Now I’m starting the much awaited mountain frame for Cleo. It’ll be here first actual new mountain bike in 20+ years of riding. It’s also going to be my first bi-laminate bike. Bi-laminate construction is a combination of lugged and fillet brazed joints. It’s a pretty time consuming process, as the frame joints are basically done twice. You’ve got a short piece of tubing brazed to the adjoining tube, with a fancy treatment cut into the other end. A smaller diameter tube is slipped into the stub, and it’s brazed like a lugged joint. Ritchey Annapurnas were done like this, and they were some of the most expensive and sought after frames of the time.
Like that. It starts out as a piece of 4130 chromoly tube 1/8″ larger than the frame tube itself. In this case, the sleeve is cut out of 1 3/8″ with a wall thickness of .058″. That gives you an inside diameter of 1.259″, which is .009″ larger than the down tube. That .009″ doesn’t sound like much, and it isn’t. It’s just enough room for the brass to wick into when you braze them together. Since the sleeve doesn’t need to be that thick, I turned it down on the outside, from .058″ to about .030″. It’ll look a lot better, and it will still have the right dimension inside.
Add brazing rodIt looks pretty burnt, but the flux starts off grey so it looks worse than it is. This is some new stuff I’m trying out, and so far I like it except that the flux is harder to get off after brazing. The Gasflux stuff I’ve been using comes off pretty good with hot water, but this has to soak for a while and seems to require a bit of elbow grease. Not the end of the world, but I like time savers. Once the sleeves are brazed on the tube ends, I’ll just build the bike like a normal fillet brazed bike. The other option is to braze the sleeves onto the head tube and proceed like you’re building a bike with regular lugs, like this example.
That one’s not my work, it’s a pic from the Flickr page of Engin Cycles. Anyway, that’s it for today. You can probably expect more timely updates, as I’m trying to get some progress made on hers and a few others.