Checking Frame Angles in a Jig, and Preparing Tubes

(this post is a continuation on the frame-building series I've been picking up and putting down over the last few months)

This is the jig, which we lower to the work table whenever necessary. I hit my head on the corners of that thing at least a dozen times during the class. 

The reason we finished the fork before touching the rest of the frame is that the fork can determine the angles for the rest of the bike. Think of it this way: if you have a frame with a head tube angle of 72 degrees that rides fine on a stock fork, then putting on a fork that's a few millimeters shorter and raked a little further out will effectively change the head tube angle to 75 degrees, which will have huge implications for the bike's handling. This is also why it's a bad idea to buy aftermarket forks: you never know how exactly the geometry of your bike might change (well, you will if you have one of Doug Fattic's frame jigs, they tell you the angles with nifty laser-etchings on stainless steel).

With the fork already made and out of the way, we can set up the rest of the bike in the jig, using the fork as a keystone for angle adjustment.

Using the jig's adjustable fasteners and angle finders, we programmed in the exact specifications of the fork. The lug, there at Doug's hand, will be moved up or down accordingly, leaving an extra 13mm for the Campagnolo headset. 

Doug, the expert, set up most of the angles for me. At the bottom of this post I transcribed exactly what my measurements and angles were. Some of the tubes needed a little re-mitering after we had everything set up in the jig.

Here, the angles are set on the jig, so we're putting in the head tube and seat tube first to see how things look. Notice, the 100mm stem is the same angle and style that I intend to use on the final bike. 

Because we want everything to look right, we're using the make and model of saddle I plan on using to be our seat-tube guide. I'm using a seat post with a 25mm setback and we want the saddle to sit in the middle, so all those things are taken into account. All the components of the bike are integral to the finished design. 

With the angles secure, we were able to do the finishing miters on the tubes and put the bike together!

Angles and lengths:
Bottom bracket drop: 75mm
Seat tube angle: 72
Stand over height: 840mm
Seat tube length: 584mm
Top tube length (actual): 544mm
Head tube angle: 72
Fork rake: 51mm
Fork brake clearance: 45
Fork crown thickness: 10
Down tube length: 607mm
Down tube angle: 69
Head tube length: 173mm
Front wheel clearance: 578mm
Top tube slope: 1
Seat tube to top tube angle: 73