ShortHopper down tube – bottom bracket gets fillet brazed

July 16, 2010 · 0 comments

The front and rear triangles are joined in this step. This will required careful alignment to ensure that the bike tracks properly.

The previous step showed the head tube brazed to the down tube. In this step that whole section is joined to the bottom bracket.

The down tube was cut to the correct length to match the drawing. The miter was hand filed to fit the bottom bracket. A compound miter was needed because the seat tube intersects the down tube at the bottom bracket. This required the bottom bracket height to be set and the front and rear triangle aligned in order to get the correct fit.

Since the front fork and rear triangle were available and complete, they were used to assist in getting the sections lined up. This was a similar tactic as used on the Chop Mountain Bike project. The front and rear dropouts were placed in dummy axils on the same center line. The center of the bottom bracket was lined up on that same line. As long as the dropouts are perpendicular to the center line, the bike should track straight.

The bottom bracket height was also set to match the drawing by shimming up the rear stays as shown in the photo.

With the frame held in the jig the joint was tacked in four places. Once the tacks were in place the fork was removed and the frame put in a bike stand for fillet brazing. The fillet can be seen in the photo.

I’ll wait until all of the joints are done before soaking off the flux and finishing them smooth. This is a labor intensive job but looks really nice when complete.

The next step is to fit the top tube to the head tube and seat tube. This will be tricky because if the second miter is cut too short, I’ll have to start over with a new head tube.

To read all about the steps for this project so far go to the ShortHopper category.

I’m still considering electric bike conversion kit options for this project. If there is a kit you would like to see me install, leave a comment.

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