Introduction: Bamboo Bicycle Frame
I found several websites with bamboo frame bicycles for sale and was very interested, but did not want to spend that much money for one. Then I found several videos and the Instructables by ayasbek (https://www.instructables.com/id/How-to-Build-a-Bamboo-Bicycle/) and xman (https://www.instructables.com/id/Bamboo-Bike-Frame/). I thought "I can do that!". I first found a suitable donor bicycle at a flea market, a 90's Trek hybrid. It had 21 gears with front and rear derailleurs, which I wanted to keep. The frame size was about right for me so I could just copy the frame geometry. The first thing was to find some bamboo. I found some locally growing bamboo, but the diameters were too small for the larger tubes needed. Searching the internet I found Fleet Farm has small quantities of bamboo in many diameters in 4' lengths. I ordered about every size between 3/8" to 1 3/4" and about twice the lengths I needed of everything. Many of the tubes were split and not usable, but fortunately had enough to do the whole frame. The bamboo was already dried, so I did not do the heat treating described in several sources. I bought hemp twine for joint wrapping but used fiberglass and resin left over from a boat repair project. You also need some epoxy adhesive to fix the joints before wrapping them. Electrical tape is used to wrap the fiberglass joints
Many of the DIY projects were single speed bikes and I wanted to use the original derailleur system, which had a 7 speed cassette and triple chain rings. I realized there were some very tight clearances in the drive chain area so was careful to watch those areas as to the bamboo tube size and joint wrapping.
Disclaimer: Death or serious injury can result from a bicycle frame failure. You are responsible for the all details that can make this successful or a failure. Be careful!
Step 1: Making a Jig for the Frame
I wanted to keep the original frame geometry, so the bike was stripped down to the frame, seatpost, fork and bottom bracket. A piece of 1/2" plywood, 2' x 4', was used for the base. The axles were simulated by 3/8" all thread rods and nuts mounted to short pieces of 2x4's and attached to the base with drywall screws. The crank arm bolts were used to locate the the bottom bracket, since it was still in the frame. More scrap wood was used to hold to locate the seatpost and fork. This frame had linear pull brakes, so another piece of wood was drilled out to match the rear brake post spacing. Now I was ready to cut up the frame.
Step 2: Cut Up the Donor Frame
The parts needed from the old frame were head tube, bottom bracket tube, seat tube clamp area and rear axle dropouts. A stub of the adjoining tubes were left on the head tube for the down tube and top tube. The bottom bracket had stubs for the down tube and seat tube. I also cut off the cable stops for the shifting cables and the rear brake posts. On these a part of the original tube was left attached to use as a mounting base.
These parts ware going to be wrapped with fiberglass, so I used a file & sandpaper to remove the paint and rough up the steel for good adhesion in these areas.
Step 3: Fit the Bamboo Tubes
Replace the head tube piece on the fork and place it in the jig, along with the bottom bracket piece and seat tube pieces. Since the bottom bracket and head tube have some bearings in them tape over these parts to avoid getting epoxy on them. Select a bamboo tube for the down tube big enough to go over the stubs on the bottom bracket and head tube. For my build, these tubes were a little oversize, so I wrapped the stub with hemp twine and epoxy to build up the diameter for a good fit. The bamboo tube was mitered at each end to get a good fit with the head tube and bottom bracket. I used a Dremel tool with a burr bit and carved up each end by hand. The same process is repeated for the seat tube and top tube. Check that the seat post can be fully inserted in the bamboo. Drill out any nodes inside the bamboo that interfere with the seat post. Once the fit was good on all three tubes, the bamboo was glued in with 30 minute epoxy. Avoid getting the epoxy on the seat post because you want that to be adjustable. I oiled the seat post in case any epoxy ended up there.
Step 4: The Rear Triangle
The area just behind the bottom bracket can be a little tricky because the clearance needed for the rear tire and front chain rings. I wanted to use the original derailleur system, which had a 7 speed cassette and triple chain rings and medium size tire (700 x 32). The front derailleur clamped to the seat tube and did not fit on the bamboo tube, so I settled for a single chain ring on the crank with no front derailleur.
The chain stay tubes size is tricky - big enough to fit over the rear dropout tube, but small enough to match the width of the bottom bracket and still leave enough room for the tire. There is also a short tube between these tubes to help maintain the tire clearance. The stays were glued on with epoxy after the trimming and fitting to match the bottom bracket and drop outs. I almost got it right - in the end I had to carve out room for the tire on one of the chain stays.
The seat stays were also a little tricky. It was not possible to get a straight tube from the dropout to the side of the seat tube and leave clearance for the tire, so the bamboo tube had to have a mitered cut. For my frame, a 85 degree angle on each side of the cut was needed. The cut was placed just outside of the tire and a short bamboo tube between the stays used to maintain the spacing.
Step 5: Wrapping the Joints
Wrapping the joints with fiberglass cloth and resin can get messy, so the bamboo that is not being used fo rthe joint was wrapped with plastic and secured with painters masking tape. The uncovered bamboo surface in the area of the joint was sanded to ensure a good bond.
The fiberglass cloth was easiest to apply when cut into strips - I used pieces about 1" or 2" wide x 6" long. The cloth was saturated with resin using a disposable brush and then wrapped around each joint. Try to apply the cloth in multiple directions to give the joint strength in all directions. Make sure each area of the cloth is saturated by the resin. Apply 2 layers and then wrap each area with electrical tape with the sticky side out. The idea is to squeeze out excess resin - the strength of fiberglass is the glass fiber and the resin just holds the fiber in place. The strongest fiberglass has a large percentage of fiber.
The joints wrapping are the seat tube-top tube-seat stay intersection, the head tube-top tube-downtube intersection, bottom bracket-chain stays-down tube- seat tube intersection, and 2 dropout-chain stay-seat stay intersections.
After the resin is cured, remove the electrical tape and smooth out the high points of the fiberglass. Build up each joint by repeating the process. I lost count of the number of layers but it was probably about 4 or 5 applications of 2 layers each. I used about 10 - 12 square feet of fiberglass cloth.
Step 6: Add the Cable Stops and Brake Posts
The cable stops were fastened to the bamboo with epoxy in approximately the same position as on the old frame. I only used the stops for the shift cables and elected to use a full length cable housing for the rear brake. The brake post location is more critical. I drilled a small block of wood to get the spacing between them correct and to keep them parallel. I measured from the rear axle to get the correct distance and tried to center them on the axle while also being supported by the bamboo. This required a large blob of epoxy to make contact with the bamboo.
After these were positioned, the extended parts of the tube were wrapped with fiberglass as done with the bamboo intersections.
Step 7: Finishing Steps
I had originally intended to use hemp twine to wrap the joints, as done by ayasbek, but was not confident that I could make the joint strong enough in the bottom bracket area. I wanted to keep the derailleur shifters and there is not much room to wrap a lot of twine around that tube. I had some fiberglass cloth and resin from a boat repair so I used that. Most builders have shaped, sanded and painted the joint wraps for beautiful finish. But I had the twine decided to just wrap that over the joints for a different finish. I just say inspired by Fred Flintstone.
I put a coat of resin on the joints and wrapped the twine as neatly as I could, trying to cover all of the fiberglass. A finish coat of resin was applied over the twine. It may add some strength to the joint but I considered it mainly cosmetic.
As the final finish I use a polyurethane spar varnish on the bamboo.
I had weighed the original steel frame with the steel fork and bottom bracket at about 10 lbs. The bamboo version at this stage was 9.5 lbs. I was not trying to save weight, but it was nice to have a small reduction.
Step 8: Final Assembly & Test Rides
The rest of the original bike parts were assembled on the frame along with new seat, tires & tubes ( the originals being too bad to reuse). I was unable to use the front derailleur since the seat tube clamp mount was too small for the bamboo tube, so only one chain could be used. The smallest chain ring interfered with the wrap on the bamboo chain stay, so it was left off. The 32mm rear tire rubbed slightly on one chain stay, so a small portion of the bamboo was filed off in that area for clearance.
And then the first ride! The first few shakedown rides were slow, making sure everything worked and the frame joint held together. My primary concern was the rear brake posts, but they have worked with no problems. Even squeezing the rear brake harder than a very hard stop (while stopped!) did not cause damage. The seat stays can be seen bowing out, but the epoxy/fiberglass/hemp wrap holds up!
After short rides around the neighborhood, I tried more stressful maneuvers - riding off of curbs, hard standing up pedaling up hills. As I write this I have ridden 450 miles with no problems. The ride is very smooth on rough surfaces and I don't feel any any indication of frame flexibility when pedaling. I am very pleased with it and have added some upgrades - new pedals, grips, shifters and Brooks saddle. I ride it as much as I can.
The single chain ring limited the range of gears so I experimented with the front derailleur mount. I was able to modify the clamp ring - rebending and adding a section so it would fit the larger diameter bamboo seat tube. I now can use the 2 larger chain rings.
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