Post tension

Was just thinking about concrete as you do. You can seriously reduce the thickness of a floor slab by running cables through it so when it has been cast you tighten them. Under compression you the tensile strength goes up.

Would this work on other materials? Could you stiffen a frame by putting the tubes under compression? I'm assuming no or it would have been done but why?

i love questions like this. my thoughts:

1. maybe some well placed ones...
   
2. my guess is the forces being dealt with in a bike are smaller than in a bridge or something, and consequently the windows are narrower.
   
3. Also, the freedom for physical distortion, before failure, in a concrete structure are much much much much larger, meaning the inherently elastic cable can actually do more to prevent failure. I guess i'm trying to say that if you distort the downtube +/-1mm, it's going to crack eventually. I think a cable will have a hard time preventing that.
   
4. In fact, in theory, now that i think about it, a tensioned cable can't prevent that. It just shifts the equilibrium towards the compressive end of things. You will still have +/- 1mm of distortion
   
5. concrete is likely very very happy when compressed, which the cable encourages, and very very unhappy when stretched.
   
   

anyway, thats my completely naive perspective

concrete is awesome.

Sometimes before I post I have a second thought and edit the post to reflect this.. then I wonder if my first post was better left alone. I hate post tension.

1. my guess is the forces being dealt with in a bike are smaller than in a bridge
   
   
   

I have done some googling and can now confidently confirm that the forces in a bridge are indeed greater than in a bike.

concrete is awesome.

+1

I've done several uni projects on concrete, including the fibrous synthetic reinforcement and recycled aggregates. Beyond that I have no practical experience. My bike building skills are limited to assembly and selecting pretty colours. I do not know the answer to your question.

Someone build a bike based on the Tacoma Bridge

I have no direct experience with concrete push bikes, but here is a protoype I had commisioned for a concrete motorbike I was working on:

Concrete is weak under tension and strong under compression. Steel cables are strong under tension. By combining the two you have a composite material that has both properties.

As a floor slab is loaded with weight it bends. As it bends into a curve, the underside is stretched and the top side is compressed. The steel looks after the tension and the concrete looks after the compresion, hence steel reinforced conrete is good for making buildings.

Carbon fibre works on the same way, ie los of thin strands that are good in tension, with a resin that is good in compresion.

There is no point loading a frame tube with tension as it is already strong in tension (steel) and it would not help those tubes that deal with a compresion load, as it would only add to the compression.

I think I have seen a bike that replaces the down tube with a cable, as I guess this tube is mainly under tension.

I have one. My evans (built out of 531) flexes at least as much as that bridge did. Of course my pastry munching arse could have something to do with it.

Someone build a bike based on the Tacoma Bridge

hmm good question..8-)

Too slow, sasmon has answered it already.

Slingshot bikes have been around for years. http://www.slingshotbikes.com/bikes/1 They don't instill me with confidence. Where's Scott when you need him, he has probably had one.

Too slow, sasmon has answered it already.

You must be really slow, it took you nearly 16 hours to type that post