Framebuilders - Back with another bamboo build using fiberglass lugs (Picture heavy)
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07-29-12, 11:32 AM
Here's my most recent build.
For the most part I copied the geometry and dimensions of an old school road bike. I added a few touring details as well.
The build process for a bamboo frame has been posted a few times already so I won't add too much of a description, but I'll show you the basic process that I used.
First step after setting up the jig is to miter all the bamboo poles. Apparently I only took pictures of the mitered joints of the front triangle. The stays each have a 1/8" slot cut into one end to fit over the tabs that I welded onto the dropouts.
This shows the front triangle mitered. Also shows the jig I designed. It does the job, but I want to improve it a bit more. The vertical piece of extrusion in the rear and the piece above the frame flex a little too much. To adjust the headtube angle I tighten or loosen the turnbuckle connecting the extrusion to the brake hole in the dummy fork. There's too much play in the turnbuckle for my liking but the error can be mitigated by measuring the headtube angle often when mitering the top tube.
After the frame is tacked I fillet all the joints in the front triangle with thickened epoxy. I used West System's 105 resin with 206 slow hardener and mixed in some 410 microlight and 406 colloidal silica.
07-29-12, 11:33 AM
After the fillets have cured, I sand the lugs to shape. Then I lay up some 2" fiberglass tape across the load paths. You can see the roughed up crossed pieces on the headtube lug.
I bought 180 yield fiberglass roving from Composites Canada. It came to me as a 45 pound spool. I made small spools by just screwing some big washers on either end of 3" pieces of 7/16" dowel. In this picture you can see three spools rolled up and ready to be used. Since wetted fiberglass is pretty much clear it makes for an ugly joint, so I mixed a little bit of dry pigment into the epoxy during the wrapping stage. I can get pretty much any color imaginable from Kama Pigments here in Montreal.
After wrapping all the lugs and compressing the fiberglass with the perforated electrical tape method I then sand and shape up the joints. I use an orbital sander with 60 grit sand paper for the big bulges and for whatever areas I can reach. For the rest I just use a metal file. I used a finer grit sanding sponge afterwards to make it a bit smoother before applying the clear coat.
At this stage the lugs are all sanded to shape. Here you can see the simple jig I used to position the brake bridge. It's positioned for use with a long reach brake for fender clearance.
The completed frame. Downtube shifter braze-ons so that regular downtube friction shifters can be used. STI adapters can be screwed on to convert them to regular cable stops. The frame can fit 2 bottle cages. I sunk a bottle cage boss into the chainstay bridge for mounting a fender. I also used bottle cage bosses on the seat stays for attaching a rear rack.
Only problem I encountered when assembling parts on it was that the front derailleur should be positioned exactly where the bottle cage boss is. Clamping the derailleur above leaves a decent gap but I think it will still work. Pictures of the assembled bike will be posted soon.
Any and all comments/criticism is appreciated.
07-29-12, 11:38 AM
Wow I am just in awe man, that is amazing! Have you gotten to ride it yet?
Andrew R Stewart
07-29-12, 02:00 PM
Aprhockey- I can easliy see why you're unhappy with your jig's stability and adjustments. Taking some of the dimensional points off the ends of what is a narrow width "c" shape is ripe for movement in all planes. Any way to close the "c"? "Window frame" jigs have been used for decades with good results. You are only a bit of 8020 extrusion length away from closing up the "c" and gaining a lot of stability and consider doubling the extrusion's cross section while you're at it. Andy.
07-29-12, 10:15 PM
Any and all comments/criticism is appreciated.
I like your work a lot. My only comment is that to the extent that you can lay your glass into the wet fillet material you will end up with chemical bonds, and zero sanding of the bog to glass step. Takes a little getting used to but normally goes smoother once you figure it out. Also, I prefer not to use 410 for structural fillets, but if it is working, and if it seems about as hard as the bamboo, should be good. If part of the reason to use the 410 is that it is easy to sand, then that problem goes away if you bond into the wet fillets with your glass.
Where are you getting the bamboo?
07-30-12, 10:22 AM
RaleighSport - No, I haven't gotten to ride this one. It was built for someone else. The next bike will be a cyclocross-ish bike for myself. I will probably have plenty of questions for you guys.
Andrew R Stewart - I think I might close it up like you mentioned for the next frame. I think I have enough spare extrusion to extend the vertical piece in the front and replace the top piece with a 4' piece like the base. I'll double the extrusion to stiffen it once I have the money for that.
MassiveD - How long would I have to wait before the fillets are cured enough to start wrapping with fiberglass? I'm using slow hardener which cures in 10-15 hours. My workspace is probably around 80F most of the time. Wait about 8 hours? I feel like this would be a delicate procedure using glass roving. It seems that I would be cutting into the fillets if I wanted to wrap tightly enough. If I were using a fabric overlaid on the joints I would be a lot more comfortable laying it up before the fillets are completely cured. Maybe I'm misunderstanding something.
I got the bamboo from Bamboo World (http://bambooworld.com/bamboo%20poles.htm). I ordered 1" diameter for the rear triangle and 1.5" for the front triangle. There's a decent amount of variation between poles and even throughout a single pole. They don't do small orders unfortunately. They only sell in bundles of 8' poles.
07-30-12, 11:29 AM
Looks great man, working on my second bamboo bike as well. How or what tools did you use to make your miters?
07-30-12, 02:47 PM
On this frame I actually used a small drill press that I borrowed from a friend. I used a 1 1/2" hole saw. I had to make a fixture to clamp the bamboo well. And I checked for any splits that may have occurred. Then I do minor adjustments with a half moon metal file.
08-04-12, 11:06 AM
Here's the bike built up. My friend was under a tight budget for parts.
So after a bit of riding he came back to me because the gear shifter boss sheared off on the left side.
Just using epoxy to attach the bosses was not nearly as strong as I had hoped. I ripped the right side boss off without much difficulty just by pulling the down tube shifter outwards.
I noticed that at the base of the threaded hole on the shifter bosses, there was an unthreaded hole about 3mm deep.
So I drilled a hole through the down tube and epoxied a steel rod in place such that there was about 3 mm protruding on either side. Then epoxied the shifter bosses back in place over the metal rod.
... I think this should be plenty strong now. And here I was thinking down tube shifters would be the easiest way to go...
08-07-12, 02:31 AM
How heavy is the bike or frame?
08-07-12, 05:33 PM
The frame was 4lbs 12 oz. About half a pound lighter than my last one, so hopefully the weight will keep dropping a bit.
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