I set myself a challenge. We hadn’t yet built a mountain bike at Bamboo Bicycles Beijing, because who in their right mind would want a mountain bike in possibly the flattest city in the world? Well, I did. I’m not going claim that it was the first time anyone thought of it. But to build one myself would be fantastic. Even with experience building road and urban bamboo bikes, I knew it would be a big challenge. To begin with, I needed to find some frame materials that we didn’t normally keep in the shop. We normally have straight headtubes, but I was planning on having a tapered steerer tube for my fork, so a larger headtube was necessary to accommodate that. And, for any serious riding, you need disc brakes – simple. Once I tracked down what I needed I set to work on my design. I knew more of less what I wanted, but I still adopted some angles and ideas from other bikes I admired. With the design set on RattleCAD, I got started with the jig. Setting up the jig (the fixture on which you build the frame to ensure the precision of the angles and dimensions) took some mathematics, which – without the help of some engineering friends – I would’ve spend a week figuring out. We normally keep it very simple, keeping both the headtube and seat tube angles the same. I wasn’t happy with that, so needed a bit of mathematics skills to figure out the correct setting.
On top of the general design I decided to add braces to strengthen where I thought should be strengthened, like the headtube junction, the seat tube-seat stay junction and between the chainstays and seatstays where the rear disc brake forces would affect the frame. It was also a bit of peace of mind and style that directed those braces, though the verdict of those is down to opinion.By far the biggest challenge I faced was trying to make the rear wide enough to fit a mountain bike tyre, while also keeping it as short as possible. It was near the bottom bracket that was most difficult. I had to make it wide enough but still have room for the chainring in front. It’s a common challenge, but one that is a bit exaggerated because of the diameter of the bamboo. And unlike metal frames, bamboo can’t really be flattened to allow for more room. I considered using more than one piece of bamboo to form the angle that I wanted and using fast-setting epoxy to stick them together. But found that it wouldn’t be easy to mirror the exact angle and length for both sides of the frame, so settled with something a bit unusual. By cutting a ‘V’ shape into the bamboo, it was possible to bend the pieces to the angle needed and keep it consistent on both sides. In bending the bamboo it essentially closed the gap of that ‘V’ too. It might not sound like the strongest plan, but with the carbon fibre reinforcing, it was plenty strong enough.
Once all figured out, I shaped and prepared the tubes and I set out to glue the frame onto the jig. From here I had the final shape of the frame, minus the reinforcement of the carbon fibre at the joints. We use carbon fibre tow mixed with epoxy to wrap around the joints in a certain pattern to make them as strong as possible. Most people find the wrapping process boring or frustrating, but for me, it’s the chance to make your bike pretty, and taking the time to wrap well makes all the difference at the end.I finished wrapping and had to clean up the frame, removing all the excess carbon fibre and coating it with a varnish. Now all that was left was to install the parts. Assembly wasn’t too smooth, though I saw some of the issues coming. I knew I needed to file the carbon fibre in some areas, like near the dropouts where the chain was rubbing and the seat stay where the disc was also rubbing. But because I added so much extra carbon fibre it was ok to take a little off. It really came down to the fact of having much thicker stays than a metal or carbon bike would have. After assembly, the first ride was nothing short of bliss. A fat smile slapped across my face, testing the jumping ability of the bike and popping wheelies. I had been wanting to build this bike for almost a year, and it was finally a reality! But because of the 5 month long Beijing winter, I haven’t yet been able to test it out on a trail. That will be coming shortly, though. Overall, having experienced many different frame materials, it seems like bamboo is a mix between them all. It’s about as light as aluminium, compliant as steel and exotic as carbon, and the strength has been tested to be on par with steel for its weight. It wasn’t without its growing pains and I certainly learned a lot and can make improvements the next time but it was quite a good feeling having designed and built my own mountain bike frame from the ground up.