Heat treating bamboo

Over the course of the last few months I’ve heard a lot about the process for heat treating bamboo. People put it in ovens, attack it with butane torches, smoke it over fires or dry it out in kilns.

Why do they do it? The main reason is to rid the bamboo of any water content so it doesn’t contract or expand with temperatures as easily.  A secondary reason is the mottled burnt look kinda looks cool yeah?

Which method is best? I still don’t know. But here’s a few things I’ve picked up that might help.

Exploding bamboo

If you do plan on doing any of these heat treating methods you’ll need to puncture the nodes before heating. Failure to do so will cause a heat buildup in the pocket of air trapped between the nodes and eventually a big kaboom! I used a metal kebab skewer and hit it with the hammer to poke a small hole, but you could use a long drill bit, or a piece of metal rod etc. Try to maintain the integrity of the node as much as possible thou as they add strength to the pole!

Don’t heat treat dried bamboo

If your bamboo is imported from another country (ie. China), like the piece of Tonkin pictured on the right above, it’s probably been dried already. Putting it in a 150degree oven for a few hours will do nothing but make it brittle and crack! It doesn’t necessarily crack whilst in the oven, it’s more likely after you take it out and it cools down. Two pieces I tried cracked in the middle of the night, long after they’d cooled. Woke the house up! I tried leaving it in the oven as it slowly cooled down, but that didn’t help either. More cracking.

Raising the temperature to over 200 degrees just ended up burning the bamboo I tried. I haven’t got a pic of it, but it basically turns to black charcoal that’s very weak and easy to break. Not good.

Heat treat green bamboo

It’s kind of a logical conclusion, but I’ll explain it anyway. Green bamboo (A piece of green Aurea is pictured in the middle above) has lots of water in it, thou the older the culm the less water it has. You can put it in the oven and you’ll get most of the water out. You’ll see it steam out of the ends!

Waving a butane torch over the surface is a better way to go thou, as it not only gets rid of the water, but also the waxy surface of the bamboo gets burned off too. This is important if you’re planning on making epoxy stick to bamboo!  The piece of heat treated Aurea (pictured on the left above) used to be the same colour as the middle piece. Cool stuff.

I’ll see if I can record a video of the butane torch heat treating process, as it’s kinda good fun and fascinating to see the colours change. Bamboo’s like a chameleon!

Real science

If you’re after some real hard science about heat treating bamboo, check out this PDF about the subject in the field of making bamboo fishing rods. Long read!

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Getting jiggy with it

I’ll stop the puns soon. I swear.

Built some standoffs for the headtube and rear end. Check em out:

The degree of accuracy of this jig is probably not within the tolerances of a well made bike, so I’ll have to resort to a lengthy process of alignment checking after the initial tack gluing. I’ve seen laser levels used with jigs to check alignment, so might go get one of them.

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Enter the jig…

Started work today on a flat wooden jig that will hold the frame in place while I tack the joints and generally put it together.
I found an old cupboard door, which to my luck had 33mm holes drilled in it from where the hinges were. 33mm is the size of a bottom bracket ID, making it a perfect fit for the PVC tube which I’ll extend out to hold the BB in place.
I’ve drawn out the centrelines for the frame, to within 1-2% accuracy I’d say. I wonder how accurate I need to be?

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Parts, parts, parts!

Just like the out of work actor Tobias Fünke mistakenly finding a Tractor Pull magazine (thinking it said Actor Pull) and seeing thousands of parts inside, I too became excited when the delivery man arrived today with my bike parts:

But what’s with all that steel tubing Mik? Aren’t these meant to be bamboo bikes? Fear not reader, as the tubes are only there for the bits bamboo can’t handle. The longest tube pictured will be cut down to make maybe three head tubes. The two smaller lengths are seat tube sleeves, which again will be cut down and inserted and glued into the bamboo seat tube so that a regular 27.2mm seat post can fit snugly and tight.

Also pictured are two 73mm bottom brackets, and three sets of dropouts of varying designs. Unfortunately for me they delivered two right-sided dropouts for one pair, so I’ll either have to bend some metal or order two left-sides now.

Finally are some cantilever brake braze ons, stolen from my steel mtb frame which doesn’t use them as it has disk brakes. I’m going to play around with finding out the best way to attach these suckers to the seat stays.

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Bamboo Bicycle Stand

So the end product of my test composite joint making is a bamboo bike stand! Here she is:

I’ve designed it to exactly fit a 700C road wheel, but it could easily be made with wider slots for wider wheels.

The final joint ended up being 3 layers and I experimented with three different types of epoxy.
The first layer used fibreglass resin, the second was the pinkish builders bog you can just see poking through certain areas of the joint and the last was Glass Coat, which is a 1:1 ratio epoxy that’s used for tabletops, pottery etc.

I’ve decided not to sand back this last coat, as it’s nice and glossy and I like the raw look of the sisal twine.

The stand can also be cantilevered back. This is a slightly more stable position for it, but requires that the bamboo sits against the chain stays, so it wouldn’t work for all bikes.

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Resin, sisal and bamboo test joint

While I’m still waiting on parts and hemp fibre to arrive I decided to have another go at making a composite joint with fibreglass resin, sisal twine and a 3-way bamboo mitred bamboo joint.

Here’s the workspace:

Here’s the initial binding of the sisal, with a coat of Timber Sealer applied (this stuff helps the epoxy resin stick to the wood):

Here’s the 2nd layer of sisal applied and it all drenched in epoxy:

It may not look too crash hot at the moment, but remember this is just the first layer. Once this is dry I’m going to use a more putty like builders epoxy to allow me to mould the shape of the join better.

UPDATE:

here’s the first layer sanded back, ready for the filler epoxy layer:

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