It’s not always plain sailing

The Samurai, my fourth frame was going well until I discovered that the seat-tube had developed some cracks around both the bottom bracket and where the seat-post sleeve inserts into it.

Luckily I hadn’t yet carbon fibered up either of those joints so I’ve been able to cut out the offending tube and will replace it with one that’s hopefully not so prone to cracking.

The cause of the cracking is probably due to the consistent cold temperates in the workshop overnight and that without coatings of epoxy bamboo will naturally dry out and become brittle and crack. It’s also due to the steel seat-post sleeve insert changing¬†temperatures¬†at different rates to the bamboo, causing the bamboo to crack.

Interestingly both the cracks were in places that would have been covered with carbon fiber and they probably won’t have propagated any further, but I couldn’t take the risk of that not being the case and don’t want to let a mistake roll out the door.

Below are some pics of the cracked seat-tube both before and after I’ve cut it out. It was interesting to see the different levels of adhesion (or lack thereof) of the epoxy to the various materials. It’s mostly only mechanically bonded to both the steel and the bamboo, making it very important to roughen up the surfaces to ensure a good bond. The aluminium bottom bracket is painted in a specific etch primer (the white paint), but it doesn’t seem to have chemically adhered properly to it. I’ll have to investigate why!

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One Response to It’s not always plain sailing

  1. Karim says:

    Thanks for sharing in this kind of details too! As for the cracks, It looks like the bamboo stretches much more than the epoxy; maybe one way of helping with this would be to mix some bamboo powder to the epoxy that is directly in contact with the bamboo, I also read somewhere that heat-treating the bamboo decreases the shrinking ratio. As for the Aluminum bonding roughening the surface might help more… I know that the oxide is a bitch for welding, but I’ve no idea how to make epoxy adhere more to aluminum chemically.

    Thanks again for sharing

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