Introduction: Bent Plywood Bicycle
After seeing this bike on a bicycle design blog I was inspired to build my own. It was made by someone named John Hobson. The concept is the same, but mine is fairly different.
Also I had some left over epoxy and fiberglass from a kayak I built and was just itching to find a project I could use it on (or maybe I was just itchy from all the fiberglass). Anyway, since I wasn't thrilled with my current bike that was a cheapo 15 year old big box store bike, I thought it would be great to replace the old frame with a wood composite frame and have something unique.
The part of the design I liked was that the top and bottom parts of the frame are like leaf springs separated by the seat tube. I thought that having a suspension system integrated into the frame was a really cool idea. Ideally vibrations and bumps would be absorbed into the frame and returned back to the wheels, without making the frame too bouncy. The frame I built consists of plywood, fiberglass, epoxy, and even paper, so there is no welding required.
Before I get started here's some bike terminology I had to learn:
Head Tube - short tube for the handle bars, and front fork assembly.
Top Tube - horizontal section that connects the seat tube to the head tube.
Down Tube - diagonal section that connect the head tube to the bottom bracket.
Bottom Bracket - This is where the pedals and crank attach.
Seat Tube - longer tube that the seat post slides into.
Chain Stays - Two sections that connect the bottom bracket to the rear wheel.
Seat Stays - Two sections that lead from the seat to the rear wheel.
Dropouts - brackets to connect the rear wheel.
Caution: Riding a bike is dangerous, riding a home built bike is even more so. If you decide to build a bike, take it slow, wear a helmet and other protective equipment, reinspect your work, and be prepared for it to break at any moment.
Step 1: Get the Materials
I looked at the 1/8 inch plywood at Lowes and Home Depot, but I wasn't very happy with the quality. I then went to a local woodworking store and found some good looking 1/8" baltic birch plywood in 5'x5' sheets that had a nice light color. It was about $20 per sheet so if it turns into a disaster I wont be too upset. I also bought a pack of birch veneer to wrap around the seat and head tube.
I bought a bottom bracket, bottom bracket shell, and crank from Amazon for about $50. The rest of the parts I plan to take from a junk bike I had lying around.
The fiberglass, epoxy, and spar varnish were left over from a kayak build. I bought these online from a boat building supplier. It is 6 oz glass cloth and the epoxy is clear, non-blushing that takes about 24 hours or more to fully cure.
Tools needed: Saw, sandpaper, rasp, a couple of pliers, drill, power sander, power jig saw, some solid wire, natural fiber rope or twine, a vise, and several disposable brushes, cups, and latex gloves. When needed be sure to wear eye protection and a mask to keep gunk out of your lungs. I also had to buy some specialty bike tools, a chain tool and crank arm remover.
For removing parts off the old bike I used a hack saw, locking pliers, bench grinder, and a Dremel.
Step 2: Cut Some Plywood
For cutting I used a coping saw, although other saws would work. Just be careful not to tear up the wood too much, especially when cutting across the grain.
In order to get an even bend, the pieces should roughly have an even width. The rear wheel section has a fork with two 1 1/2" wide sections that are 3 3/4" apart. The rear forks are 13" long, and they narrow to 3" apart where they join together. It thens tapers down to a single 3" strip. The total length is 43" The dimension don't have to be perfect since I will shape the frame once it is all glued together.
I only cut 4 pieces to start with. Two of them are a 1/2" longer to account for the extra length needed when bending. Small holes are drilled into the ends of the boards so they can be wired together. The two outer boards have the holes an 1/8" further from the edge so they can line up with the inner holes, and still come together at the end.
Wire up the boards and see how the ends look. For wire I used 18 gauge solid copper wire. At this point I had to cut a little bit off the ends of the outer boards to make sure they came together like I wanted. A temporary seat tube is made out of a piece of PVC with fabric taped to the ends to prevent it from slipping off the plywood. The fabric adds some friction to keep the tube in place, although it still slips off more often than I'd like.
Several times I held up the frame to my old $99 cheapo bike to see if the dimensions looked right. Once the frame is the right size, I cut the glass fabric to the shape of the boards.
Step 3: Initial Glue Up
I chose to layer fiberglass between each piece of plywood mainly because I have a bunch of it. Hopefully this will add durability but I don't know if doing this will make the frame stiffer or more flexible. If I were to do this again I would skip the extra fiberglass and just add a few more layers of wood to increase the visual impact of a wood bike.
Be sure you know how to use the epoxy before you start and always wear gloves. The four boards are first painted with epoxy on one side then the glass is wetted out onto two of the boards. They are then sandwiched together and the ends are loosely wired together.
It takes a couple tries to get it bent into shape and wired together, but once I got the temporary seat tube in place with the plywood bent around it, I could tighten up the wires. I tried to make a herring bone pattern where the plywood is wired together, but I was only moderately successful. I used some homemade clamps to keep the boards tight together. After it cured I removed the wires and sanded it out.
Step 4: Laminate, Laminate, Laminate
This is quite a long process. I chose to do one layer at a time, and I'm getting 2 layers done per week. The steps are. Draw and cut out 2 boards. Cut the fiberglass to match the boards. Clamp the boards on dry to make sure they are the right size. Prepare the epoxy and paint it on one side of the frame and on one side of the new boards. Wet out the fiberglass onto the new boards. Clamp it all together and let it cure for 24+ hours. After it's hard (the epoxy should not dent from pushing your fingernail into it) cut off the excess fiberglass, sand out any bumps, and do it again.
Out of the 5'x5' sheet I squeezed out 14 pieces. Thus I have seven layers each for the top tube and the down tube sections. I discovered (a bit too late) that if I make the boards a little too long, I can cut off the excess while it's dry clamped to the frame. On this frame, I placed four layers on top and one layer of plywood on the inside of the frame to cover up the wire holes, and hide any scuffing caused the clamping and reclamping.
Because of the bending, the rear chain stays and seat stays have a tendency to move closer together. To stop this I had to add a scrap piece of plywood between the stays to maintain the distance. Once all the layers were added and cured I cut out the scrap plywood. To protect the wood I added a final layer of fiberglass on the top and bottom of the frame.
Step 5: Make Tubes
The seat tube and head tube are made out of fiberglass, brown packing paper, and birch veneer all wrapped around a pipe used for a mandrel. For the seat tube I used part of an old shower curtain rod with the decorative plastic removed. It just happened to be the same diameter of my seat post. For the head tube I used 1 in. pvc pipe that had an outside diameter of 1.3 in.
I planned to attach a front derailleur to the seat tube so I had to pay attention to both the inner and outer diameter of the tube. The seat post was 1 in. in diameter and the derailleur needs a 31.8 mm (1 1/4 in.) seat tube so I had to make the tube 1/4 in. thick (1/8 in. on each side). I also integrated the old bike's seat post clamp into the tube so I wouldn't have to figure out how to put it on later.
There are some helpful websites for amateur rocketry that provide instructions on how to make fiberglass tubes, but my experience was difficult. It took 3 tries to make the seat tube. The first attempt made a tube with an internal layer of paper that was way too loose. The 2nd attempt was a very tight tube of fiber glass and paper, but it wouldn't slide off the pipe. Finally I made a tube of only fiberglass and with effort was able to pull the pipe out. The head tube is much smaller and slide off the pipe fairly easily.
Here's the steps I finally used:
1. Sand the pipe so it's smooth
2. Lubricate the outside of the pipe, with grease or wax, but leave the ends
clean so the wax paper can be taped on.
3. Wrap the pipe with wax paper, and tape the ends so it stays in place.
4. wrap the tube two times with fiberglass and epoxy. Using latex gloves I was
able to wrap the epoxy soaked fiberglass flat onto the pipe so that there no
bubbles or gaps. Don't make it too tight though or it will never slide off.
5. Let the the tube cure, but not completely.
6. Remove the pipe: Remove the tape then break the pipe free by locking the fiberglassed section into a vise and twist the pipe with pliers. Work the pipe out by pulling and twisting with the pliers on one end and push the pipe with a stick or smaller pipe on the other end. Take care to cause as little damage as possible but there will be some.
7. Remove the wax paper, and slide the fiberglass tube back on the pipe. Sand it and coat with more epoxy and wrap with several layers of paper to add some thickness and add some epoxy as you go. The paper doesn't need to be soaked with epoxy.
8. Let it cure. I used some cord to keep the paper tightly wrapped while it cured.
9. Sand the pipe again, and measure the circumference, to get an idea of how much veneer is needed. Cut the veneer so it overlaps the pipe by about a 1/4 - 1/2 of an inch and sand the inside edge of the veneer to a sharp blade (the outside edge gets sanded after the epoxy cures). The blade edge will go down first then the opposite edge will lap over it.
10. Add another layer of fiberglass and epoxy followed by the veneer layer. The veneer bends easier once it's been wetted out with epoxy.
11. Let cure. This time I used wire ties to the keep the veneer wrapped around the
12. Sand the veneer to get rid of the over lapped edge, and to remove any marks or indentations caused by the wire ties.
13. Finally apply the last layer of fiberglass.
14. Let cure
15. Sand it smooth.
The Head tube ended up being 1/8 inch thick with an inner diameter of 1 5/16 in. and outer diameter of 1 9/16 in.
I originally tried to use a 28.6 mm front deraileur, but it wouldn't fit, so I added another layer of fiberglass at the bottom of the seat tube and used a 31.8 mm derailleur. The seat tube finally had an inner diameter of 1 1/16 in. and an outer diameter of 1 5/16 in.
Step 6: Dropouts
For shaping the frame I began at the back and worked my way to the front. I plan to attach the dropouts by gluing them into slots using thickened epoxy. If it doesn't hold I may have to revert to bolts instead.
To create the slots, I dug a channel into the back the same size as the dropouts. I first cut out the area using a Dremel rotary tool, then moved on to using a hand drill and wood bit and moved it up and down the channel to route out the wood. I designed it so that the dropouts have to be rotated into the slot, opposite the direction the frame pushes on the wheel.
I chose the go ahead and glue in the dropouts before shaping the stays. To thicken the epoxy I combined sawdust with wheat flour I got out of the kitchen. With a little experimenting I was able to come close to the color of the wood. Before gluing in the dropouts I made sure they were clean and grinded in some pits with a Dremel so the epoxy can get a good grip on the metal.
A string taped down the middle of the bike helped me make sure each side was even. Then I used a handheld jigsaw to cutout off the edges along the seat and chain stays and used a flexible wire as a straight edge to help pencil in where the cuts should be.
Step 7: Bottom Bracket and Seat Tube
The bottom bracket section needs to be shaped such that the frame has clearance between the rear tire and the sprockets on the pedal crank. To do this hold the bottom bracket with the crank loosely installed to the frame and measure where the cuts should go. Make a small cut to the frame then measure again. After doing this several times it should fit. Then cut the other side so it is symmetrical. The down tube ended up being 1 7/8 in wide.
To determine where the bottom bracket and seat tube should go I held the bike up to my old bike to get an idea. I drilled a hole for the seat tube using a 1 1/4in hole saw. Once the seat tube hole is made, the edges can be cut off leaving 3/4 - 1 in. of the frame on either side of the seat tube. Then taper the top tube section down to 2 in wide.
Once the seat tube is glued in place I glued the bottom bracket shell on the opposite side the down tube. I went ahead and wrapped some extra birch veneer around the bottom bracket but it wasn't really necessary.
Since the bottom bracket is basically hanging from the bottom of the frame, it needs a lot of reinforcement to keep it on. In my first attempt I tried to fasten it with a paper and fiberglass composite, but it didn't look that great. Also I neglected to reinforce the connection between the seat tube and the bottom of the frame. It cracked on a test ride, and gave me a really wobbly crank.
In the next attempt I used strips of fiberglass cloth wrapped from the seat tube over the frame to the bottom bracket and back, and then I wrapped the whole area with many lashings of a natural fiber rope I had in the shed. Everything was wetted with epoxy before applying. I used wire ties to add some compression to the joints, but in retrospect I probably should have used some plastic or electrical tape to get rid of the lines left by the wire ties.
It might be possible to place the bottom bracket on top of the down tube, but you would lose ground clearance, and you may need to use a more sophisticated bending technique than that of a stick and some wire.
Step 8: Head Tube
The hard part about attaching the head tube is getting it in the right position. I used my old bike as a template to get the forward/backward angle right, then I was able to use my hole saw and drill it out. When checking to see if the hole is lined up right, I found it helpful to insert the PVC pipe into the head tube and see if it looks true.
Unfortunately the left/right angle wasn't right, so I had to do some sanding and used a couple nails to temporaryily hold the head tube in the right spot while the epoxy cured. Once it cured I could remove the nails and add more flour and sawdust thickened epoxy around the tube.
Step 9: Cable Stops and Head Tube Cups
Now that the headset, seat tube, and rear dropouts were in place I could spend some time finalizing the frame shape and sand everything smooth. It should now look like a bicycle frame.
I created the cable stops out of left over pieces of the frame. I cut a rectangular piece and drilled a hole halfway thru about the size of the cable shielding, then a small hole all the way thru for the actual cable. Carefully I sawed a slot to the hole for adding and removing the cable, and rounded off the corners.
I pressed the headset cups from the junk bike into place by hand. After a little sanding they fit inside of the head tube snugly. To make the old parts look better I taped off parts of the bike and spray painted silver on the headset cups, seat post clamp, and rear dropouts.
Step 10: Finish and Assemble
Once everything was glued up and looking good, it was time for the varnish. I used some spar varnish and gave the bike 3 coats.
Assemble the bike. I ended up putting the rear brake below the frame because I didn't want any cables running along the top of the bike. It's far from ideal though. If I can get my hands on a free or almost free rear disc brake set, I'll install it, but for now this will do. For the front derailleur I drilled a small hole behind the bottom bracket to run the cable thru. It attaches to the bottom pull derailleur.
That's it. It's actually a lot stiffer than I anticipated, but it's a good ride. I don't ride a whole lot, but it will work for what I do, either riding thru the desert or on the canals or down to the local bar (and walk it back of course). I'm pretty gentle with it, but I've gone up and down curbs slowly. When you build your own bike be prepared for it to break at any moment until it's been thoroughly tested and visually inspect it from time to time. I've only taken it out a couple times and I'll get more aggressive on it in time. I'll update this instructable if anything breaks. I got a couple looks from people who notice bikes, but a majority of people it seems see it as just another bike on the road.
There are a couple fills in the bike where I had to add thickened epoxy, but it's not too noticeable. You can probably spot them in some of the photos. The frame by itself weighs about 6.8 pounds and all together it is 32 lbs. Anyway, I wish you the best of luck if you decide to build something like this!
Update: I broke the seat tube just above the frame when I went to a shorter seat post. It didn't go far enough into the seat tube, so it was weaker. I redid the top of the seat tube so it is stronger now and use a longer seat post. Also the bottom bracket shell broke free and started sliding left and right. I was able to reset it with gorilla glue and it seems fine now. Finally I replaced the rear side pull brake with a center pull calliper brake. The side pull was uncomfortably close to the crank arms.