With a bike jig in place and your head tube, bottom bracket, seat-post and dropouts secured in place, the next step is to start measuring out the bamboo and cutting it to size. You can get the metal components from a frame-building supplier like Nova Cycles or harvest them off a junk frame. Once cut each joint needs to be mitered down so that it fits perfectly and snugly around the component you will be epoxying it to. My bamboo was pretty hard, so I ate through a few dremel bits during the mitering process.
Once each piece of bamboo has been cut and mitered to relative perfection, it’s time to start heat treating the bamboo poles. Bamboo is a grass made up of sugars and starches so heating the poles caramelizes these sugars/starches and strengthens the poles, this helps to add additional strength, to the frame and it also helps remove moisture and the waxy outer layer from the poles, while darkening the poles aesthetically. Through a little trial and error I found out that if you don’t punch a hole through the solid sections inside the bamboo culm, pressure will build up inside the culm and the bamboo will actually explode. Though this can be very exciting… in the interest of not having pressurized bamboo shards ripping your eyeballs apart, I’d suggest removing these first. I did so with a long piece of rebar and a hammer.
Once they have had a few minutes to cool down you can start tacking the poles together on the jig with a mix of high strength epoxy resin and a thickening agent to reduce running. Initially I didn’t use a thickening agent because I thought I could get by without it. I was wrong, all I ended up with was a big mess and a few hours with some sandpaper. Mix in some high density filler until the epoxy has the consistency of peanut butter. After lots of product comparisons I chose AeroMarine 300/11 Epoxy Resin Adhesive because of its exceptional tensile strength and bonding properties.
If everything goes well, after 48 hours the epoxy should be fully cured and you should be able to remove the frame from the jig. If things don’t go well… feel free to start again.
If designed and constructed properly, the frame should be relatively strong at this point, but your going to need to reinforce the joints with wrapped lugs. There are a variety of materials to choose from. Originally I had planned to use carbon fiber, but I found that it’s thermal expansion rate is significantly lower than that of bamboo, this can cause de-lamination over time which will eventually result in catastrophe and or embarrassment as your bike crumbles apart. Instead I chose to use raw hemp fiber. For one, hemp fiber just exudes a more organic, hippy, sustainable resource vibe, the sort of vibe befitting of a bike built out of grass, but it is also very strong when wrapped properly, and it is much less likely to result in de-lamination. I bought my hemp fiber from some place in Compton California. It came in a large cardboard box marked “Cannabis Sativa“, and unsurprisingly it was delayed at customs.
When wrapping the lugs it’s important to soak the hemp fiber in epoxy resin so it saturates every nook and cranny of the joint. It’s very important to wrap the joints in such a way that the hemp fibers will properly support the frame through a variety of different pressures and forces, other wise they will fail. This is a great piece of reference material from Zack at Stalk Bicycles. I overbuilt each joint and then wrapped them tightly with electrical tape to squeeze out any excess resin and remove all internal air pockets.
After 24 hours you can remove the electrical tape from the lugs and inspect the joints. The joints probably wont look pretty yet as there will be stray bits of fiber and resin strewn about. Simply take out your dremel and start mitering each lug down until it has a smooth flow and finish. You can then reapply a light coat of resin to fill in any imperfections, wait for it to dry for 48 hours then start sanding with 100 grit sand paper before doing a final wet sand for a smooth glassy finish. If you don’t wait for the resin to fully cure before sanding, it will still be tacky, and you’ll mess up the finish.
Ideally you don’t want a bunch of resin settling in the bottom bracket threads (as depicted here). That was rather painful to remove and probably could have been prevented by taping things better. Once the lugs are looking good you can sand down the frame and stain, gel-coat or varnish it however you’d like. Just remember that if you want it to be water proof, gels and stains should be finished with a coat of polyurethane. I found a nice dark chestnut in a gel-coat that worked for me. Once your frame is complete, the rest is simply a matter of picking out the right components and putting the bike together. BAM