bamboo bike Part 2: fitting the tubes

After building the jig that holds the lugs in place, it was time to fit the bamboo poles together.

I found that Orchard Supply Hardware was the easiest place to get bamboo (neither Home Depot or Lowe’s near me had it). You can definitely order bamboo online, but I found OSH to be much cheaper, and I like being able to inspect and see each piece of bamboo before I buy it.

I marked a lot of the angles I wanted to cut by holding the poles up the the jig and roughly eye-balling it. I found it definitely helps to do the first cut long, so you have room to fine tune. Probably it helps to do some practice cuts, but as you’ll see, I was pretty sloppy about it. I did find it very helpful to clearly mark out what I’ll call the “spine” of each tube – basically the line on each tube that I wanted to be facing up. Since most bamboo poles are slightly oval in cross section, I found it just feels more polished if you align the major diameter of the oval vertically. Also, the spine helps keep you oriented when sawing the fish-mouth miter joints.

I also roughed up all the ends of the poles so they will hopefully bond better with the epoxy.

The seat tube on this frame is thicker than on most metal frame bikes since I wanted something large enough that the aluminum seat tube could fit into. This still involved quite a bit of sanding to open up the inner diameter of the bamboo tube.

When I was happy with the fit, I tacked the tubes in place with 5 min epoxy. As you can see below, for the really big gaps (since I was so sloppy with mitering), I mixed really short (1cm or less) hemp twine fibers with the epoxy so that it would fill the gaps better.

Below you can see the support I added (more scrap wood!) to support the seat tube. There was lots of eye-balling to make sure things were straight, and some impromptu shimming to make the angles look right.

For the rear triangle, I was really concerned about having enough clearance to the rear wheel. I didn’t want to build in lots of clearance if I didn’t need it, resulting in an unnecessarily long frame. But I definitely didn’t want my rear tire rubbing on the bamboo. So I carefully measured my rear wheel to make a cross section template. From this image you can see that had I built my frame with the dropouts where they were, my rear wheel never would have fit! I think in CAD, I didn’t model the seat tube big enough, and my angles were likely off. Regardless of the cause, I just moved the rear dropouts back by ~3cm  and the wheel cross-section template suggested that things would fit.

Adding chainstays and seatstays:

When I sawed up my metal bike frame, I left some of the tubing on the rear drop outs so that there would be more length to engage with the bamboo. I did my best to shape the ends of the bamboo to interlock as well as possible.

I used electrical tape to hold things in place while the epoxy set. This mostly worked, except that one it pulled one of the seat stays crooked while the glue set (more on how I fixed that in the next post!)