indestructible frame (alu + cf)

[vspa]

In cable tv i saw the construction of oxygen tanks for firefighters, which were capable of sustaining an internal oxygene pressure of up to 4500 PSI ! ( keeping a low weight )

It was high quality aluminium going through a range of different treatments.
Once they had the desired qualities and the desired shape they covered it with carbon fiber all over. Finally with glass fiber to protect the CF coat,

I was imagining a frame constructed in that way would be near to indestructible, for 1 or 2 pounds of added weight ?

 

[elithrar]

The shape of an oxygen tank is different from a bike, however, and doesn't need to withstand twisting forces nor does it have to fit within a certain shape/volume.

A big, round cylinder is easier to produce and manufacture than a bike frame, given the same materials.

 

[dabac]

Originally Posted by vspa .

In cable tv i saw the construction of oxygen tanks for firefighters, which were capable of sustaining an internal oxygene pressure of up to 4500 PSI ! ( keeping a low weight )

It was high quality aluminium going through a range of different treatments.
Once they had the desired qualities and the desired shape they covered it with carbon fiber all over. Finally with glass fiber to protect the CF coat,

I was imagining a frame constructed in that way would be near to indestructible, for 1 or 2 pounds of added weight ?

Not really. An oxy tank is fairly short and stubby compared to the separate pieces of a bike frame. Then there's the fact that the biggest force acting on the tank is the ability to withstand expansion. Bikes has to resist several different forces.
If' you're willing to accept a 1-2 lbs weight increase, a frame can be built pretty much "indestructible" today already, by only upping the wall thickness of the tubes.
Not that lack of frame longevity is much of a problem anyhow.

 

[Volnix]

Gas tanks are cool arent they? /img/vbsmilies/smilies/biggrin.gif

In my search for a good design for a frame that is supposed to last steel was my first choice. (The strong one like cro-mo so the frame will be light too). I checked some fully equiped cro-mo touring bikes which were around 12kg which is not that much really.

Aluminium does have some fatigue issues, but apparently after searching a bit about it there are some "warning signs" that the frame is starting to brake like some "creaking sounds" etc. A good corrosion resistant design also helps in avoiding that.

Carbon fibre is supposed to be a totally "fatigue damage" proof material, as in it wont lose any of its strenght characteristics or brake due to normal use in its design loads, but then again I am unsure if that is actually true for a carbon bike. For example is the epoxy that is used in gluing the carbon fibre sheets fatigue damage resistant aswell? I also dont like very much that carbon fibre sheets sell very very cheap and the bikes made out of them are so expensive. Its not such an energy consuming procedure to build a carbon frame either and there arent so many manufacturers that say what makes the frame "special" in design. Some very high-end ones do but then the prices are too expensive for a bike to be used for normal use anyway.

There are also some very high end cro-mo frames with very good corrosion resistant design (since corrosion is a bit of an issue for steel).

 

[dabac]

But making a CF frame can be very labor intensive. With steel, you're just about limited to butted tubing, but with CF you can add layers just where you want them for a strength/weight optimized design.
One frame I read about started life as 400 differently shaped patches of CF prepreg cloth. These have to be laid out and then cured.
Also, while a frame made from tubing will support itself during assembly, CF has to be laid up in molds, or around templates(which have to be removable).

That curing takes time, and for that time the mold is occupied. Metal, you get a away with a fixture/jig, and you can remove the frame seconds after the braze/welds have cooled.

Epoxis aren't all created equal, but I'd consider the life of a good epoxi(given properly UV protection) as indefinite.

 

[Volnix]

Originally Posted by dabac .


Epoxis aren't all created equal, but I'd consider the life of a good epoxi(given properly UV protection) as indefinite.

I cant say that I checked too much into cf frame manufacturing but on what I checked whas it was basically layers of cf layed on the mold and then pressured with some kind of pressure medium like air.

There is lots more ofcourse, like designing the frame etc but what I was saying was that the "cheap" cf frames are still too expensive if they lack a design and are just made in the same principle as a metal frame.

Is the epoxy the stuff that glues the cf sheets together? Wont that be susceptible to fatigue damage?