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Thread: bamboo fork

  1. #1
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    bamboo fork

    Another bamboo question. im building a bike from it and am wondering if a triple crown fork with bamboo arms would be strong enough. would it flex too much? where could i get the necessary metal parts, not sure the name but the brace things that hold the arms and steer tube, and wide, disc compatible dropouts. i know i could bond the bottom brace with epoxy and fiber, but the top has to be removable, and clamping bamboo seems a little risky, but i guess if im trusting a frame with it it should be rigid enough for that. any thoughts from anyone? thanks

    jon

  2. #2
    Senior Member Allen's Avatar
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    For a lazy Sunday afternoon bike, maybe.
    Otherwise I would think that the bamboo would be far too flexible.

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    im thinking of using tam vong poles for it since they are solid and pretty rigid. If i can just find the parts making it i will let you know how it goes.

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    Senior Member Allen's Avatar
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    Right on.
    You will most likely need to have the pieces for the crown custom CNC cut.

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    If I was doing it, and I think it sounds a little light, I would just cut the plates with a hole saw, then use epoxy to cast rings around the fork material so that it is socketed in the plates with a washer of epoxy. Something like WEST system with some cabosil and microfibers.

    For the top end if hollow I would hardware bond some threaded rod in there or drill out the solid material and hardware bond the rod in. The top of your fork material would shoulder to the plates, and the rod would actually penetrate them. Hardware bonding is the best method to take loads off wood like stuff.

    I would use heavy aluminum plate so that the socket bonded area would be substantial.

    One advantage to hardware bonding is that the parts can float a little before they are frozen in the epoxy washer. A fork needs to be accurae, but your tubes aren't likely to be given the material.

    With the rod I would bind the outside of the fork material with carbon or glass tow, to stop it from spliting. If you space the nodes on bamboo carefully, you could make this build-up look like a node.
    Last edited by NoReg; 08-12-09 at 09:43 PM.

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    Let me see if i understand that last post clearly. i should make a fork jig (can this be made inexpensively out of wood and a dummy axle like a wood frame jig? or does a fork need more accuracy)

    on said fork jig, I should fix the dropouts and the two plates. then i should insert a metal sleeve into the bamboo at the top that sticks out the width of the top plate.

    i should then insert the top of the metal sleeve into the top plate and tighten it. the second plate should have holes big enough to allow a bit of space around the fork tubes. then use epoxy and a filler to fill this space and fix the perhaps not straight tubes into a position that correctly orients the plates with the dropouts.

    then wrap where the bamboo tubes enter the dropouts with carbon or glass.

    that sounds like a good solution except for the medal sleeve. this would work for other bamboo varieties, but tam vong is solid. hence why its attractive for fork material, at least in its rigidity if not its weight. I could drill out a bit of it to insert the metal sleeve, but as tam vong is supposedly stronger that steal, i am not sure that this is nessesary.

    tam vong is know for bending a bit over the length of an entire pole. the curve should be minimal over the length needed for a fork. but still i like the idea of a little play that can be fixed by epoxy washers to ensure straightness.

    this still leaves me the issue of aquiring the plates. i dont have any machining tools at me disposal. i could borrow some time on a drill press but thats about it. how thick of plates should i use? could i cut it with a hand jig saw and a metal blade? what if i could use a band saw? how expensive would it be to have them machined?

    i think the best solution may be to buy plates already made for the application. choose poles slightly smaller in diameter. fit a metal bit over the top of the poles that is the correct diameter for the plates then use the above method for the bottom.

    thoughts? places to buy plates?

  7. #7
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    The upper connective in the tripple tree would be a threaded rod or machined better equivalent, hardware bonded to the center of the TV. The reason you would do that is that the steel or Al will interface better with the top tripple tree, and make it possible to endlessly remover the top plate.

    As far as the wraps are concerned I was only suggesting one at the top of the TV where the rod is bonded inside. There will be a bending moment there, and the rod could split out the TV, or even it could split due to moisture issues, where the TV dries out, and cracks as a result of the material in the center, though hardware bonding will minimize that. all these wood materials are real strong in certain respects, but poor in split resistence.

    The way you hardware bond a stud into some wood, etc... is if it is 1" diameter, then you might drill a 5/8" hole in the end and use a 3/8" stud, the rest is taken up with epoxy. You want to leave as much wood as possible but also leave at least 1/8" for the epoxy. This fastening system is used in high performance aps like wind turbine blade up to 160 feet in diameter held to hubs by studs bonded into the endgrain wood fiber, and all sorts of sailboats etc..

    I've been doing wood and bamboo comPosites for 30 years, but your ap is all new to me, and I don't Know this TV stuff.

    "this still leaves me the issue of aquiring the plates. i dont have any machining tools at me disposal. i could borrow some time on a drill press but thats about it. how thick of plates should i use? could i cut it with a hand jig saw and a metal blade? what if i could use a band saw? how expensive would it be to have them machined?"

    If you are going to do it with tripple trees you are stuck with the plate issue. I would use 3/4" Al, on the assumption you want to look beefy, and you need significant socketing to hardware bond the TV in there, might not even be enough. You can cut Al with a wood bandsaw, or cut it on a metal bandsaw. You can cut it with a high qualityu jigsaw and correct blade, though 3/4" is pretty thick. You can hole saw that with a drill press. In my neck of the woods CNC work or machining are both very expensive. CNC the costs only come down with volume. There are a lot of products involved, CAD drawings, G-code file, machine set-up, then running the parts. Manual machining is slow, and also expensive.

    It could be done in composite, though the design work gets more difficult and you would have to be careful to maintain accuracy.

    Forks need to be real accurate, or the bike doesn't ride correctly. However, wooden jigs can provide that accuracy. Particularly where stuff like hardware bonding or composites are concerned, because there aren't any heating forces pulling at things.

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    how far into the top of the TV should my threaded rods go?

    I think i would rather use slightly thinner aluminum so as to make cutting easier, yet sand the bamboo surface and wrap the bottom plate' joints with fiber and resin in order to give a contact patch larger than just 3/4 inches.

    where can i acquire small pieces of aluminum alloy?

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    The other option one has got for the crowns is carbon fiber plate.

    It's quite a bit easier to cut than metal, is similar in many respects to very hard wood.

    One could also wrap carbon onto the bamboo stanchions to get the joints snug, and then further wrap carbon weave onto the crown & stanchion.

    Carbon fiber plate at significant thickness is not cheap, but you wouldn't need a huge area to make a couple of crowns out of.

    Regarding the mast-style end-pin fixing, I'm not sure how strong the material forming the wall divisions in the nodes of bamboo are, might be best to bore the tube out slightly and then fit an aluminium or carbon plug into the end of the tube, possibly even fit a few inches of aluminium tube inside the end of the bamboo tube to help with the compressive loading crushing the tube from the top crown and prevent splitting.

    Removable carbon steerer tube FTW

  10. #10
    coprolite fietsbob's Avatar
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    Truss fork like a Dursley-Pedersen? , Ie , a triangulated, structure.

    http://en.wikipedia.org/wiki/Pedersen_bicycle

  11. #11
    Randomhead
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    I assume the OP made a decision some time ago given that this thread is 4 years old

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