unterhausen

Not quite a bicycle, but I want to make dishing gauges with my discarded chainstays, in the style of EVT. I made a couple of them before, and had a lot of trouble with distorting the center tube. I also distorted the plates, but I think I have a solution for that.

Andrew R Stewart 

Like a frame, can you do after brazing finishing (like reaming or facing)? Andy

unterhausen

It's not super important, but I would rather not have it distort as much as it did.

bulgie 

I used a thing my co-workers called a heatsink, for large thin eccentric BB shells. I didn't think of it as a heatsink because I made it to sink as little heat as possible, I just wanted to keep the shell roundish. Since the shell was relieved inside, only clamping the insert at the outer 1 cm on each side, I didn't care if the center distorted. So my anti-distortion plug ("heatsink" is easier to type!) was made of two big washers maybe 1/8" thick. At whatever diameter, I forget, a bit under 2" I think. Connected to each other with a 1" tube down the middle to make sort of a spool shape. Soaks up fairly little heat (good), and the shells did come out round. Made prolly a hundred tandems with that thing, almost all fillet-brazed.

Remember there will still be some distortion, so give yourself a way to get the thing back out, lest it become a permanent part... Maybe three segments that collapse inward towards each other, tightened by cones at each end like a bar-con in the handlebar? With a lot of mass and a lot of contact area, you will be getting a true "heat sink", as in needing more fuel and time to get the thing hot. I have not tried this, pure speculation.

Mark B in Seattle

unterhausen

Thanks Mark, I figured if anyone had done this it would be you. I am worried about getting the heatsink stuck in there. I wonder if "heatsink" isn't a misnomer, I don't think brass works all that well as a heat conductor and that's what they seem to be made from. Okay, I looked it up and brass has a little over 1/10 the thermal conductivity of copper depending on the alloy.

I brazed the bottom end in before I attached the stays, and the hole in that end actually got distorted enough that I had to ream it. I might put that in later. I mostly just care about the ends, so I may just make something like what you made.

I was lucky with my tandem, I had no idea about distortion when I made it. But the eccentric fit in okay.