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2nd Bamboo Build. . . questions on carbon fiber alternatives

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2nd Bamboo Build. . . questions on carbon fiber alternatives

Old 05-15-12, 12:51 PM
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aprhockey
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2nd Bamboo Build. . . questions on carbon fiber alternatives

Hey, I posted several months ago about my first bamboo build that ended up cracking at a few of the joints. You guys pointed out that I likely hadn't compressed the hemp enough and possibly didn't wet it out completely. So, with that in mind I built a second frame. This time I went for carbon fiber rather than hemp and so far with 350+ kilometers, it's riding perfectly well.







The second top tube was added because a) I had a spare pole, b) the bamboo that was delivered to me was not as large in diameter as I had asked for, and c) why not?

So, now, I want to build some more frames to test according to pages 46-52 of the European standard EN 14764.

I guess I got really lucky in finding some 12k carbon tow, because now that I'm looking to buy more, I can't find anything at nearly the same price. I was able to get a 4lb spool off ebay for $65 with shipping and now I can't find that size spool for anything less than about $90. So, I want to try out some alternatives. I found that working with carbon tow was much much easier than with raw hemp fibers. For one, I didn't need to comb it out to get workable 'rope' to wrap the joints with. Also, I found that I used much more epoxy when working with hemp, though that may just be due to inexperience. Another thing I didn't like with hemp compared to carbon tow, was that the raw hemp fibers were not continuous.

So, my question to those with more experience is what are my other options?

Is fiberglass continuous roving adequate to use for the entire joint? I understand that there is S-glass and E-glass. Would S-glass be required? I'm not too familiar with fiberglass, but if I understand correctly it can be used just the same as carbon tow (i.e. no special epoxy or prep work is required).

I would also love to use organic fibers such as manila or flax. Regardless of the fiber I use for the joints, I will first put some fiberglass tape across the major load paths as the Bamboo Bike Studio shows here. If using manila or flax, would that be sufficient for the whole joint, or do you think I would have to mix in some layers of carbon fiber or fiberglass? I found this site which sells low-twist flax roving in which they show that the tensile strength is less than 1/3 of E-glass.

Correct me if I'm wrong, but I'm assuming if I use a natural fiber, it wouldn't be a good idea to use twine, yarn, or rope since that would be too thick and lead to gaps between the fibers.

My knowledge of composites comes almost entirely from the two frames I have built, one with raw hemp fibers, the other with HexTow IM7 12k carbon tow, so please point out any errors in my thinking or anything I'm missing.

Since I couldn't land myself an internship this summer, I will be spending the next few months experimenting with bamboo frames.
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Old 05-15-12, 08:46 PM
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Did you use 4 pounds of carbon already? Carbon is still relatively cheap for the actual amount you use on a bike.
Any type of glass will work although the S glass is stronger. Aircraft Spruce sells large rolls of S glass. The trouble with glass is that the lugs look pretty bland when finished.
An underlayer of glass/carbon and then a wrap of Manila looks good and is easily strong enough. The first bamboo bike I built several years ago was made this way. I just untwisted Manila rope from Home Depot.

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Old 05-16-12, 10:02 AM
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If my scale is accurate, I used about 3/4 lbs of carbon tow. I'm interested in finding a cheaper alternative that is still sufficient for long term use.

Using fiberglass I would apply a outer layer of hemp/manilla/flax, maybe even carbon tow. Or, I would like to try out using dyes in the epoxy.

When you got the manila rope from Home Depot, did you look for anything specific? Also, how did you treat the fibers before using them?
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Old 05-16-12, 02:40 PM
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I've worked with composites for a long time, and carbon always seemed like overkill on this. Think of steel lugs, they don't weight that much, laying down a ton of carbon just doesn't make sense. All carbon used to wrap is basically wasted. Carbon is lighter than glass so for a similar volume it weighs less, but glass would be fine for the hoop strength, and is often substituted for carbon when hoop strength toughness is desired. Of course the carbon is much stronger than glass but it shouldn't take much to get where you want to go. I would save the carbon for where it will do the most good which is anywhere where it crosses over the joint in the tubes. Wrapping is something you could leave to other materials.

That said. Carbon is becoming ubiquitous. It is still expensive to buy, but it also shows up surplus a lot. I haven't been this lucky, but a pal down the street got 100 pounds for free in a 12K linear knit tow. He gets lucky a lot. But just keeping your eyes open you may find better deals. If you can get it in your price range, there isn't a lot of downside in using it, but while you are looking I would substitute the glass or even hemp for the hoop strength.

There are a few traditional structures where bamboo, carbon, and glass are used. Fishing rods, arrows, and bows come to mind. Fishing rods are tubular for the most part, and all three materials are popular. Bamboo holds it's own in certain niches because of damping, design possibilities, and quality in execution. It is however the lowest performing in raw numbers. Overall continuing bamboo structure levels with glass sounds like a good bet. Not an engineer though.

There are a few things you can do to further save on Carbon:

1) Use it aligned to the loads, this is what we have been talking about but you can get even more explicit about it. Really noodle out your load paths and get good tight, flat fiber alignment.

2) Use good epoxy. The internet is the home of a lot of talk about how all epoxies are the same, and you should exclusively shop by price. This is not true, but it can work out if your structures have high redundancy, or are lightly loaded. Any time carbon enters the fray, you need a carbon equivalent epoxy that will do for compression loads what the carbon does for tensile loads. In part to even things out, but also because the carbon is going to be superb at transferring those loads because it is so stiff. WEST is one such good brand, there are others, but expect to pay a little for them. You will need to mix them carefully. WEST started out as a boat building epoxy, and was instrumental in early pioneering of carbon/wood structures. There are a lot of other boat building epoxies out there that are not as good, but west is not the only one. Avoid ones marketed for their flexibility.

3) Use meaty coves. There is some degree of epoxy coving that would allow one to dispense with carbon entirely. Coving is the art of mixing silica, and micro-balloons, to create syntactic foams. The putty is applied like fillet brazing. And works the same. The coving could take some or all of the loads, but practically given the flat faces of the sides of the tube joints, one is going to use some fiber. But getting good meaty radiused fillets in there will add a lot of strength, and maximize the effectiveness of any carbon you use.

Last edited by MassiveD; 05-16-12 at 02:44 PM.
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Old 05-16-12, 06:11 PM
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I plan on using West Systems 105/206 epoxy. As for point 3), that was the plan. First tack the poles together using epoxy thickened with 404 high density filler creating a small fillet. Then build up around the joints to create large fillets with epoxy thickened with 410 microlight low density filler (I guess this is called coving). Then apply fiberglass tape across the load paths, longitudinally between poles like in the Bamboo Bike Studio link above. And finally apply the carbon/fiberglass/hemp/whatever it is I decide to use.

Does that make sense? Should I apply the fiberglass tape before coving? For the coving, should I use a higher strength filler?

I did a rough practice joint with this method and it seemed to work well. I can try to get a picture uploaded soon. Also, as a note, in the frame pictured above, I didn't form any real fillets between the poles and found that I wasn't able to easily compress the lugs. That was the initial reason I looked into using the 410 microlight filler. I didn't think it would do much for the spreading the loads.
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Old 05-17-12, 11:51 PM
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MassiveD
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410 is great stuff, and has a variety of secondary uses. If you already have it and want to use it, it will probably work. But it is really intended as a bondo type product that one uses to fill/fair surfaces, not fill structure. So amending it with a tougher filler would be a good idea. Without real data, and few people have that, I don't actually know if it will be hard enough given that what is under it will be bamboo. But most of the west stuff is wood, which is softer than bamboo in the type commonly used, so I would not choose it myself. But if you have it, see below.

What I use for most coves are one part fumed silica (west 406, but there are other cheaper version that work every bit as well), and 3 parts q cell. A better product than Q would be 407 phenolic. Even when you fix on a mix, you can vary physical properties a lot by how much you extend the mix with the fillers, the more epoxy the harder it will be, I am not recommending that, just saying you have a powerful kit there.

So the 410 with a lot of silica in an epoxy rich blend will also be darn strong. It will just cost you more epoxy, but that is not likely a problem on this scale of build, and it will allow you to get by without buying more filler. In fact you don't have to add silica, as long as you can get an epoxy heavy mix with little 410 that won't slump. 410 is a silica substitute to some extent.

That is a long way around, but might help you depending on what is in your paintbox.

I pretty much never use 404, other than some hardware bonding. I would probably glue down using just silica and epoxy in a mayo consistency. 404 is a stronger filleting blend, but that does not make it a stronger adhessive. The raw glue is as strong is it comes, you just add silica to adjust the viscosity. Also 404 won't make your overall fillets stronger, since they would have to fail before you would get and tweak on the 404 fillets. Where it is stronger is if yoy try to shoot a bullet through a fillet, or rip a piece of hardware out of a fillet, it is not going to really get you are stronger structure here. But as long as you don't use too much, it can't hurt anything. In fact you could make the whole fillet out of it, then it would be uniform and load sharing. not sure what that does to your weight, but if it is still on budget, and you have the stuff, cool.

Definetly use the carbon across the joints. Or use rutan style glass roving, AS sells. What you wrap for hoop strength is probably not that critical. Fiberglass tape normally refers to:

http://www.clcboats.com/shop/product...lass-tape.html

This is the good stuff, though that price is terrible:

http://www.aircraftspruce.com/catalo...es/sstrand.php

I know I bought a smaller amount from them cheap at one point. You can pretty much get a piece of woven roving and just pull it apart.

Last edited by MassiveD; 05-18-12 at 12:13 AM.
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Old 05-23-12, 07:09 AM
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Have you considered making stronger bamboo joints before wrapping? The basic joint is just one piece butted up against another which is about the crappiest joint you can make. With a wood lathe, you could drill one piece and glue in a smaller internal lug that fits inside the joint. There are a lot of other pretty fancy bamboo joint techniques out there.

http://www.guaduabamboo.com/bamboo-pdf.html
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Old 05-25-12, 09:13 AM
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I looked at a couple of the articles on the site you linked to and it seems that most of the joining techniques that may be better would be overly complicated for me to do. Many of the joints involved drilling holes in the bamboo which I don't think would be a good idea for a bike frame.

As for using wooden plugs, let's say we're talking about the bottom bracket joint, how would I connect the wooden plug to the bb shell? Would it not be the same butted joint that is used normally with just the bamboo poles? It would be a bit more surface area for bonding, but I don't feel that it would be worth it.

Massive D: thanks for all the info. I think I'll definitely get some of the 406 colloidal silica. I had to add a decent amount of 410 before the mixture was thick enough to use. I have been in contact with a company that makes s-glass and I'm trying to get a small quantity from them but they normally sell a minimum of 300 lbs. We'll see what happens.
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Old 05-25-12, 11:07 AM
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If someone welded an inside diameter stub onto the BB That might help.

Particularly since you have carbon for the parallel strands, and some people are wrapping with hemp, I am sure you could get away with e glass. If you can't find spools of that, for like a chopper gun or something you could try just taking a little roving apart.

406 is good, but if you find colloidal silical, or cabosil, it is basically the same stuff. Be careful not to inhale. What I do is establish a mix, and put the parts in a coffee can then shake that up for later use. This reduces the volatility of the silica. Though 410 has a binding agent in it that is a great thing, but does mean you can't just shake it. This way, when you take spoonfulls out, the material isn't as volatile. Gently place on top of your epoxy and slowly stir in a counter clockwise direction (northern hemisphere) until it is folded in and no longer able to get into the air.

When working with any of this stuff, be really careful. Sanding dust is like spreading asbestos around your workshop. I work clean, with no sanding, or only sand out of doors.
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