# Helium in a foss tube will leak?

Dumb question?

Probably.

Also yes, since it will provide very small weight savings for the price and effort. You can either do the math yourself or google it to find someone that has done it already. Bottom line is, you probably wouldn’t be able to measure the weight difference if you put the unicycle on a scale afterwards.

Helium leaks very easily. I doubt it would stay inflated for long.

I assume you are thinking of saving weight by using Helium in the tube.

Air has a standard density of about 1.2 kg per cubic metre while Helium is about 90 grams. Raised to four atmospheres (60 psi) gauge, amounts to a difference of about 5.5 grams per litre. (Assuming all the air was removed, we multiply by five rather than four since the first atmosphere in the tube doesn’t register on the pressure gauge.)

A big 36 inch tyre with say a 7.5 cm (3 inch) wide circular cross-section would have a volume of about twelve litres so you would be saving about 65 grams of mass by using Helium instead of air.

Someone might want to check my logic and maths.

I was bored too and did the math with very similar numbers, and got 68 grams of weight savings. More than I expected, if you have a pretty accurate scale you might actually be able to measure that. (most home scales are accurate to 0.1 kg).

Thermoplastic inner tubes (which I think FOSS tubes are) should, I think, stay inflated with helium longer than butyl tubes. One of their advantages is that they keep air pressure better so presumably this makes them less permeable to helium as well. Quite how they would compare to the mylar foil used for helium balloons I have no idea.

Some info on thermoplastic tubes:

Quite whether it is a good idea or not, looking at more aspects than just weight saving, is explored (somewhat tongue-in-cheek) here:

I was just wondering about the rotational weight saving for a light road 3.8 fat tire with a fat foss tube.

Not meant to improve overall weight, intersted only in the rotational mass.
Not interested in butyl tubes, nor in tubeless setup, nor in heavy spokes or rim…

Why not a fat termoplastic tube filled with helium?

The last link seems to tell me: helium is not so important… but if you use a big air tire… made to be ride not at low psi… not in a butyl tube… you could save the weight in the most important part a uni…

Can you help me with the math for a 3.8" inflated at 40PSI?

The volume of the tube is its cross sectional area times the average distance around the tyre measured in centimetres to give a result in cubic centimetres which is the equivalent of a millilitre. Air weighs about 1.2 grams per cubic centimetre. Multiply this by the number of atmospheres pressure plus one.

The cross-sectional area depends on the rim as well as the tyre and there are irregularities but it approximates a circle with a diameter equal to the maximum width of the mounted tyre.

The area is pi times half the width of the tyre squared.

The length is the pi times slightly less than the nominal wheel diameter. This is best measured in the individual case as we know that tyre sizes are just numbers.

A trails tyre might be lightened by around thirty grams by inflating with Helium. That is not an insignificant difference in a high jump that only requires the tyre to stay inflated for a short time.

Should it be outlawed? Are people already doing it?

What about inflating with Nitrogen? It is slightly lighter than air due to the twenty percent content of the fifteen percent heavier oxygen molecules in air.

Nitrogen would reduce the weight of the air in the tyre by three percent. This is about a gram in a trials tyre.

BTW Planes have their tyres inflated with nitrogen but it isn’t about the weight.

Isn’t helium a smaller sized element that would leak out of anything much faster than air or nitrogen? Don’t foss tubes already have a problem that air leaks out regularly, all the time??

It’s not significant. I’m willing to bet you could give any competitor a unicycle that is 200 grams lighter than his current one, and competition results wouldn’t change. We are doing a ridiculous niche sport, not formula one. The big difference is riders and their technique, not equipment.

How much would it suck to not want to pump up your unicycle on the trail because you didn’t bring your helium bottle. Spend the time riding and building muscle, that will do much more than any rotational mass saving.

~10 grams of weight saving. About as much as a pencil weighs. I guarantee you would not notice it.

Spend the time you would have spend getting helium with riding instead, and enjoy. Spend the money you would want to invest into parts in traveling to unicycle conventions, and become a much better rider. Equipment is the smallest limit we face.

I think Vogelfrei80 is interested in the effect the lighter gas will have on reducing the overall moment of inertia of the wheel. Reducing that means you can accelerate/decelerate the wheel with less torque, and hence force on the pedals.

If you assume that the gas is a torus and you calculate the change in mass as in the post above, you can work out the change in moment-of-inertia using the lighter gas. Here is a handy calculator (it is amazing what folk put on the interweb ) :

So for a particular linear acceleration of the unicycle you can work out the corresponding angular acceleration of the wheel and hence the extra torque you need to accelerate that difference in moment of inertia. Once you know the torque you can get the force on the pedals for a particular crank length.

This will only be the change in force due to the different gas in the tube, you’d have to judge if this is significant, or model yourself and the unicycle in more detail to figure out the absolute force and then determine if the change is significant.

Or at least that is my current thoughts on how to work this out

I think one of the main reasons they put nitrogen in tyres is that it keeps pressure better. As far as I know this is because the N2 molecule is very slightly larger than the O2 in the air it replaces. Putting helium in has the opposite effect, hence the question on using a FOSS tube instead of a rubber one since the polyurethane material is apparently less permeable.

When considering the rotational inertia or moment of inertia I don’t think it is as simple as figuring the change in weight because the air or whatever gas is free to “flow” inside the tire. If the gas was not free to move inside the tire it would be pretty straight forward.

It may interesting to see how nitrogen works in a Foss tube. I have a Foss tube in my 36er and have not really noticed as much leakage as some report but maybe for some that leak more, nitrogen may help. In auto tires the nitrogen does slightly reduce the amount of leakage but some think it is not worth the added cost or bother.

You may well be right on that. The article I linked above (in Cyclist Magazine) does actually mention the gas movement in the tube, however it states that it will quickly get up to the same speed, estimating the time to be a few seconds, so as long as the wheel isn’t accelerating too fast it is probably a reasonable first approximation. Also, assuming the gas doesn’t flow would (I think) give an upper bound on the pedal force difference.

Folk say they can feel a significant difference when running a FOSS tube versus a butyl tube, so if you compare the tube weight difference (FOSS vs butyl) with the gas weight difference it should give an idea of whether the difference would be felt.

What goes in must come out.

I believe that they actually put Nitrogen in high performance tires because the molecules are larger and leak less. The 21% oxygen in normal air are the molecules that search out the small passages and escape. Also they corrode or oxidize the rubber in the tube/tire. I see helium as not having any real benefit. As has been discussed, the weight difference is not that great.

I think the helium would not take long to escape past a schrader valve, requiring a top up fairly often.

I was wondering just because I own a heavy hunirex, it has a 26" 3.8" road tire in a tubeless setup. I could easily access a helium can, I think I can also spend 20€ for a tube to insert in an already partially sealed tire. So my dumb question…

I will achieve a better result if I get slim, if I train more or if I spend 300€ for a carbon rim… but tomorrow I could spend 20€ in 5 minutes… not the same as getting slim, train for 10+ years or spend 300€…
But if it is a nonsense I will be glad to save 20€ too!

Thank you for your help. If I’ll ever stumble upon a new tire ready to be helium filled at the same price of a normal tire I will have my test and let you know. Right now I won’t use any leaking helium valve

this (kind of broken) video shows a TdF mechanic from Belgium using nitrogen + H₂O + most (“oil”) filter - to purify the nitrogen.
You can see the pistol and filters after 1:42 or so.
It is meant to keep the temperature of (glued) tubes low, during long breaking in long down-hills.

While I know the idea is very impractical, at least with the current equipment available, I still like the thinking of ways to improve on things a little bit, here and there.
A lot of those ideas might be crazy, but the occasional one can be groundbreaking.