Monday, July 28, 2008

Day 9: Finishing the Front Triangle, Starting on Lug Filing

Allow me to speak for all three students in the class and say that today was the best day so far. For one thing, we're finally all at the same point — we all have our front triangles brazed together. And for a second, this point is a very satisfying one: it's awesome to see our frames really coming in to shape. (At right: my main triangle, with my Phil Wood hubbed rear wheel.)

This morning I did my last two lug brazes: first, the downtube into the bottom bracket shell, and then the seat lug. The seat lug was a bit tricky since the seat tube isn't double-butted. It's only butted once, and the thin butt is at the seat lug. It's thus easy to overheat it. If you get it cherry red, it bulges and deforms, and your frame is wrecked. It did get red easily, but the braze went fine. (Later in the day Mark, who worked with Doug for ten years, paid a visit and showed us a frame he had ruined in just this way only a few weeks ago...) In any case, before lunch I was done my main triangle — very exciting!

And — somewhat astonishingly! — Dan and Robert also did their main triangles today. Dan's persistence in sticking to his 700c wheels resulted in a compromise geometry that ended up totally working. Robert just kept on with his steady progress and is also on pace. Now we'll all get the same lesson tomorrow and will all be working on the same thing. This will help us all finish our frames on time: we're good at teaching one another and helping one another along.

I spent my afternoon and evening filing my rear dropouts and my seat lug. Lug filing is very time consuming and very satisfying. I'm again super happy to have chosen Henry James lugs — they pretty, and they're small, which makes them easy to braze and file. Filing is not just about thinning but also about giving shape. Lugs in their pre-filed shape bulge out and are non-parallel with the surface of the tubes, and don't dip evenly into shoulders. Filing takes care of that.

Today's note of advice: I am very, very happy that I chose a simple frame. While the bike I most wanted going into the class was not a fixed gear (what I wanted was a bike built around the Grand Bois Hetres with a rod-operated front derailleur and a Simplex SLJ rear derailleur...), it makes a lot of sense to do something along those lines. By picking simple lugs, I got easier brazing, minimal profile filing, and a small area to do shape-filing. By using horizontal dropouts, I've made the chainstay braze much easier on myself. And instead of being stressed out all week and constantly rushing to catch up, I've managed to stay relatively relaxed. The point, after all, is to learn to build other frames — not just to get a new frame. (We discussed with Doug the option of having students build bikes for his Ukraine project — I wonder how the experience would change if you weren't even building for yourself...)

And today's highlight: the very reticent Doug telling me, while I filed better than he expected — remember, last week he took one look at my filing and said "You're going to have a hard time with brazing" — "If you stick at this, you'll make some nice frames." I take what I can get!

Tomorrow morning all three of us braze our chainstays into our bottom brackets and get to work on our seatstay attachments.

[More photos here.]


NB said...

Why isn't the seat tube double-butted? Just no need for it?

AH said...

My best guess is either that's it's because it's a low-stress area or that the seatpost picks up the slack.

whip said...

Because the seatpost needs to fit. The butt would make it too tight

brian jenks said...

Whip has the right idea. A 0.7mm wall is plenty thin enough already for a seat-tube, and 0.7mm is exactly the correct thickness for a 27.2 post to fit in a 28.6 tube. The tube is usually a little thicker at the BB of course, making the tube "butted". In order for it to be double-butted, the middle section would have to be thinner than 0.7mm. (not the seat-cluster end thicker)
Although some tubes' middle sections do come thinner than that, there are two reasons for not further thinning the middle: The first is it would be unneccesarily thin for most seat-tubes. The second is top-tubes and down-tubes have butt lengths to accomodate the vast majority of frame sizes. This is not reasonable with seat-tubes, especially when considering classic horizontal or sloping top-tube options. Seat-tube lengths simply vary so widely that it makes sense to make one 28.6 seat-tube, butted at the base, that works for everyone.