Somewhere between woodwork and framebuilding: bamboo bike #2

**PLANNING

**I always end up spending way more time planning and researching my projects than actually making them. There's so much great information out there on the internet which has been utterly vital. All of it is easily found so I won't bother citing too many inspirations apart from where they are a bit more esoteric.

Bamboo choice
The first decision I had to make was the variety and size of bamboo to use. My previous bike, like almost all bamboo bikes you'll see on the internet, used quite oversized tubes for maximum strength and stiffness. Because it's a natural, highly variable material, bamboo is usually sold in width bands, and the 40-45mm band is generally used for the main triangle. However, my first bike was a bit over 2kg, and I fancied being lighter this time. Also, I am certain that the vast majority of a bamboo frame's stiffness comes from well laid-up joints made with a suitable material - 45mm tubes seem massive overkill. My last frame was bendier than I liked, but that was because my joints were shit (see below), not because there was any real flex in the tubes.

The next size down is 30-35mm, which is much closer to the tube size of steel frames. They are still extremely stiff and strong. I'm going to push the boundaries of hobbyist framebuilding a bit this time, and use the thinner tubes - if it comes off, I should get a frame that's significantly lighter than my previous effort. If it's too noodly, I'll rework it slightly to add a second top tube a la big Rivendell frames, but I'm hoping it won't be needed. For the chainstays and seatstays, I used 20-25mm poles. If I could handpick the poles, a smaller diameter might be possible, but the smaller you get the more weaknesses there tend to be, so I prefer to overbuild the rear triangle as I don't need cassette clearance.

Bamboo variety is something that's discussed a lot by framebuilders. The general consensus is that the strongest species are iron bamboo, tonkin, henon and guadua. However, in the good old UK it's very difficult to get any of these in the right sizes. Moso and black bamboo are widely available - moso is your classic green/yellow bamboo, while black is a very similar species with darker surface coloration. There's also tiger bamboo, but it's hard to tell whether it's *real *tigerbamboo, which is the same species as black, or something like moso that has been selectively scorches. It's also a lot more expensive. I'm sure that moso and black are easily strong enough, so I'll be using moso as I'd just sand off the black layer anyway.

I bought my bamboo from UK Bamboo Supplies - unfortunately they've just introduced a new shop system with a larger minimum order (10 poles of each size) - if you call them up they might be able to do smaller packs (it used to be 5). Their shipping is expensive and slow, but there's not much choice in the UK. The Bamboo Bicycle Club also sells bamboo kits as well as running courses. They are much more expensive, but they do hand-pick good bits and treat them well. I'm cheap so went for the UK Bamboo option, but you run the risk of getting wonky, cracked or funny-shaped bits so you need to buy with that in mind. Both times I've bought one 5-pack of thick poles, 1m long, and one pack of thin 1m poles, and that's worked out. But you may need to be crafty with the bits you choose.

***Lug choice
***Because I was being even stingier last time, I didn't use carbon fibre for the lugs. Instead I used sisal twine. In hindsight this was a poor idea, because while it's stronger than the hemp tow most non-CF builders use, the tight weave of the twine makes it hard to wet out. You also need to really, really overbuild the joints, which end up being very big. This time, I'm going to use 12k carbon fibre tow from East Coast, which is about 12p/m. I'll need several hundred metres, I think, but I need to research that part more.

If you read up on bamboo framebuilding, sooner or later you'll come across Craig Calfee. As well as his better-known carbon frames, he also makes some great bamboo frames, and his sideline Bamboosero sets up framebuilders in Ghana. Calfee's of the opinion that CF shouldn't be used for lugs because the coefficient of expansion of the two materials are too great and will cause the joints to fail over time. I can see the science behind this, but I have never seen a CF joint on a bamboo frame fail. I have, however, known several cases of hemp / other natural fibre joints failing. I'm sure that Calfee's hemp-lugged frames are as strong as CF and longer-lasting, but it's very easy to make a shitty natural-fibre joint - they're harder to wet out, prone to internal voids, and difficult to get the fibres running in the right directions. It's not just a matter of building up material around the joints. CF is much better suited to epoxy wetting-out, and has a greater margin of error in terms of actual strength. I don't think the thermal expansion issue is a problem considering the temperature changes the bike will experience. Watch this space!

For the glue I'll be using West Systems 105 epoxy. It's strong, odourless, not too unhealthy to have about, and easy to work with. I'll also be using bamboo sawdust to built up joint fillets before wrapping with CF.

***Metal bits
***Unless you're brave or very inventive, you'll need a few metal bits - dropouts, BB shell, and headtube. I'm getting these from Ceeway. Last time I used normal horizontal dropouts and found that they were fiddly to build up strongly, and I made the chainstays slightly too short and had to deflate the tyre to get it in. I'm not a track end fan, so will be going with the Ceeway sliding dropouts. This will allow me to adjust the effective chainstay length, easily run mudguards, remove the wheel and replace it without changing the chain tension, and change the inserts if I ever want to run derailer gears or a Rohloff hub. They're a bit heavier though, and a bit dear.

***Geometry
***Before I could do anything, I needed to decide on my geometry. Last time I think I just copied the Steamroller frame design; this time, I've done it from the ground up. Sheldon Brown links to numerous websites which have sensible modern bike sizing and geometry advice, which I used to get an idea of what I wanted. I then used BikeCAD to draw up my design:

Bike geometry is boring as fuck if you don't care about such things, but basically it was built to have short chainstays (for fat tyres and full 'guards, anyway - I wanted to stiffen up the rear - a wishbone seat stay will be used for the same reason), stack and reach which suit my size, and trail of about 55mm, maybe a bit less. This is a relatively low amount of trail which will give fast handling. For that you sacrifice high speed stability but I'm not going to be doing many 60mph alpine descents on a fixed gear...