# How are single tube bicycles prevented from collapsing?

From time to time I come across bicycles with a particular geometry (or rather a lack of). Most of them have no seat and a single tube which simply connects wheels and has the bottom bracket (if that still makes sense) in the middle of it.

What I'm talking about are bicycles like these:

Shouldn't the frame split in two by the force put on the "bottom bracket" while pedaling? Is the "bottom bracket" stay strengthened in any way? Does the lack of the seat tube imply there is less load on the frame? I assumed that the "diamond" shape of a bicycle offsets the bending and compressing forces that act on a beam structure.

• It doesn't collapse because it was designed not to colapse. Just like any beam. I don't understand what you're asking. Commented Jul 7, 2017 at 14:48
• @Wasabi Yeah, that was essentially what I was wondering: how such a structure is designed. I'm pretty new to engineering, so forgive my ignorance :)
– user12075
Commented Jul 7, 2017 at 14:52
• @dotbitcode "how such a structure is designed." - the same way as every other structure. First figure out the loads, then choose the materials and dimensions you need to carry them. Note, you can't design structures until you have learned how to analyze them, so most introductory books and courses are almost entirely about analysis, not design! Commented Jul 10, 2017 at 12:02
• @alephzero I think you're talking to me like I'm an engineer or anything like that but I just came here out of curiosity. It's not like I know how every other structure is designed. I have no training or degree in the subjects we're talking about. Although I'm saddened by the circlejerking I want to thank you anyway for improving my knowledge since your comment and the other 2 answers were a very good starting point for me to continue researching on my own. Otherwise I would've been lost. Cheers! ;)
– user12075
Commented Jul 10, 2017 at 18:29

While a single tube like that isn't as structurally efficient as a triangulated frame there is no reason it can't take the required loads as long as it is chunky enough. Note that, especially in the top picture the beam is clearly a lot more substantial in section than the tubes in more standard cycles. Equally you can't see how thick the walls of the tube are.

So the chances are that any weight saving from having less tubes is offset by the fact that the ones you do have need to be more substantial but here I suspect that the main aim is to make the whole thing as compact as possible.

Also, in most cases the limiting factor in bike frame design is stiffness rather than outright strength and most failures are due to either impact damage, manufacturing defects or long term fatigue.

From experience something like 25x50x2mm rectangular mild steel tube will support an adults body weight over this sort of length with no trouble whatsoever.

The beam is simply made to be strong enough to support the live load of a fully grown man pedaling. Modern materials can be made stronger than back when the diamond frame was first designed.

The wheel base looks to be shorter than in a standard road bike (thanks to the smaller wheels) so the bending forces are reduced.

In the first picture the beam is pretty wide vertically and the axle for the pedals is put through the middle. That helps resist the bending load.

The second bike doesn't have any reinforcements against braking at the pedals except the welds that attach the hub. but I expect that an engineer rand the numbers and was comfortable with the results.