Somewhere between woodwork and framebuilding: bamboo bike #2

***BAMBOO SELECTION AND PREPARATION

***
Poles by comatus1649, on Flickr

Preliminary drying
The bamboo arrives in a mostly-dried state, which is a good start, but needs some stuff doing to it before it's useable. It's important that you let it stabilise to the humidity of wherever you're working, or else you risk it changing size or cracking later on. I recommend buying the bamboo a reasonable time in advance so you can leave it to dry - you don't want to put it on radiators, in airing cupboards, or anywhere that's really warm, just leave it inside somewhere with an even, warmish temperature. You should also break as many of the internal nodes as you can to encourage even drying, and to help with later steps ...

From what I've read, I don't think you should heat treat it as many people recommend. Everyone talks about caramelising the sugars in the bamboo to make it stronger, but all the evidence I've found suggests that it reduces the strength by about 20% and makes it more brittle. I found a very useful paper called 'Bamboo in the Laboratory', which was the only properly scientific test of heat-treating bamboo I've seen. It needs to be taken with a pinch of salt because the needs of bamboo rodmakers (who split culms into smaller bits before reforming them into rods) are different to those of framebuilders, but the evidence is useful - heat-treating bamboo up to about 150 degrees C (I estimate - I don't have it to hand) permanently reduces cellular water levels in the bamboo, making it lighter and stronger. Beyond that point, after which the coloration changes, the bamboo is internally scorched and made weaker. (This is why I wouldn't recommend using 'tiger' bamboo unless you can check that it's the species with the natural markings, as some is just burnt moso or similar.)

I don't have a big enough oven to heat my bamboo properly, and all you can do with a propane torch is burn it. So I decided that just letting it dry thoroughly was the best option. Over time the bamboo turns from green to a nice tan colour - if you buy it from the internet, it will already be well into the tan stage, and will need little further treatment. Lots of hobbyist builders have used propane torches and had no long-term problems, but why reduce the strength of your key material?

***Checking the bamboo
***It's quite likely that your bamboo will have some problems. The initial checks are to find and discard poles that are too curved, cracked, or insect-nibbled. Curved poles can be useful if they're curved in one plane (chainstays in particular are better if they're curved) - if they're curved in two directions, like a very stretched out spiral, they'll be very hard to use. Curved culms often grow due to a restricted or light-starved position and may be weaker another reason to beware of extremely-bent poles (and those that are much greener on one side than the other).

Look out very carefully for cracks, both at the ends and on the nodes. Cracks are bamboo cancer. They will only get worse and spread. If, by the way, you find a crack in a pole when you've already built your bike, don't totally despair. Drill out the crack at either end to prevent it spreading, and wrap the pole tightly with CF tow. That should stop it spreading much further. Likewise, if you find any cracks on any of your poles (even ones you don't think you'll use), cut after the crack to keep the rest useable.

Also look for woodwork, rot and other grass maladies. There's often surface discoloration that's not a problem, but sometimes there are bits around the nodes on smaller poles that concern me slightly. Avoid using them if you can.

Marking and cutting by comatus1649, on Flickr

Marking, choosing and cutting
As I go I mark up all the problems with a soft pencil or Sharpie. This way, they're easier to spot when I'm trying to choose poles for particular parts of the bike. In the picture above, I've marked up the bit where the harvester has torn off the leaves, leaving open grain. This isn't a problem for me, but it increases sanding. If I was going for a natural finish on the bamboo, I might chuck this bit.

Choosing the right poles for the different parts of the bike is the real art of this stage. Looking at bike FEA models, the majority of the stress is around the bottom bracket, especially on the downtube. This joint wants to have the sturdiest, densest parts of your tubes.

Take into account your seatpost solution as well, by the way - on my first bike I used a normal seatpost, so I needed a seatpost that had an internal diameter of about 28mm (bamboo in that size has a wall thickness of maybe 4mm or so). It also had to be bolt straight, and I didn't want too many nodes or I'd just have to internally rasp them away (faff!). Some people cut a slot in the seattube for a seatpost clamp on a normal bike - I think this is a terrible idea. I used a double clamp made by eXotic which clamps around the uncut tube, and also onto a seatpost - I shimmed it down from 27.2 to whatever size I had that fitted into the bamboo tube.

However, this time I'm using 35mm tubes that have an internal diameter of about 22mm, so a normal seatpost is out. I intend to extend the seattube almost as far as the saddle, and bond in a 22mm BMX seatpost for the last little bit, with a plain-top saddle clamp. Unless I find an affordable integrated seatpost topper before then ...

As for the chainstays and seatstays, the clearance in this region is pretty tight, but you don't want to use very thin tubes. It's a hard balance to get right, but a very important one. You'll find that smaller tubes have flat sides, which end up being rather like the crimps on steel chainstays.

Once I think that I know which tubes I'll use, I then cut the pieces roughly to length, making sure that I have plenty of leeway. A pull-saw is the ideal tool for this job but anything will do really. The skin may splinter slightly when you cut it because of the longitudinal fibres (the reason it makes such great frames), but resist the urge to peel off the splinters - often if you go overboard they run the whole length of the node and look rubbish. You could wrap the pole in tape before you cut to prevent this but it's no big deal really.

In the next installment, I will cover my top-secret bamboo frame innovation, and my first great error of judgment ...

Dennis looks on approvingly by comatus1649, on Flickr