I know that helium balloons float because it is less dense than air. I'm not expecting my bike to float, although that would be pretty cool. I just wanna know if replacing normal air with helium in the tires will produce a noticeable effect on its weight. Will the helium 'lift'/reduce the weight force on the bike?
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asked Mar 19, 2017 at 11:02
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4$\begingroup$ If you're planning to ship your bike and want to save a few bucks on by making the bike weigh less, I'm afraid I have to disappoint you. The price you'll pay for the helium will outweigh the marginal profit you could make by inflating your tires with helium, even adding helium balloons won't save you enough. I know someone somewhere already tested this out, however I have forgotten where I read this or saw the video so i can't name a source anymore. $\endgroup$– BlueCactiMar 19, 2017 at 22:31
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6$\begingroup$ Just think about how little air the tires displace. Even with a perfect vacuum inside the tires (while still magically keeping the tires’ shape) that’s all the lift you could get. $\endgroup$– MichaelMar 20, 2017 at 8:36
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4$\begingroup$ It should be kept in mind that helium will leak through the sides of the tire much more rapidly than will normal air. Ie, the tire will go flat quite quickly. $\endgroup$– Hot LicksMar 20, 2017 at 21:03
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$\begingroup$ @GroundZero one could possibly have some unused helium lying around (indeed, I do have some unused helium lying around in my spare bedroom), making it a sunk cost. I don't think I'd bother all the same, though. $\endgroup$– Jon HannaMar 21, 2017 at 15:06
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3 Answers
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It will make it lighter, but the effect will be very small. The volume of the tube is probably less than a liter. One mol of an ideal gas is 23 liters at atmospheric pressure. So you have about 0.2 mol of gas in there at 4 bar pressure. Helium weighs 4 g/mol, nitrogen about 28 g/mol. So for 0.2 mol, the weights are 0.8 g and 5.6 g. Cleaning off the dirt from the frame will have a greater effect.
Helium atoms are smaller than nitrogen molecules. Therefore there is a greater rate of diffusion through the bike tires. Your tires will become flat quicker than normal. Therefore it is not really a good idea to use helium.
answered Mar 19, 2017 at 11:08
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$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– rob ♦Mar 20, 2017 at 7:53
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2$\begingroup$ Through the tires would be effusion, not diffusion, no? $\endgroup$– SamuelMar 21, 2017 at 1:52
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3$\begingroup$ According to another answer (Dmitry) your math is off. The concept is valid, but he calculates the voume to be 2.4 liters, containing 11.5 liters of gas, effecting about 12 grams difference ;) And he has a link to justify his math - it seems you seriously misunderestimated the volume of a typical tire. $\endgroup$– TomTomMar 21, 2017 at 13:48
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$\begingroup$ Martin's volume estimation is closer (Dmitry used the wheel size, which is much larger than the tube). The tube is seated into an odd shape but it's a bit less than 1" wide and a bit more than 1" tall. 29" * Pi * 1" * 1" is a good approximation = 1.5 L. Probably skinny road bicycle tire tubes (which is where people care about eight) are closer to <1 L and fat MTB tires are closer to >= 2 L $\endgroup$ Mar 22, 2017 at 7:22
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2$\begingroup$ @RyanCavanaugh Road bicycle tubes are smaller but are inflated to significantly higher pressures (6 to 9 bar), so they have roughly the same amount of air in them. $\endgroup$ Mar 22, 2017 at 8:19
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Helium has been used in racing bicycle tires for indoor track (velodrome) events.
The helium will decrease the overall weight of bike and rider only slightly, and it will somewhat reduce the angular momentum of the tires.
There's also the possibility that helium is more "elastic" than normal air or pure nitrogen, which would reduce rolling resistance, but I haven't found any articles about this. Helium is slightly less compressible than air, but I don't know if this affects "elasticity".
Depending on tire technology, the helium leaks far too quickly to be useful for a longer period event such as a road course. This could change if tubeless tires could be made to not leak helium. I don't know if the rules for bicycle racing have changed to disallow helium in tires anymore.
Update - there may be another factor, heat dissipation.
"We used helium because it is such a good heat conductor. Our rims and tires were so thin heat buildup was a hazard and the helium helped keep them cool."
Scroll down a bit into this forum thread:
I recall decades ago someone mentioning that using helium allowed somewhat higher pressure to be used in the track bicycle tires, which could be related to the heat dissipation, but I haven't found an article to confirm this yet.
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7$\begingroup$ I wouldn't use the word “elastic” with gases – both air and helium are very elastic in (adiabatic) purely compressive processes (they are well approximated as ideal gases), so as far as compression is concerned, the rubber deformation would almost certainly dominate the energy dissipation. $\endgroup$ Mar 19, 2017 at 21:39
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$\begingroup$ Unlikely IMO but possibly also relevant for rolling resistance is incompressible gas movement along the tyre. I'd conjecture that this is a pretty laminar flow, in which case we'd need to compare viscosities... in that regard, air and helium are pretty similar. If turbulence needs to be considered, helium might have an edge thanks to lower Reynolds number. $\endgroup$ Mar 19, 2017 at 21:41
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1$\begingroup$ Just a though among others : since Helium is lighter than air, you could augment tire pressure thus rigidness - linked to the density of filling matter ? $\endgroup$– BenjMar 20, 2017 at 13:28
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$\begingroup$ @Benj - I would think compressiblity would be more of a factor than density in terms of tire deformation. The density would affect momentum associated with deformation, but I assume compressibility would have a more significant effect. $\endgroup$– rcgldrMar 20, 2017 at 20:05
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The most common wheel size for mountain bikes is a torus with R = 307mm and r = 27 mm. Assuming about 7mm of rubber on the rolling surface, this amounts to about 2.4 liters of volume. For a typical pressure of 70 psi, you'll pump 11.5 liters of air in your tire. That air would have a mass of about 14 grams, and an equal volume of helium would be about 2 grams.
For both wheels, replacing air with helium would give you about 24 grams of weight reduction. Not really worth it, unless you're about to set an Olympic record and every gram counts.
Also, if your bike had tires big enough that filling them with helium would make the bike float, it would float upside down ;)
answered Mar 20, 2017 at 9:46
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7$\begingroup$ Don't forget that in addition to the mass savings, you're saving angular momentum, since all that mass is in the outer part of the rotating wheel. $\endgroup$ Mar 21, 2017 at 13:48
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6$\begingroup$ For those of us who do not really remember how much 70 psi is, it is about 480 kPa (or 4.8 bar). $\endgroup$ Mar 22, 2017 at 10:41