Almost 9 months ago I built a frame jig for a bamboo bike. Between work and life, I didn’t actually finish the bike until Thansgiving, and I’m only getting around to blogging about it now.
Here’s the finished bike:
I’ve wanted to build a bamboo bike for a long time. More than a year ago, my bike (aluminum frame) was run over by a car (thankfully I wasn’t on it). The frame as a whole was in sorry shape, but the lugs were perfect for a bamboo frame.
I started designing my frame by using Bikecad: http://www.bikecad.ca/1327254791539 You could also use Sketchup or any other CAD program. But its also just really easy to use a ready-made parametric model from Bikecad. I later used the dimensions from Bikecad to CAD a frame-building jig. A roughly dimensioned drawing of that jig can be found here: bamboo
I chose to use mostly 2 by 4’s since I had lots of scrap wood and a miter saw. If you’re welding, wood probably isn’t the greatest idea. But I’m just gluing sticks together. I think you’ll see that this is certainly not the most accurate of jigs, but its good enough for what I’m after.
In the image below you can see there are two places where I didn’t have scraps that were long enough, so I spliced some pieces together.
Here is a time lapse of me building the jig:
The head tube is supported by a threaded rod. There are two PVC tubing end caps inside the steer tube that keep the it concentric with the threaded rod. I needed to drill centered holes into the end caps so I could put them on the threaded rod. Without a lathe, I put various concentric things into the end cap until I had something small enough to serve as a drill bit guide. I used layers of tape to build things up to the right diameter. Depending on the size of the steer tube you use, you could also use PCV end caps as spacers, or just build up small ones with layers of tape, until they are the right diameter.
I also used a hole saw to make plastic fender washers for each end of the head tube to keep epoxy from getting inside.
To hold the rear drop outs I used a rear axle, but you could just use a threaded rod, too.
I found some PVC pieces that work well for holding the bottom bracket, too.
Before making the actual frame, I made a test joint to give myself confidence that it would be strong enough and to try out the process. The materials I used were:
- 2-part epoxy from ebay (search: EPOXY RESIN MARINE BOAT BUILDING HIGH IMPACT 32 OZ.KT). This wasn’t as low-viscosity as I was expecting, but it worked fine
- hemp garden twine, since all the raw hemp fiber I could find was really pricey
- Harbor Freight electrical tape ($3.99 for 10 rolls!), for compressing the joints. I went through like 4-5 rolls, so definitely get the cheap stuff
Anyway, for the test joint, I made a huge mess, made a really ugly joint (forgot to take a picture of it before sanding), sanded/dremeled it down, and coated it with a thin layer of epoxy (not really necessary). The resulting joint was surprisingly stiff. I pulled as hard as I could and couldn’t feel any flex.
Later I cut it open, just to see how it looked inside. A few things to note:
- There are a number of small voids from poor compression. I tried to avoid this by applying thinner, more even layers of hemp and epoxy
- There is a lot of epoxy in the joint (probably hard to see from the photo) compared to the volume of hemp fibers. I forgot to perforate the electrical tape that I wrapped around the joint for compression. Perforating the tape lets the excess epoxy seep through.
- A pool of epoxy had developed inside the bamboo tube (especially visible on the right side where the epoxy flowed down the tube. This would be a waste of epoxy and make the frame unnecessarily heavy. When building the frame, I was careful to seal all the seams with 5 min epoxy (gorilla glue would probably work well, too) so this couldn’t happen.
- The actual miter joint between the bamboo tubes isn’t great, but that doesn’t seem to matter much. For the actual frame, I was pretty messy about mitering the tubes.
Stay tuned for the actual building of the frame!