# crazy idea! Pressurized bike frame to decrease weight and increase stiffness



## t-ruh (Jul 8, 2013)

I thought about this idea today and could not get it out of my mind. My idea is to pressurize a bike frame with helium to lighten and stiffen a bike.I think you would be able to build a frame lighter and keep it as stiff or stiffer because of the air pressure. I thought about it today when i tried to crush a sealed pop can and couldn't get it to budge. Then I opened it and could bend and crush it really easily. So...would this work or would it crash and burn. I think it would be really useful for hard tails rear ends that way you can make them stiffer or compliant by adding more pressure or decreasing pressure. Could you build something like this?


----------



## Drew Diller (Jan 4, 2010)

Can't really say as to stiffness. Probably negligible difference. The vessels I can think of handily that involve air pressure all flex quite a bit.

As for weight, you are unfortunately confusing the function of weight - or as I prefer, _mass_ - on the trail. The point of a light weight bike isn't really to be able to pick it up easily, which helium would certainly help with. The point of a light bike is to make it have less inertia so you can accelerate faster, brake faster, "flick" the bike around easily, etc.

Not that I know anything of the sort, all my bikes weigh more than 30 pounds.

Anyway, it's likely not worth your while.


----------



## Kiwiplague (Jul 22, 2011)

I can't imagine that you would actually achieve much, if anything, with pressurizing the tubes. How are you going to get the helium in there? How are are you going to seal the tubes around the bb shell, seat tube etc. What happens if the bike gets a crack? Explosive ruptures due? Also, the amount of helium you get into the frame is going to make no difference to the weight of the frame.
By the way, a sealed soda can has fluid in it, which can't be compressed. If it was full of gas only, then you would be able to crush it (although with some difficulty).


----------



## AZ (Apr 14, 2009)

Fill the tires with Helium, no need for special construction.


----------



## Eric Malcolm (Dec 18, 2011)

On my first MTB that I had, I went on a weight weenie campaign. I friend who was gullible, was totally sold out on seeing a valve that I had propped into the Down-tube water bottle boss that he thought I had actually filled it with Helium. He really thought my bike was lighter. Neatest trick I've pulled in a long time. 

Eric


----------



## Clockwork Bikes (Jun 17, 2006)

In Chem. 101 we calculated how much weight would be saved by filling a 700x23 tire with 100psi of helium instead of atmosphere. I think it was only 4g.


----------



## shiggy (Dec 19, 1998)

t-ruh said:


> I thought about this idea today and could not get it out of my mind. My idea is to pressurize a bike frame with helium to lighten and stiffen a bike.I think you would be able to build a frame lighter and keep it as stiff or stiffer because of the air pressure. I thought about it today when i tried to crush a sealed pop can and couldn't get it to budge. Then I opened it and could bend and crush it really easily. So...would this work or would it crash and burn. I think it would be really useful for hard tails rear ends that way you can make them stiffer or compliant by adding more pressure or decreasing pressure. Could you build something like this?


Compressed air weighs more than ambient air in the same volume.
I have weighed a 26x1.95 tire (mounted on a wheel) inflated to 30psi and at 60psi. The latter was ~15g heavier.

Part of the reason the sealed pop can will not crush is liquids are not compressible.


----------



## MCS5280 (Mar 14, 2008)

A similar approach has been used on the aerospace industry:
Balloon tank - Wikipedia, the free encyclopedia

The issue is that as soon as you lose pressure the whole thing will collapse, but being able to use extremely thin tubing may allow for some weight benefits.


----------



## RCP FAB (Jun 15, 2011)

Its been done. Didn't really work then either.


----------



## dr.welby (Jan 6, 2004)

Reminds me of the Checkered Flag Special.


----------



## compositepro (Jun 21, 2007)

Google porches 917 admittedly if memory serves the permanent pressurisation was gauged to detect frame failures a loss of pressure indicated a cracking somewhere

There was also a theory running the pressure increased its rigidity though it has been discussed ad nauseum


----------



## Harold (Dec 23, 2003)

eliminate the weight of ALL gases inside your frame tubes! vacuum seal them!


----------



## Oldfatbaldguy (Nov 4, 2010)

I believe the appearance of strength in a filled beer can is the result of the incompressibility of the liquid, combined with the extreme thin-ness of the can. The thinnest spots of a drawn aluminum can are usually less than 1/5mm with almost no resistance to bending, but they still have tensile strength. To crush the can, you'd have to "stretch" it somewhere.

Even using the thinnest wall tubing on a ridiculously light track bike, the force required to bend or crush the tubes is measurable. Pressurizing the helium enough to meaningfully resist bending loads would have to be very near liquid state, necessitating even thicker tubes to resist the pressure. 

The weight of the Helium would be negligable compared to the container. Many of us have experience with gas tanks for welding (oxy, acety, argon, C02) and the tank weighs more than the gas, even when full.

But experiments have been conducted with auto and aircraft frames made of thin aluminum or esin-bonded fiber and hard, closed cell foams. There might be a future for a foam core, carbon fiber bicycle even lighter than current carbon bikes.

Not for me, though. I'm happy chuffing around at modest speeds on a time-worn steel framed bike.


----------



## zerodish (Jun 17, 2006)

Not so crazy one NASA scientist said early spacecraft looked like massively strong structures but were nothing more than inflated tin cans. Nasa no longer uses this type of spacecraft but is experimenting with isogrid structures which they invented. I think it is the NOVA episode making stuff stronger which shows this. Also download the youtube video riding the booster with sound and you can deformations in the skin of the shuttle and tanks as it takes off. Pressure tanks are welded from the inside and outside simultaneously only the most skilled humans and robots can do this. It would be difficult to do this in the confines of bicycle tubes. Porter is using silver paste on the inside of tubes which melts and flows into the crack when the outside of the tubes are brazed. Tubes buckle when the structure moves outside the line of force, too much pressure any you would create the type of failure you are trying to prevent. Bicycle tubes have been pressurized both as an energy storage device like a flywheel and just to save air to inflate tires.


----------



## TimT (Jan 1, 2004)

The weight you would save would be nothing to the weight you would gain with the valves.

Tim


----------



## MrCookie (Apr 24, 2005)

The weight of the frame is only around 20% of the weight of the bike. Lose the weight elsewhere... bearings, tires, crank axles, etc.


----------



## unterhausen (Sep 28, 2008)

when I lived in Dayton almost 30 years ago, the compressors failed at the Air Force museum and they lost a batch of rockets that were being held up by compressed air -- they ended up looking like deflated balloons. I had no idea that would work for something so large. I think it's a matter of scale and required strength. The amount of dynamic pressure that a rocket experiences in flight is actually fairly low. So you can counteract that with pressure on the inside. It's not like they need much strength in bending. Given the loads on a bike frame, it's not likely to help much


----------



## Testmule (Jul 27, 2013)

compositepro said:


> Google porches 917 admittedly if memory serves the permanent pressurisation was gauged to detect frame failures a loss of pressure indicated a cracking somewhere
> 
> There was also a theory running the pressure increased its rigidity though it has been discussed ad nauseum


Gonna drive the Porsche guys crazy when you don't get the spelling correct - lol

I am old enough and fortunate enough to have watched the great Mark Donahue win the CanAm series in the 70's in his Porsche 917-30. The frames were titanium and were pressurized to monitor any cracks. In qualifying form the flat-12, twin-turbo cars produced close to 1,500 hp. As a kid I would hang around the start of the straight away and listen to the big Porsches come out of Moss's corner at Mosport and go through all the gears. Still sends quivers up my spine.


----------



## MrCookie (Apr 24, 2005)

Testmule said:


> Gonna drive the Porsche guys crazy when you don't get the spelling correct - lol
> 
> I am old enough and fortunate enough to have watched the great Mark Donahue win the CanAm series in the 70's in his Porsche 917-30. The frames were titanium and were pressurized to monitor any cracks. In qualifying form the flat-12, twin-turbo cars produced close to 1,500 hp. As a kid I would hang around the start of the straight away and listen to the big Porsches come out of Moss's corner at Mosport and go through all the gears. Still sends quivers up my spine.


That would have been awesome to witness! Thanks for sharing.


----------

