Methysticum

Aloha, everyone! This is a DIY riddle, and it'll sound ridiculous. I live in Hawai'i, and I'm building a bamboo road bike for fun; I'm no racer. The frame was prebuilt in Ghana by Booomers, and it's a bit wacky for what they consider a "road bike.". The BB shell is 68 mm, but the frame is unconventional in its various tube diameters-- including the chain stays-- and the bulkiness of its joinery, which is epoxy-soaked grass lashing. In fact, because of the bulk around the rear dropouts, I've had to adjust the limiter screw to ignore the highest gear on an 11-speed cassette entirely. It sounds klunky, but I've been on two 20-mile test rides with it, and it's quite glorious. And that's also how I stumbled embarrassingly upon the issue at hand (don't ask-- just don't ask).

Where my issue lies is with the crankset. I have two-- a 24mm and a 30mm, each a double-ring-- and BBs appropriate to the shell size. That's all good-- if this were a standard frame. The puzzle I present to you is this: The inner crank ring impacts the chain stay before the crank can properly seat in the BB (likely because the frame may actually be designed for a single). I can alleviate this with spacers, of course, but I then short the left side crank arm of its ability to adequately mount to the spindle end. That's 7.5 mm to clear the chain off the chain stay. I know! I must be mad, you're thinking; but the chain alignment is fine, but at its limits; that's how wacky this frame is-- it's like a Mobius strip or an Escher drawing of bike frames. I'll get dizzy and throw up if I think at the geometry too hard.

The 24mm crankset is a Shimano Ultegra, and the 30mm is an FSA SL-K carbon light with a BB386EVO bottom bracket. The crank arm on the former uses a combination of splines, screws and a cap for proper seating, so if the driver's side isn't seated perfectly, the left side won't mount securely whatsoever. In my case, an ideal spindle would thus need to be 7.5 mm longer. The FSA crankset uses a simple extraction bolt, so even if shorted a bit on the left side, the crank arm will still mount, but not fully, leaving just a little crank arm slippage under casual pedaling torque.

I need puzzle-solvers!. There is a solution. Both cranksets are two-piece; the spindles cannot be replaced, yet they need to be 5-7.5 mm longer for the left crank arm to set properly..

I woke up in the middle of the night with this thought in my head: the FSA spindle has a tapering, splined end (see photo). The crank arm socket does, too, but because the spindle isn't long enough exiting the BB, the tapered grooves of the socket aren't meeting the splines of the spindle until it's fully seated. What if I placed spacers inside the crank arm socket-- let's say 5 mm worth, half the depth of the socket-- and then I removed 5 mm from the splined end of the spindle? I realize this is lunacy, but technically speaking it would artificially seat the crank arm "fully." How do you think this would affect the integrity of the crank arm installation under riding stress? I imagine increased wear over time, but... what could possibly go wrong?

My other thought was-- as the frame was probably designed-- to remove the inner ring entirely, whittling 20 gears down to a measly 10. Boo.

Your not losing any money on this thing, so your radical insights would be appreciated. Mahalo!

Bill Kapaun

I'm not up on anything but sq. taper, so be warned-
"The BB shell is 68 mm"
That infers stand BSA threading?
I would have gotten a Sq. taper crank and extra long BB.
Maybe something that spec's a 113mm BB would work with a 122 or 127?

Andrew R Stewart 

Option #3 is to get a tapered square crankset and a really long BB.

Mt opinions don't really matter but... I find the suggestion that the frame was "designed" for any specific crank to be questionable. The majority of Bamboo frames I've seen and read about seem to be much more "seat of the pants" WRT designing. Second is the focus on gear count with a bamboo frame being somewhat silly. Andy

Broctoon

Have you considered trimming some material from the bamboo chain stay to provide an additional 8mm of space? If you’re worried about that tube’s integrity or water intruding into it, you could trim a little extra and then patch over the hole with some fiberglass and resin. Calfee sells bamboo bike DIY kits that use medical fiberglass tape to lash the tubes together (like the stuff they make casts out of). Something like that could help maintain the stay’s strength in spite of a slot cut in one side.

...Just an idea, if you can’t get the crank arm moved out somehow.

*Scuba 

One solution would be to get a set of Shimano Hollowtech 2 crankset that has a longer spindle. For example, their MTB doubles or a road triple. The MTB Shimano Hollowtech 2 crankset are designed to be used with a 73mm BB shell, you actually need to use spacers if your frame is 68mm.

This would require you to get a different crankset than the two that you already have.

joejack951

Put a 1-2mm spacer on the drive side of the axle and I bet you can make that 11th cog work just fine.

If I'm picturing this correctly, it's not going to do what you think it's going to do. Once you trim the spindle, you now have less protruding spindle to engage the crank arm. You won't have an issue of a loose fit any more; you'll have no fit at all.

Depending on how desperate you are to use those cranksets, you *could* find a local laser welder to cut the spindle and splice in an extension to get the 7.5mm of extra clearance. Probably cost a few hundred dollars and wouldn't be as strong as the original, but it would make for a cool story and photos.

Or, go with the longer square taper or MTB crankset suggestions. Or something fully custom?

dedhed

Those are both hollow steel spindles?
CAREFULLY cut the spindle, have someone machine a steel rod to the proper OD to fit inside the spindle, lengthen the spindle the proper amount, weld the spindle ends to the rod where needed to accomplish that, grind/file welds as needed to match spindle OD to fit through the bearings.
Know the right people and it can be done for the cost of a couple cases of beer

jccaclimber

1. Square taper with a longer spindle.
2. MTB crank....with a longer spindle.
3. Give up on the extra ring.
4. If you cut and weld that spindle at a traditional weld shop I'm betting it isn't going to be straight anymore. Might be close enough you can't tell and not matter given everything else that seems to be going on here, but FYI. If you already know the guy who can do this it's a favor. If you don't, be prepared for it to cost more than options 1-3 above, assuming they want to do it in the first place.
5. I might clearance and build back up my own frame (and may have done so in the past on a recumbent project), but my imaginary lawyer prohibits me from advising you to do so.

Mad Honk 

You want me to think outside the bambo parameters?? Well here is my take on this:
Since you likely don't have an overly stocked Co-Op, I would spend a bit of time researching square taper bottom brackets from the vintage time frame. And then opt for a triple spindle the correct size for your needs.
Second you need to contact a few co-ops here stateside to get the right bottom bracket spindle size and cup set. And if needed ask about a crank that matches the bottom bracket.
Three or four places come to mind quickly, but I would contact the St. Louis co-op who does have an internet presence. And the Old Spokes Home that has a pretty large supply of vintage parts. Be sure you know which spindles you want by spindle identification numbers before you contact them. They will likely be able to help you, IF you have the information when you contact them. Converting the bottom bracket and crank arms should get you where you want to be. And from those two co-ops it shouldn't be that great a cost. Far less than trying to buy new stuff in your location. Just my thoughts. Smiles, MH

icemilkcoffee 

6. Put spacers to space the crankset to the right, and then have a bike shop face the left side of the bb shell to shave off the same amount.
In effect you are moving the whole crankset to the right.

3alarmer 

.
...you could set the whole thing on fire. Problem solved. The only guy I actually know who built a bamboo bike was Allen , but I think he's banned now. Not sure if he left any contact information. I agree with the previous sentiments expressed that worrying about chainline on a bicycle constructed of bamboo and lashed with resin soaked grass is kind of silly. But I also think that an 11 speed rear cog setup on such a bicycle misses both it's original purpose and the limitations of the frame material. You can get a perfectly adequate gear range with fewer cogs in back, and cadence for racing would only apply if you have a local bamboo frame criterium society...which is unlikely.

Me, I would just go with the idea the thing was probably designed with three piece cranks in mind, find one used somewhere out in the garage or at the co-op, and install a spindle long enough to clear the stays. But what do I know ?

Edit: you do know that they sell a slightly longer spindle for two piece cranks to use with Italian standard BB's, right ? Not sure it will have the reach you need, though.

seibaatgung

How do you get banned on a bike forum? This isn't an abortion good/bad messageboard.

Methysticum

Ha! It's a baby only a mother could love.

3alarmer 

...I could explain it to you, but then you would get banned.

dedhed

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seibaatgung

I don't see him. Probably switched to riding tricycles.