately, such hi tech bikes also tended to have lightweight tubes which were very permeable to those little bitty helium molecules. I don't recall seeing any real discussion of what the tradeoffs were including the increasing rolling resistance as the race went on.

Cool idea.

I don't know much about competition bike construction, but isn't the frame hollow? Why not fill that with helium? Or better yet, a vacuum.

The idea isn't that good: The differencewill be too small to be sensible.

I think it would work just fine for races but not on everyday travel. A rider can inflate just before a race. The wall of the tire is much thinker than a balloon so is should hold the helium longer. Since the race will end in a short period of time you would not notice a great deal of pressure loss. Even if the race went a couple of hours you ar still good to go.

However I don't think it would be a good idea on the biker who rides to and from work (as an example). Helium would be lost over time and may not hold the same pressure when leaving as it did when arriving at a destination.

It would definitely make watching the Tour De France more interesting when Lance Armstrong goes up in Flames while coming in for first place. Oh, the humanity!!!

I doubt it would make a positive difference, perhaps a zero pressure tire like what is in testing now for automobile spares. from what I've read about them they are easier to make lighter and of course you don't have to maintain pressure. If they work as auto spares and are being tested for normal tires they should easily work on a bicycle or motorcycle now.

"...when Lance Armstrong goes up in Flames while coming in for first place..."

Helium is non-flammable- in fact, doesn't like to bond with any thing at all, which is why helium molecules are monatomic. You're thinking about HYDROGEN, a very flammable gas.

Pure nitrogen gas would be a better gas to inflate tires. It doesn't absorb water, and isn't flammable at any temperature and pressure that a bicycle is ever likely to encounter outside an active volcano cauldron.

Dose'nt pressuerizing a gas make it heavyer. ie fitting more of it to the same space. If pressuerizing the tires to 45psi would make the tires lighter. Then it would stand to reason that the tank wich is at over 1000psi would need to be anchored to the ground?

Since air is mostly nitrogen filling the tire exclusively with nitrogen is hardly worth the trouble.

Race car tires are filled with nitrogen because using nitrogen from bottles excludes any water vapor from being injected into the tire. While this isn't a really important thing for your standard passenger or bicycle tire it is a big deal for racing applications because the amount of H2O in the air in a tire affects the pressure when the tires heat up. And at 200 miles an hour you really do want to eliminate as many variables on the handling of the vehicle as you can.

Although helium might be lighter because it's more buoyant, I think the mass is the problem in a bicycle, not the weight. You have to overcome inertia every time you change speed--you only have to overcome weight when you're going up hill.

I did the math -- you'd save over 8 g per tire, of rotational weight. This is definitely worth trying, at least in races where you have ready access to maintenance equipment.Here's the math: assume a wheel diameter of 700 mm and a tire diameter of 28 mm. The cross-section of the tire is 14*14*pi = 616 mm^2 and the volume is 2 * pi * 350 mm * 616 mm^2 = 1,350,000 mm^3 = 1350 cm^3 = 1.35 l. Air weighs about 1 kg/m^3 = 1 g/l at standard pressure so at tire pressure, which is 8 times normal pressure, it is about 8 g/l so you have 10.8 g of air in your tires.With helium at standard pressure you get 1 mole / 22.7 cubic liters = 4 g / 22.7 l = 0.176 g/l. At tire pressure that is 1.41 g/l so you get 1.90 g per tire with helium, saving a bit over 8 g.

Using nitrogen gas has a similar effect, motorist have sworn it feels like they have installed a new power steering pump or have skinnier tyres.