Tuesday, January 24, 2012

A Bicycle is a Series of Tubes


In my last post I mentioned that I was seeking clarification on where to feed filler and how to flow it.  I'd been seeing a bunch of posts on this topic, but it often wasn't clear if they were talking about using silver as a filler or bronze and suspected I was making some false assumptions about handling bronze differently.

And I was: as with silver, the goal is to add filler on one side of the lug, and then flow it to as much of the lug as possible from that point.  By only adding filler to one side, it's possibly to figure out if you have at least partial penetration of the filler into the lug.  Basically, if you put bronze in one side and it comes out the other, you can assume that there's some in the middle.

Old Tubes

After a lot more practice, mostly with sleeves of tubing rather than actual lugs, I decided to give the first joint on the actual frame a try.  It did not go well.

I overheated the joint and managed to flow bronze over the top of the lug... making it impossible to tell if any went all the way through.  Then, I mucked things up even more trying to ensure that I did penetrate the lug with bronze.  Basically, I overheated the lug a lot and put bronze everywhere, without being sure that I actually got it underneath.

I learned another lesson: I'm more limited in how much I can modify lug angles than I thought.  This lug is designed for a 60 degree angle, but I need it to be 59.5.  Not a big deal, but I wasn't able to get it perfect.  There was a bit of extra space at the point of the lug that had to be filled.  Based on that, I don't think I have the skill to do the 3 degree lug angle change that I designed into this frame to slope the top tube downwards towards the seat.

New Tubes

So, since I have a joint that I'm unsure of and a design problem that I can't correct without replacing another tube, I'm basically at the point where it makes sense to order some more tubes and lugs.  It'll set me back another $250 including shipping, but it lets me use the remaining lugs from the old set as practice, and I'm using the opportunity to grab some extra lower quality lugs to play with as well.  Practicing with sleeves of tubing helped, but I've got that lesson pretty well down, while doing it with lugs still presents obvious issues.

On the up side, I'm getting some sweet Nova Slant Six lugs that will give me a very nice sloping top tube, ideal for my long torso and short legs (and without the need to monkey around with major changes in the angles).  I'll be using the light version of Nova's oversized road tubeset again; it's thin without being too fragile for me to work with, and is made of chromoly (a nice strong steel alloy without too many surprises).


So, I'll have to go back to the drawing-board, adjust my blueprint, re-miter the tubes, and get back to brazing.  That means that I'll have the whole process up here, a bit more streamlined than last time.  A win for posterity!

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