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Old Feb 24, 2012, 01:10 PM
bwill6 is offline
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Help!

Ideas for how to mold this shape


I'm looking into making a mountain unicycle frame from carbon and I'm interested in ideas for how to mold this shape. I'd like to make it a one-piece frame for aesthetic and structural reasons, but if i can find a good way to join several pieces then I may go with that. Bladder molding would be difficult because of the fork, but not impossible. Expanding foam is expensive. Any ideas would be quite welcome. Thanks!

Brandon
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Old Feb 24, 2012, 02:51 PM
Ward Hagaman is offline
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You want to make many? Or just one or two?
Old Feb 24, 2012, 02:58 PM
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I'll need to experiment some, so the first mold may only have a few come out of it. Ultimately I may do a production run if things go well.
Old Feb 24, 2012, 03:35 PM
AirBornOne is offline
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Quote:
Originally Posted by bwill6 View Post
I'm looking into making a mountain unicycle frame from carbon and I'm interested in ideas for how to mold this shape. I'd like to make it a one-piece frame for aesthetic and structural reasons, but if i can find a good way to join several pieces then I may go with that. Bladder molding would be difficult because of the fork, but not impossible. Expanding foam is expensive. Any ideas would be quite welcome. Thanks!

Brandon
Wow,if you think expanding foam is expensive your in trouble son!
I'm thinking that to do the job the original is designed for,you need a completely different animal.
First,you will be using different materials so the geometry of the fork will need to adapt to the new material's properties.
Are you thinking of copying someone else's work?
Hmm.
J
Old Feb 24, 2012, 04:41 PM
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Haha I realize that the other materials are going to be expensive. I've priced carbon and epoxy as well as what it would cost to make the mold. The expanding foam is just another cost, another mess, and I think it would be simpler to do a hollow tubular construction. I am not planning on copying someone else's work. I just posted the pic to show the basic shape I'm needing to mold. I'm starting this pretty much from the ground up and doing the engineering work myself. I posted the question just to see if anybody had ideas I hadn't considered for manufacturing this. Thanks for your response.

Brandon
Old Feb 24, 2012, 06:53 PM
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I posted the question just to see if anybody had ideas I hadn't considered for manufa


How about starting by listing the ideas of yours?
Old Feb 24, 2012, 07:35 PM
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I've considered the following:

1) two piece mold with a wet seam. I'd need to use a bladder that conformed to the shape of the mold. That would mean making my own y-shaped bladders. The problem with this is that I'd have to make the bladders over the stem. If a bladder breaks, I can't make a new one in time to save the layup.

2) two piece mold, vacuum bagged and the two halves joined post-cure. The seam would have to have several layers of carbon over it, causing an obvious discontinuity in the finish. It would work, but cosmetically not the best option.

3) Different molds/mandrels for the forks and the seat post. The unicycle frame would then be 3 pieces that would have to be joined together after being cured. The joints would have to be highly reinforced and the same cosmetic issues as #2 exist here.

4) 2 part mold using expanding foam to press out excess epoxy in the layup and form a solid core. I believe this would be the most difficult method and it may not work well anyway.

Brandon
Old Feb 24, 2012, 08:27 PM
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Brandon, I'd be tempted to try making a prototype using hot wired foam and sock material from Soller Composites. The finish wouldn't be very nice without some extra work, but in my opinion you'd get a part many, many hours earlier.

Ward
Old Feb 24, 2012, 09:19 PM
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Brandon-

Things I would think about:

The seat clamp/seat post is a high shear loading area that composites will need help with as are the tips of the forks.
The fork itself is compression loaded so fiber path will be important(think column buckling).
How will you connect the load (you) to the axle of the wheel(point load).
How weight critical is this structure?

I would also be concerned about shock loads(you go over bumps right?).

The center of the wishbone puts the each side of the fork in shear from the seat post, figure 200lb. person- 100lb. per leg spread over a 1" diameter leg with a wall of say .080"( .135 inch^2 or roughly 740 lbf/in^2.

Most fibers can handle this load but remember to take the tensile strength of carbon and reduce it by 40% assuming you are using a wet lay-up resin(low modulus).

You may want to think about a big wishbone design with an upper tube holder of machined aluminum.....

Cheers
Old Feb 25, 2012, 08:24 AM
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Why not go to a bicycle forum


Your asking airplane modelers how to make bicycles. Why not got to a how to make a composite bicycle forum?
Old Feb 25, 2012, 09:02 AM
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Im think maybe tubing in this case is better suited for the alloys and maybe a plate with gussets or a laminated structure might be best for carbon. They have some assume lamination in things like landing gear for large RC now.
Old Feb 25, 2012, 09:58 AM
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Hey,
Try this link http://www.instructables.com/id/How-...ome-and-a-bam/
I'd say for starters this is on of the best ways to get started as it is important to have everthing in the right place.
You can do it like guru bikes do take look at this youtube video
Ανθρακονήματα Carbon fiber How its made Guru bicycles Anthrakonimata (5 min 2 sec)


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