It is just air. Helium has been used but the cost benefit ratio is huge and helium leaks out of tires quickly.



Styrrell

What about nitrogen? It would leak out of tires more slowly and keep pressure more consistent.

I think i remember reading some track cyclists were doing this in the 1984 olympics. the weight savings is small. the problem with helium is it will leak through something that is "air tight". it is used for leak checking because it can fit through very small gaps which is why helium balloons deflate. you could go to a party supply shop and get a tank to try out.

I thought they were CO2?

~Matt

ntirogen is 96% as dense as air. we are talking about a weight savings of grams anyway. a bit of dirt stuck to your tire will weigh more.

If I did the math right...likely not, your complete weight savings for both tires at 120 psi @70 degree F in a roughly a 700 x 21 would be around 12g.

~Matt

Wow you got the volume of the air space in the tire and calculated the weight? Well 12gms diff in a wheelset can translate to quite a few $$ when buying different makes and models.

But as smtyrell says helium leaks, so I guess that's the end of that.

No. That has nothing to do with rolling resistance. Rolling resistance (measured in terms of power cost) is proportional to the total mass of the rider and bike, and has no relation to the distribution of that weight on the bike or rider or wheels.

I thought the whole reason behind using nitrogen at the track was for more stable pressure relative to tempurature changes when running the higher tire pressures often used at the track.

I don't know the exact physics behind it, but the outer circumference of the wheel, it has to do with centrifugal or centripedal or sumpthin forces of momentum and thus weight there has the most effect on 'rolling resistance' or at least momentum and speed. I read about it a couple of times in different places when reading about bicycle wheelsets and performance. From what I read basically shaving weight on the outer circumference on the wheels is the best place for performance benefits.

I was a business admin student not exactly the rocket science hi IQ science savy type. -:)

The weight of the rim basically boils down to the wheel's moment of intertia ("MOI") (and has zero to do with rolling resistance, with has to do with the rubber in the tire). You can think of MOI like you think of the bike's mass. The bike's mass adds inertia to the bike, which has to be overcome in order to accelerate it. Not to keep it moving, but to accelerate it. MOI is a wheel's version of mass, or inertia. MOI determines how much force it takes to accelerate a wheel up to the desired speed.

So yes, in theory, a higher MOI has to be overcome in order to accelerate the bike. The question is -- once the bike is moving along, just how big an effect does an MOI of "X" have versus an MOI of "Y" that is just a little bit higher? Most prognosticators just stop at the theoretical point and "conclude" that higher MOI is bad and that "weight at the rim is hugely important." But, they never have bothered to do the math.

When we do the math (which I am going to skip), we find that the range of MOI of various bike wheels will affect the average speed of a bike over a rolling course with various turns in a vanishingly small amount. So small that there exists no method of even measuring the effect in the real world. In other words, we can calculate the effect, but we couldn't actually observe it if we tried.

Conclusion: Fuggedaboutit.

Around 1996/97 there was some finding about using Nitrogen it was found that using it reduced rolling resistence,it may have been in Velonews.

'air' is what, 76% nitrogen anyway?

I see your point. Let's forget about the Helium since the weight savings is so negligable. But what I do know is just based on pure weight savings on the bike, besides the frame the next thing which varies in distinguishable weight the most is the wheel set.

When was the last time you weighed air? Not that heavy.

Helium filled balloons can lift pretty heavy weights and so can hot air. So besides being not heavy, air and gases can actually reduce weight to the extent of being lighter that weightlessness. But that's big volumes of the stuff of course.

The helium thingy started with the maybe not so viable idea that it can reduce a liitle weight in the rim area due to floating properties. As mentioned here it may have some effect since track cyclists actually use it.