Category: bamboo

High gloss

Two layers of top coat later and the frame’s all finished!  It’s not the smoothest of finishes. There’s a few carbon fibre/cat hairs stuck in there and imperfections of my sanding back are clearly visible. Still, it’s just a prototype and doesn’t look too bad from afar.

The final processes haven’t been without drama though. During the drying process a few hairline cracks appeared on various parts of the frame. To mitigate further crackage I’ve wrapped the affected areas in carbon fibre tow (which are the black bands you can see on the down tube and chainstays).  The cracks are caused by the expansion of the bamboo at high temperature during drying (ie. around 80 degrees). I don’t expect it to crack any further, unless we get a severe heatwave…

It’s now off to the shop to get parts fitted. Hooray!

Finishing touches

Today I’ve put the first of the final touches to the frame – a carved bamboo head-tube Cognitive logo. It’s a bit simplified, but that’s ok as it will probably change down the track anyway.

The only things left to do now are two more layers of carbon on the seat tube and then to unwrap all the tape, sand it back properly, and add some final clear coat as a protective varnish layer.

Carbon fibre wet layup – stage 2

Unwrapping the first layer this morning and it’s all gone well. The cling wrap’s left lots of wrinkles in the resin, which isn’t ideal, but for these base layers it’s ok as it’ll all get sanded back. Hopefully by the time I’m on my final layer’s I’ll be shrink-wrapping with better precision.

I’m now onto the 2nd and 3rd layers. Each layer I’m making from a slightly smaller template so the edges of the carbon fibre will gradually taper off rather than being a sharp drop off to where the bamboo is.

Freshly unwrapped
Freshly unwrappd
You can see the wrinkles left by the cling wrap
I finished off the rear end with some tow wrapped around each stay
Sanded back
The templates get gradually smaller to taper the edges

Mitre’s well show you

Here’s the results of today’s mitreing. Pretty happy with the results so far. Just have the top tube to go and then into the more finicky work of the rear triangle. The main thing I learned today is that getting both sides of the mitres straight and level (ie. so the BB sits at exactly perpendicular) is much more important than getting the curves right. Sure getting the curves right is a good thing too, but it’s ok if there’s small gaps as it will allow the epoxy to fill into the inside of the tubing and strengthen the join.

Choosing the bamboo for your frame

This might seem obvious, but choosing the exact bits of bamboo you’ll use for your bike frame is one of the most important things to do in the frame building process. You need to know a lot of stuff before you can choose the right bits thou! Read on for a few pointers I’ve picked up along my bamboo selection journey.

So you’ve got your geometry right and you know what lengths of each bit of bamboo you need. So off to the shop/woods right? Wrong! You’ll need to know what diameters suit which bits of the frame and then work within a set of tolerances that you know won’t cause problems (or be willing to fix those problems in creative ways!).

The rear chain-stays are by far the hardest bits to work with, followed closely by the seat-stays, then seat-post. The top and down tubes are fairly open to how much trust you have in the strength of bamboo. You could go for skinny poles if you want a retro steel looking frame, or fat chunky one that might weigh a bit more, but are probably stronger, like on an aluminium or carbon frame.

The main problem areas (and questions to find answers to) are:

  1. Tyre clearance – what’s the widest tyre you want to put on your frame?
  2. Chainring clearance – how many chainrings? How many teeth?
  3. Crank-arm clearance – what’s the Q-Factor (width) of your cranks?
  4. Seat-post – what diameter and length will you need? Are you going to use a metal sleeve inserted into the bamboo for the seat-post to go in?
  5. Disc rotor clearance (if you’re using them) – how big are the rotors?

Only once you’ve got all those questions solved are you then ready to go in search of bamboo with the right diameter!

A rough diameter guide

The figures below are a rough guide to what you might need for a road or cyclocross frame. The figures in brackets are what I’ve used on my first frame.

Top tube: 26-36mm (35mm)

Down tube: 35-45mm (41-43mm tapered)

Seat tube: 40mm (the inner diameter needs to be more than the metal sleeve, which if you’re using a 27.2mm seat-post is around 30mm)

Seat stays: 20-25mm* (22mm)

Chain stays: 20-25mm* (25mm)

*Tyre and chainring clearances end up being very precise things, so try to find bits that will exactly match your spec.

What to look for in choosing your bamboo.

So you know what diameters you need. What else is important in your bamboo selection process?

Nodes

I still can’t find out decisively if nodes are strong or weak points in bamboo. Books tell me one thing, the internet tells me others. It’s confusing. If anybody has any definitive information on node strengths & weakness please let me know. What I think is right is that they add strength from crushing forces, but cause weakness from bending forces.

Wall thickness

Bamboo varies wildly in wall thickness from species to species, and due to its tapering nature will often be thick at one end and thin the other. Use the thick ends for areas of your frame you think will be under lots of force I guess. Where are those areas? All over the place!

Straightness

They don’t have to be dead straight, but my feelings tell me that any bends and kinks in bamboo will only make it weaker and more likely to fail when put into the triangle formations of a frame. A triangle with a bent side can easily be crushed!

Roundness

Bamboo is often not round! Lots of the pieces I’ve dealt with are quite oval shaped. This can be a good thing. Oval shapes provide more strength in certain directions. Use them on chain-stays and down tubes to your advantage.

Defects

Wood borers seem to love bamboo and often you’ll find pieces with trails left by these little critters. Most of the time they just eat the surface “skin” of the bamboo and don’t do much structural damage, but this surface is the strongest part or the culm, so if they’ve eaten away large chunks of it, or ring-barked it, be warned!

Cracks, scratches and splinters are also things to look out for. Remember this thing is going to be on a bike for a long time. Find the best bits you can!

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!

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.

A visit to Bamboo Australia

I’ve flown up to Queensland this week to see Bamboo Australia and their amazing supply of bamboo.

Arrived there today and spent a good 3 hours breaking the ears off the lovely staff and asking possibly too many questions.  Here’s my good buddy Nat at the entrance to the farm:

Here’s the cutting shed, where I gathered various sizes of Tonkin and Aurea species bamboo poles and had them cut to 70cm long each (mainly so they’ll fit in my bag, but also because the bottom tube is never really any longer than this).

I came away with about 20kg of poles, both green and dried, as well as 5 pairs of bamboo socks and the bible of bamboo: Bamboo, The Gift of the Gods

Here’s hoping it’ll make it back to Melbourne without splitting from the various temperatures and humidities of flight.

Selecting the culms

I’ve selected the 7 culms (fancy word for a bamboo pole) for the first frame. Here’s a pic of them laid out into the respective parts of the frame:

And here’s a view of the thickness of the downtube:

I also ventured to my local hardware store and picked up some epoxy resin, glass coat,  borax, beeswax and some other glues and finishes to test the curing and joint making processes with:

I haven’t managed to find Boric Acid anywhere yet thou. I suspect I’ll be able order it online somewhere, or it might be available at a chemist.