Reading about the Kirk magnesium frame on another thread, which is a truss/beamed frame and thinking of folding bike design has me wondering if fabrication cost is the reason that tubes are overwhelmingly chosen as material for bike frames. Does anyone make, say, titanium frames out of girder like material?

I chose titanium because it would seem (in my mind at least) well suited for that kind of a design and I'm pretty much convinced that aluminu is an unacceptabe material for the higher stresses of a folding bike.

TIA for any reply,
DG1

Tube frames are basically trusses anyway. You have two parallel cords and diagonal braces, just not very many of them. Once you shrink frame size vertically so that the unit can be made more compact you give up huge amounts of structural potential, your strength goes down by a the square, and stiffness by the cube. So if you go from a 20" separation to a single tube of 2 inches, the tube starts to want to be a little heavy, so a deeper truss made out of lighter elements starts to be attractive.

Tubing is pretty wonderful stuff and as nice as it is, the only reason it is cheap is because so much of it gets used for so many diifferent applications, that it ends up reasonably priced.

Round tubes are easy to fabricate, have excellent strength to weight characteristics, and take torsional loading very well, which is what you primarily get in a bicycle frame.

I-Beams are hopeless at dealing with torsion - they're only really designed to take bending in one plane.

What perterpan is talking about in terms of a smaller 3D truss is well illustrated in the Moulton folding bikes.

Tubes seem to be a poor way of constructing odd-shaped/assymetric bikes such as folders. You may get a better strength/weight using monocoque construction. Moulded carbon fibre is probably the easiest way to do funny shapes but pressure formed Al such as in the Biomega is another good way of forming really light, stiff structures.

"strength/weight using monocoque construction"

If that was true why would they require the wire downtube on the "Boston"?

Nothing wrong with Monocoque. It's never as light as a tubular structure though.

I wouldn't be using Biomega as an example of good engineering though. That's what happens when Industrial Designers' egos are allowed to run amok.

"The splendour of monuments"

"attain inner fulfilment though sheer velocity"

Get your hand off it Mac Newson you ******! Hide the cableing but then not care about standover clearance for shorter people, and conveniently ignore the biggest torsional loading experienced in a bicycle frame - that between the head tube and the bottom bracket - by attaching the 'top tube/boom tube'.......to the seat tube?

Idiotic.

I take it then, that designs with lattice braces, like the Moulton, are strongest. What about multi-shaped tubing and tubes that are drawn with patterns, like Colnago's Gilco set or Coppi's 'For Three'?

I suppose that what I'm asking is, if one were to make a good, stiff structure for a folding bike (with 406 or 451, 20" wheels), what's the best way to do it? I've seen evidence that aluminum isn't good enough to take the stresses of such a small frame
and I'm thinking that titanium is the more obvious choice. I'd like to avoid steel because folders are commuting bikes and are subject to the wet more than most other types of bikes are, and because steel weighs alot (a lightweight folding bike is a blessing because it gets taken places other bikes never go and there's usually a number of staircases along the way).

I've posted a photo of the discontinued Soma Ti-Journey below. What do you think of it, in terms of its design?

Thanks,
DG1

A tube is the monocoque for a cilyndrical form.

Ti has obvious advantages as far as weight and corrosion is concerned. I'm looking into building a first Ti frame, the tubing isn't all that expensive as far as bike stuff goes. I need to find out more about how to bend or machine it.

I dont know why this would be specific to folding bikes, but any engineer or construction worker will tell you that the strongest shape is one where the edges have the farthest average distance from the center. This means that a circle, where all the edges are the same distance, is the stongest. Triangles are also very strong, being used primarily in bridges, and you can see (two) triangles on bikes, which is where the strength in several directions comes from. I-beams are decently strong, but can only take stress in several simple directions, which is why they are used in building, because in a building, the building can only sway in 2 ways (hopefully!) and doesnt need anything special. Because of the strength (equadistant edges) to weight (hollow) ratio, tubes are very good to use on bikes.

I dont know enough about carbon to comment on monocoque.

Peter

Wha? The raw materials to build a Ti frame is three times as expensive as the best steel.

Right so your project cost is right up there with an incredibly expensive discount surly. Big deal. Also I found some tubes at about twice the price. Not sure there is enough stout stuff yet for what I want at a price I want it for...

"This means that a circle, where all the edges are the same distance, is the stongest."

It's really an efficiency issue. anything can be strong if your materials budget is big enough. What the tube does is gives you the best deal when the load can come from a lot of different directions, Like a fishing rod, where you can wiggle the thing in almost any direction, and when you do the down and dirty to land big fish you are loading it in all kinds of directions, so a tube is a good choice. I guess another example is bamboo. wind probably comes in all directions even if it predominates in one. Now a bow is not a good structure for a tube because the string determines that all the load comes from a narrow range of angles.

"Triangles are also very strong, being used primarily in bridges, and you can see (two) triangles on bikes, which is where the strength in several directions comes from"

Triangles are great because they support each other in such a way that they don't shift the corner angles. The triangles you see in bridges keep the top and bottom trush cords appart in such a way that the truss comes very near having the same strength as if it were solid, but with a fraction of the material.

There are well trained aerospace structures engineers who are rolling in their graves or rolling their eyes at the bastardized marketing of the word "monocoupe". Please stop! A large backbone structure is technically not a monocoupe.

Alright then, I've learned alot but I'd still like to know what I might start looking at specifically, were I to try and have a durable, yet lightweight tiny frame made. As I've said, considering how the thing is to be used, titanium is the material of choice. It'd be great if it folded, but at the very least it needs to have a folding handlebar set (something Dahon calls a handlepost). I would like a design where the wheels don't tuck under as I find that this is when grease arrives at clothing. My other desire is that the final product can withstand abuse and survive a minimum of 10 years daily riding.I suppose that means not so light that a dent will kill it. Probably 20 inch wheels; 451's seem right but 406's would do.

Whatever your thoughts I'll appreciate hearing them. Especially the custom builders who do this stuff so well.

Thanks,
DG1

bikes are made of tubes, because those wierd shapes arent race leagal, and very few companies want to make bikes that racers wont buy, because that would reduce their customer base. wierd monocoque shapes were allowed for a while, but then they were banned. look at the time trial bike lance rode in 2001 i think... not sure on the year exactly, but it was a lotus. and biometrics is gay.

Uhhh, you mean monocoque... (from french for single shell)

Monocoupe is a radial engine, fabric and tube airplane:
https://www.monocoupe.com/

basic geometry and physics says the circle and the triangle (and their 3-dimensional cousins) are the strongest shapes (they do the best job of redistributing force throughout their structure) that's why a tube-framed bike with the basic standard frame shape (a few dimensions changing based on size and preference) is so successful. more exotic structures probably rely more on the material characteristics than on structural strength.

I saw, what looked like a downhill, free ride bike at a LBS and it was mostly box section, weighed a ton too...