Sunday, June 24, 2012

Front Quadrilateral


And, at long last, I have a nice big chunk of the bike done. The front triangle (quadrilateral to all of you literal minded geometrists) is a unit now. 

I started by getting the jig set up. I used my little angle gauge to determine that all of the tubes were in plane and clamped stuff in place. Unfortunately, I discovered that the cold set I'd done earlier hadn't actually taken, and was unable to get the down tube/head tube junction at exaclty the right angle. This will make the bike a bit shorter (I trimmed the down tube based on how things fit together before I realized what was going on), and make the seat tube angle a lot slacker. I may have to shove my seat forwards a bit, but it'll still be rideable. Summary: I messed up a fair amount but I think I'll still be able to ride it with pleasure until bike #3 rolls around in a few months to replace it (#2 will be a single speed to take the place of an existing steed).

Next, I tacked the down tube to the bottom bracket and layed it out on the alignment table. The big block you see is a theater weight. It's heavy enough that it pins the plane of the bottom bracket shell to the table. Then I just drag a height gauge around to see if the tubes tip one way or another. 

It took a lot of tweaking, but I got it in plane. At one point a tack popped and nearly made me jump out of my skin, but it didn't damage anything and I just melted it back together. 

I don't have pictures of the actual process because it happened all in one big rush. I basically powered through right before the shop closed. It was pretty uneventful and predictable. I was nervous, so my brazes weren't the best, but they'll do. The only big hiccup was putting a hand down to steady myself while peering into the bottom bracket looking for filler and burning a hole through my shorts with the still hot brazing rod.
Fear not, no genitals were torched in the building of this bike. 

When I checked the alignment, I found that it was fair, but not perfect. I'm pretty happy with it as a first attempt and will make better alignment a big goal on bike #2.

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