Building Bamboo bikes
Bamboo Bicycle Technical Issues
Written by Craig Calfee
The technique of joining bamboo with fiber reinforced epoxy wraps came from our carbon fiber joining methods. These were originally developed in the late 80′s for joining carbon fiber tubing with epoxy soaked carbon roving. This method was used for bikes whose geometry was too far from our normal geometry and did not fit our metal die pressure molds. Over the years, the fiber wrapping patterns were developed and evolved to attain the ideal fiber orientation for each joint. In 1995 we made our first bamboo bike with carbon wraps, using the same wrapping patterns used on carbon frames.
The bamboo on that first bike was selected to look similar to a carbon bike as it was destined for Interbike, the big annual trade show for the bicycle industry. The bike was well received and achieved the purpose of being written up and photographed for the Interbike Show report in Bicycling Magazine. The bike rode nicely but it was too flexible for serious racing. But we could see the potential as it had amazing vibration damping. So we built a second frame with larger diameter down and top tubes. That brought the stiffness up to the same level as we were building for racers like Greg LeMond and his team. The bike could descend at speeds over 50 mph without feeling wobbly. With the wide variety of diameters and wall thicknesses, we realized that we could specify the optimal tubing for each rider depending on rider weight and stiffness requirements. This was especially nice for lighter riders as they could get frames made that were not so stiff as the “lowest common denominator” for carbon bikes. And heavier riders could go as stiff as desired without having yet another custom carbon tube ordered. Tandems and triplets were even possible!
In researching bamboo, we came across numerous references to the strength, hardness and toughness of bamboo. Most of this came from the flooring industry. But there were a few references to whole bamboo tubes, particularly for the thick walled fighting sticks used in various martial arts. And of course the bamboo fishing rods demonstrated tenacity and are still held in high regard when compared to modern carbon fiber fishing rods. Especially when comparing durability. Generally speaking, carbon fiber wins on the stiffness to weight ratio side, but bamboo will beat the brittle carbon for toughness when abused or crashed. But managing bamboo’s tendency to split and other natural challenges is another story.
Bamboo splitting and moisture content
Whoever invented bamboo was pretty clever in designing a plant that would withstand a lot of abuse yet maintain structural integrity and survive the worst storms. The failure mode of bamboo is to split. But the fibers stay together and the splitting allows the plant to bend even more to let hurricane force winds blow through. The plant can still absorb water and nutrients even after splitting. For bicycles, we prefer that it doesn’t split and it’s very rare to crash a bike hard enough for this to happen. But splitting can also happen when changes of moisture content happen too fast. During the initial drying process, bamboo must dry evenly, otherwise, it can lose moisture on one side, shrink unevenly, causing stress within the tube and lead to splitting. It can also happen after drying if moisture is allowed to reenter the vascular bundles of the bamboo and then dry out again unevenly. Repeated raising and lowering of moisture content can proceed with no harmful results if the changes are slow and not extreme. But the world is not like that, so the bamboo must be sealed from changes in moisture content with a waterproof polyurethane coating. And not all polyurethanes provide a decent vapor barrier, so the correct one must be used. Also, the inside of the seat tube must be sealed up because water and humid air can enter and soak the bamboo.
Bamboo has a fairly high coefficient of thermal expansion (CTE). That means it expands and contracts with changes in temperature. We stopped using carbon fiber for joining bamboo because carbon has a very low CTE. This is also why there are no more carbon tube/aluminum lug bicycles being built. They didn’t last very long. After a few years building with carbon lugs, we saw some frames have the bamboo come loose from the carbon wraps. These were well sealed from changes in moisture content which also cause expansion and contraction. When we switched to natural fiber for the wraps, we stopped seeing this problem. We did have to compensate a bit with less stiff fibers like hemp, but the slight weight gain was worth not having the frames come apart after a few years. There are other ways to deal with dissimilar materials but it gets complicated and difficult. Luckily, hemp fiber composite has a CTE that is closer to aluminum, and that makes it a little easier to join. The dropouts and other metal parts still need a good mechanical retention with the fiber for long term durability. The metal parts should be designed specifically for this purpose.