Boson's bakfiets: fabrication challenges and methods.

Boson's project presents a couple of pretty interesting problems in construction, as there isn't really a jig to hold all of the bits aligned in place in order to braze them together accurately. When you build a frame or fork, there are jigs that allow for any configuration within the traditional diamond frame design. The bakfiets, however, falls more into the genus of the tandem and the recumbent, weird creatures who live beyond the constraints of traditional frame building equipment. Given that I do not have the money, time, or expertise to fabricate custom tooling just to hold the bits of this one particular project together, other means are needed. In order to keep all my angles as accurate as possible, I split up the construction into several smaller sub-assemblies, little chunks that I could align and clamp and hold down on the welding tables at the Foundery where I work. Then, once I have the sub-assemblies together, then I can carefully and craftily align them in the bicycle repair stand and quickly tack them together. Once tacked, I can complete the brazes and hopefully retain the alignment needed to keep everything centered around the centerline of the bicycle and the cargo bay perpendicular to the vertical centerline of the bike. That way, when it is loaded up, it will be rideable hands free and will track consistently and steer predictably. Here is how it has gone thus far:

Here is the first sub-assembly, the cargo bay floor. I will still add the mounting tabs that will allow for the final installation of the plywood floor. Doing a simple square meant easy miters and an uncomplicated brazing sequence that I could do all on the welding table. This project is also great brazing practice for me!


Here, we see the second sub-assembly, the bottom bracket mounting bracket, that I have brazed onto the cargo bay floor assembly. I calculated the angle that they join at from the original blueprint, and then did some trigonometry and was able to mock it up accurately on the welding table. All of the miters thus far are very simple, and the brazes were all very accessible, which kept the actual assembly pretty easy. I was also able to clamp everything down to the rigid table top, which kept things aligned while I tacked them in place. 

Next, I attached the first two sub-assemblies to the bottom bracket of the rear donor frame. To get the alignment here correct, I made some wooden blocks to hold the cargo bay floor at the appropriate height (again, from the blueprints) and clamped a dummy wheel into the frame to attain the ideal riding position. I supported everything as best I could and quickly tacked the struts to the bottom bracket shell. Once they were holding, I completed the brazes.

There they are, in all their raw blobby wonder! After attaching the first two assemblies, I could begin building up the cargo bay. First, I attached the rearward vertical supports for the upper rail.

As you can see, it required some very crafty alignment and more applied trigonometry. The beginning of the flare to the sides of the cargo bay is visible nicely here. Once the vertical strut was attached, I cut and mitered the lateral support to fit the down tube and the vertical and then brazed it in. 

I did the same for the other side, and then moved on to the front subassembly. The front sub-assembly will get its own post soon, as it was a whole lot of trickiness all by itself. The front assembly contains the most critical angle of the whole project, the head tube angle that steers the bike. It must be centered, vertical, in phase with the centerline of the rear bike, and set to the correct angle within a half-degree tolerance in order to maintain the desired steering characteristics, so there was a lot of planning to get everything right.