Bicycle Mechanics - Square seat-tube/post

So, as I was adjusting my seatpost earlier today on the trail and trying to get it perfectly aligned, a question popped into my head...

Why do seatposts have to be round? How about a square post and seat-tube? Or a triangular or hexagonally shaped one? How about a D-shaped cross-section? I've seen spinning bikes with square-shaped seat-tubes and posts. Okay so at first there will be limited availability if a frame manufacturer actually tried it but everything new has a ramp-up period.

Ready? Ready? Discuss!

Note to mods: I'm not sure this thread belongs in this forum but I couldn't think of anywhere else to post it. Perhaps we need a "tech, design and concepts" forum?

Apart from historical legacy, round holes mean that the saddle can be precisely aligned as the user wants (which may not always be in-line). With non-round posts, you would need to have the seatpost precisely aligned during manufacture. This may be possible on expensive workshop bikes, but the frame alignment of factory bikes is often way off.
The clamp on a square system would not apply even pressure, so you need quite a thick post-wall. Spinning bikes are very crude and weight is not a factor. For a reasonably lightweight seatpost, round is sound.

Good question Khuon,

I suppose there are two issues. Compatibility and engineering. Once you have a few standart seatpost sizes, there is no technical issue for any manufacturer to make what is basically a round pipe to fit those standards. If we started on square, oval or whatever profiles for seat-tubes there would be a real additional complexity in manufacture and compatibility. This would certainly increase cost and could spark another branch of the Campy / Shimano debate :)

Technically, a circular profile tube has good strength in all directions. I would imagine that it is also the easiest shape to clamp in place. Plus, after many years of riding, it is very helpful to be able to twist the post to aid removal if it becomes seized in place. You could not do this with any other design.

Just a few thoughts.

Cheers,

Ed

Basically a round shape has a better strength to weight ratio than a square one.

Originally posted by Bobatin
Basically a round shape has a better strength to weight ratio than a square one.

hahaha.... so true.... the simplest answer is probably the most relevant one!!