Tuesday, January 26, 2010

Bicycles also abhor a vacuum

Things have been going great with my 4th bike, though I still keep learning the hard way. I have been getting a lot more systematic in the way that I lay up the carbon, rather than just going on my gut instinct as to what will be strong enough and then overbuilding, I actually did some 3rd grade math and came up with a target for thickness and I went from there. I try to do my layup based on my semi-intuitive understanding of the forces for each joint and the notion that carbon is strongest in tension. I even had the forethought to cut all of the pieces of unidirectional carbon to size and shape before beginning the layup process, and going so far as to separate all of the pieces that would go on the tops and bottoms, lefts and rights of each joint. This has made the process MUCH faster. I've also been able to capitalize on the benefits of using the vacuum pump to a great degree, and occasionally to TOO great a degree.
I did the bottom bracket on Sunday night and laid up the whole joint at once, wrapped the release fabric and the breather ply, then sucked all the air out. Things seemed perfect. I unwrapped it the next morning and it looked great, stiff and solid. Then I tried fitting a wheel in. DOH. The force of the vacuum had distorted my rear triangle together, spacing the rear wheel at something like 120 mm. Bad news. I should have put a spacer in, but I thought that I had folded the bag in such a way that the dropout spacing would be unaffected. Wrong.

I'm weighing my options to proceed forward, and have had a modest bit of success in heating the BB joint and spreading the chainstays, but I'm not sure about the future strength once I do that. Epoxy is only flexible to a certain degree, and I hope I'm not creating too much internal stress. I'll give Gougeon Bros a call to double check and then hope for the best. Worst case scenario involves a gargantuan amount of sanding and then re-setting the joints. That should be only about 5 extra hours of work, maybe.

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