Somewhere between woodwork and framebuilding: bamboo bike #2

MAKING THE FIRST JOINTS

Bottom bracket and seat tube

The first joint in my process was the ST/SP to BB junction, a simple right-angle T. I have taken several photos which can be found in my Flickr photostream, but I'll only briefly describe the process here for clarity. Because bamboo poles are not entirely straight in all planes, you need to decide which way they will point. My ST pole was only very slightly bent, but I wanted it to bend away from the rear tyre and be straight when viewed from above - by comparing it with a straight edge and just rolling it in my hands I found and marked its forward face, and made sure to mitre it correctly.

Also be very sure to double check the BB orientation as well - most shells have a groove cut on the inside (past the threads) on the right hand (drive) side, but double check with a BB as well. I have read a build report where the guy had it all completed before he realised he'd put the BB in backwards, and had to run it as a self-undoing left hand drive.

Cut the mitre roughly with a saw first, as coarse rasps are prone to leaping off and taking bites out of the user if they don't have something to guide them. This whole process is of course much easier if you have the piece in a vice (either with rubber jaws or, better, a pipe clamp (which can be improvised from bits of wood)). Check regularly as you go - you want to end up with nice tight mitre that doesn't wobble, holds the other piece at the right angle, and doesn't have any visible spaces. It's not the end of the world if you're no good at it (I'm not, but I'm getting better...), as it will have very little effect on the joint's eventual strength, but it makes the initially glued joint stronger and easier to work with.

A pretty good mitre by comatus1649, on Flickr

I was pretty happy with that one.

Next stage is to have a dry run before gluing it. With my jig, it's easy - the short perpendicular section at the end of jig part 2 holds the BB exactly parallel to the longer aluminium section made up of part 2 and 3, joined with part 5 and some nuts - the BB is held in place with bands, as is the ST, in such a way that the joint is pulled together at precisely the right angle.

ST and BB shell in the jig by comatus1649, on Flickr

Having measured and remeasured and satisfied myself that it all worked fine, I applied a thin even layer of Gorilla Glue to the bamboo, having first misted it and the BB shell with water. Held in place, it had 80% of final strength in two hours, and was ready for the next stage of gluing.

Chainstays to BB and dropouts

For attaching the chainstays, the jig needs to be changed to configuration 2. Parts 3 and 5 (the long piece of aluminium and the short piece of rod) are removed from part 2 and replaced with part 1. Part 5 is used to hold the dropouts in place in part 1. The chainstay length is changed using the threaded rod part of part 1, screwed into part 2. The BB shell and attached ST can be rotated, and is held upright by the tension on the bands, as seen below:

Quick pre-glue layout by comatus1649, on Flickr

The BB/CS joint is very hard to get just right at this early stage because of the small mating area, the numerous angles to get right with the mitre, and the tight clearances. Bamboo's greatest weakness as a bike material is that it's quite straight and fairly uniform in cross-sectional shape, whereas for chainstays you want a bent, ovalised or crimped stay to create space for the tyre and chainring. Smaller bamboo poles have alternating flat sides which can be used to some extent. You also need to leave enough space outboard of the chainstay to build up the joint without interfering with the BB cup.

A great deal of measuring and head-scratching took place at this point, along with some test fitting of the chainring. I eventually satisfied myself that it would probably fit, though I would most likely have to file away some of the bamboo on one or both sides. I should be able to strengthen this just fine using CF, but if you're using other materials for the joints you may wish to consider using a wider BB shell to increase clearances (up to 100mm is available, but this limits BB choices and confuses chainline; I decided to make a standard 68mm shell work somehow).

The seat tube needs to be set to the right angle - this means that the rear axle needs to be elevated. I planned a BB drop of 55mm, so put DVDs and CDs underneath the right-angle section of part 1 until the centre of the axle was at 95mm from the table, compared to the BB's 40mm (in the centre, of course). I could then use a large protractor to set the angle of the ST. The inner tube bands hold the BB firmly enough when you pull the slack to the correct side - in other words, as it was leaning to the left, I needed to make the band tight on the right hand side. In jig configuration 2.5, parts 3 and 4 are used to provide a somewhat haphazard support as well, just in case:

Jig configuration 2.5 by comatus1649, on Flickr

Eagle eyes will note even more inner tube bands around either ends of the chainstays - on the BB side, I arranged them to overlap the edge of the bamboo by a fraction of a millimetre in the hope that it would stop them slipping down the BB shell. It didn't work very well, and I should have just scratched up the shell more. I didn't because it was a machined aluminium shell with a light textured finish rather than a smooth cast steel one, but I had several problems with poor adhesion. On the dropout end of the chainstays the bands were used to pull the chainstays tightly against the metal to improve the joint. Because the tubes were filled with foam the joint was much easier to get right this time around - the tabs on the ends of the dropouts are much smaller than the insides of the tubes. Some manufacturers make dropouts with plug ends rather than tabs, which I would imagine would work very well.

By the way, the angle of the dropouts was something I wasn't sure about. Eventually I went for an angle similar to horizontal dropouts which made the bottom of the aluminium inserts parallel to the floor. I think this looks fine, though you loose a little horizontal travel because it's diagonal. It does (in principle) mean that with a rear rim brake, the rim would stay aligned with the pad regardless of chain tension, which is the reason why horizontal dropouts are really a bit diagonal and track ends are actually horizontal.

Seat stays to dropouts and ST

Now for the final stage to complete the rear triangle - attaching the seat stays to the dropouts and the seat tube.

[By the way (as we shall see...), when I say 'complete', you shouldn't have any great expectations of strength for a frame only tacked together in this way with glue. It is reasonably rigid, but requires very gentle handling. The purpose of this stage is to get it all held together; epoxy filleting strengthens the joints enough for further modifications, a brief fiberglass wrap prepares the way for a durable CF wrap, and the CF wrap is what actually gives the joint all of its strength. Because the CF wraps entirely around the whole joint, what is inside it is almost of no importance.]

I wanted to do a wishbone seat stay design for this bike to make the rear triangle a little stiffer, to allow good tyre / mudguard clearance even with compact geometry, and to make it look cool! Making 6 joints all line up at once would have been a real headache, so I did it in two stages - first I connected the seat stays to the horizontal part of the wishbone, and secondly I connected the vertical part of the wishbone to the ST and to the horizontal part, and connected both dropouts. The first stage was done in situ to make sure it was all aligned correctly, with the horizontal part of the wishbone resting against the ST. After that had set, I trimmed the horizontal part to size and rounded the edges so that it would eventually have a more natural shape:

Roughed out wishbone by comatus1649, on Flickr

After that, it was trivially simple to rasp the tiny vertical part of the wishbone to shape and glue it in place. Glue can be a bit frantic to work with because it needs to be held in place just right and you're constantly fretting about it going off or dripping onto things, but by doing it in two stages this was made very straightforward. Again, inner tube bands came to the rescue holding it in place at the top, while I used hair bands for the bottom (see Flickr).

A couple of hours later I was feeling pretty happy with the whole thing, and wanted to take it out of the jig to set up the next configuration to attach the top tube. And then this happened:

Oh snap. by comatus1649, on Flickr

The right hand side had broken at the wishbone bridge and the bottom bracket - I forgot to remove the hairbands that were tensioning the joint before I took it out of the jig, and it snapped the two tiny joints just like that. With better preparation (roughing of the BB shell in particular) this might not have happened, but it was mainly a human error. However, it was very frustrating to have to awkwardly reglue two joints which had just been made so easily, and broken even more easily. I had been overkeen in trimming off the excess glue as well - this weakens the joint, but allows the next stage (filleting) to take place properly (it is stronger to have a joint with excess glue trimmed off and an epoxy fillet made around it than a fillet over the top of excess glue). Though I'd still advise trimming excess glue, leave it until you're just about to do the fillets, and be very gentle with the frame.

On the other hand, this was nowhere near as frustrating as what happened next. As you, oh patient reader, have read above, I precisely measured the ST angle with the correct rear axle elevation. However, because it didn't seem to be broken and I thought it had already set firm, I didn't remeasure it after a visiting friend knocked the damn jig off the table. Remeasuring didn't even occur to me until I had carried on, glued the wishbone seat stay and fixed the rear triangle semi-permanently in place. When I did remeasure it, I found that I had created a bike with a 69.5deg seat tube, an accidental bamboo version of Bauer's stealth bike in the 1993 Paris-Roubaix. It was such a massive error (3.5deg) that I can't believe it was anything other than it getting knocked over and me not thinking to check it.

This required a major redesign; I could rip it all to bits but I'd put in a lot of work and would have to remake some parts. I decided to work around it.

My original design had a very high BB, with a drop of 55mm (which would be high for a track bike, let alone a road bike). I decided to raise this by 10mm - this still makes it a little higher than a Surly Steamroller and will be just fine - it may even have some handling advantages but I'm not so sure about that. More importantly, it will make the ST angle about 71.5deg - still slack by modern standards, but I will be using a Brooks saddle. Because of their rails, they often need a layback saddle on modern frames, but should suit this frame fine. It seems perverse to design a frame to suit a saddle, but that's part of the fun of custom framebuilding!

I had also made a couple of other minor changes, such as making the rear triangle more compact. I couldn't get a headtube cut to a sensible length from Ceeway, so with the freedom / hassle of cutting it myself, I went for a 147.5mm HT, the strange result of aiming for 150mm and having more trouble with squaring it off than I had expected. Maintaining the same TT length for the sake of argument in BikeCAD gave me a slightly shorter reach or front centre which actually on reflection is no bad thing, so I kept it. It should tame the otherwise fairly long wheelbase a bit. There may be a touch of toe overlap but I can deal with that. Here is the revised design:

Revised geometry by comatus1649, on Flickr

That's all for now. In the next few days I will be working on the front triangle, using still more jig configurations. I'm going to order the composite bits today, so may wait to fillet the rear triangle before I start on the front, to avoid repetitions of previous 'Oh snap!' moments...

Thanks for waiting ;)

Ben