Somewhere between woodwork and framebuilding: bamboo bike #2

Next update. This is brief because I can't be bothered to type it all out again. Look at my Flickr for more notes and process photographs.

MORE CF

Finishing the tape layup
Last on my hit list was the BB. I chose an aluminium one because it's much lighter than the steel equivalent I used before; however it's best to isolate carbon fibre from aluminium due to the potential for galvanic corrosion. So I skinned it with a thin layer of fibreglass first, then built it up with the carbon fibre as before. I took my lead from the wrapping guide already mentioned but had to change it quite a bit because my 25mm tape is much wider than tow. After the peel ply (hard to wrap tight around this complex shape) came off, this is what I had:

After curing by comatus1649, on Flickr

Looks pretty neat and strong. But if you look at my other pictures on Flickr you'll see that there are voids in the layup on top of the chainstays. These could have been avoided by building the fillets up more, or possibly by thinking about my wraps a bit more carefully. I'll have to fill these later, but I don't think they'll affect the strength too much.

Cosmetic skinning
So the tape laid up already is structural, and the bike is finished in that respect. However, I wanted to put a layer of carbon fibre cloth over the top to really finish it off - UD tape just looks black and uninteresting, and you don't have the nice weave texture people associate with CF. I also chose a carbon/aramid blend which will add a degree of crash protection.

This is a totally different process to the wet layup already shown. The skinning technique is generally used to cover car parts or things like that, and it's much much trickier on complex parts like the joints on a bike. It is also a tiny subset of the tiny community of bamboo bike builders who use carbon cloth, so this is rather undocumented territory. I was helped enormously by this guide, but I have a few tips of my own. First, use a thicker epoxy like the one CarbonMods / Easy Composites sell in their kits. West Systems is a bit thin really. And really do let the epoxy go off until it doesn't come off on your finger when you poke it - only then will it properly stick to the fabric. Otherwise it will drive you mad by coming off, plus the epoxy may soak through the fabric and spoil the look of the weave. You may need to use tape to hold it down while it cures, especially on the smaller tubes (you could alternatively use a very lightweight cloth (90gsm is available), but it's very very expensive).

Working with woven CF cloth is not very easy. It frays easily, is expensive, and the weave distorts if you look at it funny. Easy Composites will sell you 'ProFinish' cloth for £5 more per metre; it's just been sprayed on the back side with spray mount. I used the cheapest stuff I could find on eBay and it clogged endlessly. Post 3M stuff might be better. You want to slowly build up a thin layer on the back. If you're buying from Easy Composites you can cheaply buy a sample of their ProFinish cloth to see how much you need to apply. This makes the fabric a little stiffer but much easier to handle, cut and look after - no longer does it instantly fray wildly.

You need black epoxy for this so that bamboo can't show through the weave of the cloth. I used graphite powder because it's cheaper than pigment and has other uses. You want a smooth, very dark black mixture - mix the graphite into the epoxy before adding the hardener, mix, then transfer to another pot to avoid lumps of unhardened epoxy or graphite. Paint it on everywhere you want the fabric to stick:

Black base coat on dropouts by comatus1649, on Flickr

When it is tacky, you can then apply the cloth which you have already cut to shape, and hold it in place while it cures. At first I had it overlapping, and I then cut back the excess. For larger parts or those where you need a very neat symmetrical seam, I pinched it like this:

New improved skinning technique by comatus1649, on Flickr

When it is almost cured I trimmed back the flap and pressed what little excess there was left into the tacky epoxy. This gives you joins as neat as you are ever going to get. They will never ever be as good as the seamless appearance on moulded carbon frames, but they will look alright once the clear coats of epoxy have gone on.

Another note on complex shapes - I tried cutting the fabric into a shape that would let me do the joints with one bit of cloth, but it was a disaster. The best way, as it seems to me, is to do it in parts - first do the bits that could be considered not the end of a piece of bamboo, then when they're done, neatly do the bits that are the ends of the lugs. That sounds confusing, but the pictures may help. This is the second (and final) lot of black epoxy being put on the HT/DT joint:

Second application of cloth to the HT joint by comatus1649, on Flickr

That's all for now. At time of writing I'm just letting the epoxy cure for the last few bits of cloth to go on around the BB. I've fucked up a couple of bits but it's surprising how well the clear coats of epoxy cover it up (or that's the hope anyway). I will update with that whenever I manage to do it!

Thanks for reading, and for being patient!

Ben