Wrapping & Heating

I’m continuing to wrap the bottom bracket joint today. I’ve now done a total of 9 layers, which I’m pretty sure will be enough for a sturdy and stiff BB.

After wrapping up each series of layers in cling wrap and PVC tape I place it inside my homemade autoclave. It gets up to 80 degrees C in there, with just a regular old hairdryer. 30-40minutes is all it needs at that temperature to cure the epoxy enough that the wrapping can be removed. It will continue to cure at room temperature over the next few days.

Wrapping the bottom bracket

The headtube’s pretty much done so I’m now starting work on laying up the bottom bracket area. It’s a much more complex shape to work with, which made the template cutting all the more fun. I’ve figured out a better way to make the templates is to wrap smaller pieces of cloth around each part, then stick them together with tape, then cut strategic slits to remove it from the frame. Then lay that all flat and trace over it to make one bigger template. I also add areas of overlap around where the slits were cut to add strength to those areas.

Here’s some pics of the finished headtube and the first layer of the bottom bracket:

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.

Carbonfiberizing

So tonight I’ve started down the road to the final wrap.  I didn’t document last night’s fibreglass layer, but it’s very similar to this process, just with white fabric instead of black. The layer of fibreglass is used between any steel parts to prevent the carbon fibre from corroding the steel (that’s a very long PDF on the topic, probably best you don’t click that link).

So, with an array of carbon fibre, some dish cloth and a penchant for late-night carbonfiberizing I got stuck into it.

First I used the dish cloth as a template around the joint I was wrapping (in this case the head tube). The idea is to use as long a piece of carbon as possible without cutting it. I managed to make something that looks like a space invader. It’s done the job splendidly thou, with very little overlap and good coverage around the awkward tight areas. The comments within each picture explain the process.

Seat post in

I’ve cut, drilled and notched the seat-post sleeve and epoxied it into the seat-tube. Hopefully the carbon fibre will arrive tomorrow and I’ll start the mega-sisal-carbon-bamboo composite process!

Tune in, drop out

I’ve completed the wrapping of the drop-outs with sisal. I’ve also added stay support bars near the bottom bracket and seat-post to give the frame more stiffness. Here’s some pics:

The clearance between the last cog and the chainstay wasn’t something I thought about that much. Luckily for me there’s about a 3mm gap to spare. This will be a single speed bike, so there will be plenty of room anyway.

The chain tension screws were also a very close fit. I actually had to cut away at the joins a bit to make them screw in properly.

That’s a wrap

Started wrapping the sisal twine around the joints tonight. Here’s the process for the seat-tube. I’ve done it in two layers. The first using lengths that run lengthwise down each pole, following the curves of each filleted corner. The second layer then holds these strands in place by wrapping around and around them. I finish it off with a few diagonal turns across the main joint and give it a haircut to neaten it up.

There’s no epoxy on any of this. I’m mainly practicing the wrapping process but am also considering making some sort of mould that I could then inject the epoxy into the sisal and joint as it is now. I’ve tried brushing and drizzling epoxy into twine like this before and then wrapping it in PVC tape to set and it sort of works, but due to the tightness of the wrap and the double layer it might need more pressure than what PVC can provide in order to make its way to the bamboo and not have any air bubbles in it.

Sanding back

Today I’m sanding back the tack epoxy to a smooth finish. I figured I probably shouldn’t have used as much epoxy for this stage as it’s not very strong, due to the micro-fibre filler that’s added to make it more paste-like. The final wraps will use undiluted epoxy along with sisal twine and should be the main force-bearing layers.

It’s messy work sanding this stuff back. You need to wear a mask as the un-cured epoxy is still toxic. Only after it’s properly cured, which takes 4-5 days is it harmless.

Tacky

So it’s taken approximately 3 months but I’ve finally tacked together my first frame!

Here’s the process from start to finish:

Surface preparation

I roughen up the bamboo by using the edge of a Dremel sanding wheel. For the steel I sandpaper first with 80 grit, then follow the same sort of deal as with the bamboo. This will provide more surface area and texture for the epoxy to key into.

I then coat the steel parts in a thin layer of pure epoxy and set them aside to get tack-dry.

Tack glue

Starting with the chainstays, I fill the rear end with epoxy and then squish onto the threaded rod I’ve attached to the drop outs. I then position the bottom bracket and stick the other ends of them to that (forgetting to sand back the epoxy on the BB… oops!?). Next comes the seat-post and the down-tube. The glue’s pliant for about 30minutes so it’s pretty easy to move stuff around if you make a mistake.

I then go to the pub for dinner and a few beers with mates while these parts take hold.

Upon returning and checking that all is well I then attach the head-tube to the down-tube, along with the top-tube to the seat-tube. The angles of the head-tube won’t be exactly 72°, but it’ll be close enough I’d say. I really need a jig that holds the head-tube in place!

Finally it’s the attaching the seat-stays to the seat-tube. Proving harder than it should be I find out that they’re not exactly even in length. I make a few adjustments and compromises on the seat-post angle and set them anyway. If it’s wonky in the morning I can always cut thru the epoxy and try again. Fingers crossed I won’t have to!

Let’s set something straight.

Sharks with frickin’ lasers. That’s what I’m talking about. Ok, well minus the sharks. It’s more like jigs with frickin’ lasers. Close enough. Check out the hi-tech laser-level action below.

The mitres are all done, so now it’s a matter of setting my somewhat cumbersome frankenstein of a jig (can someone buy me a gift voucher for Maytec please?) into place for each pole. I’ve made a series of height adjustable stand-offs to hold up each piece as well as come conical shaped rubbers from some chair legs for the head tube placement. It’s all a bit too dependent on the table being level thou, making it quite painstaking to set.

With any luck I’ll have it all set tonight. Then it’s on to roughing everything up so epoxy will stick to it. Good times.

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!

Mitre 10

Ever wonder where Mitre 10 got its name from? Neither have I. But mitres are what I’m working on this week.  I’ve printed off my mitre templates from the ever awesome BikeCad and wrapped them around the poles I intend to use for the frame. Here’s a pic:

Tomorrow I’ll use my trusty Dremel to cut, gouge, grind and sand those curves into things of beauty. Joy!

What to do with cracked bamboo?

If you’ve read my previous post you’ll see I’ve cracked a few bits of bamboo.  Not wanting to throw them away I’ve made use of them by splitting them into two and then cutting them down further to form two less-than-semi-circle pieces that can then be glued together. I got the idea from Brendyn, another Melbourne bamboo bike maker, who’s done something like this in his latest build. Sweet stuff!

Below are some pics of what could be an oval shaped chain stay or seat stay. You can see in the first picture where the PVC tape I used to wrap it all up as the epoxy dried has left small amounts of epoxy behind. This would be sanded off later.

Next time I try this I’ll spend more time making sure the two pieces have bigger flat edges facing each other. Bigger area means more epoxy bonding area!

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!