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Bottom Bracket Selection
#1
This fixie I've been working on has got me stuck. I'm designing custom crankarms and am having difficulty figuring out what bottom bracket I will need for the chainline. I know the chainline is 45mm and the shell is 68mm. I've read over Sheldon Brown's articles on the topic but it was too confusing for me. Is cartridge style the way to go since that's what's mostly sold today?
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#2
You're designing them, as in... you'll also be machining them? Very cool!

Axle width is determined by the crankarm and its chinring-to-frame clearance so, you may be getting ahead of yourself if I read that correctly.

Rob
Wheelies don't pop themselves. (from a QBP fortune cookie)
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#3
(06-24-2012, 11:41 AM)RobAR Wrote:  You're designing them, as in... you'll also be machining them? Very cool!

Axle width is determined by the crankarm and its chinring-to-frame clearance so, you may be getting ahead of yourself if I read that correctly.

Rob

Yes I'll be machining them CNC. I got some help figuring it out which was simpler than expected. Now it's getting the design down.
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#4
(06-25-2012, 05:17 PM)capner2112 Wrote:  Yes I'll be machining them CNC. I got some help figuring it out which was simpler than expected. Now it's getting the design down.

Why not design them with a removable or extra thick center section, you could then CNC different offsets to bolt into that section or simply re-machine after you discover the correction required. So say you started with a shimano 68mm UN-55 113mm (or whatever) BB, fit it, install you cranks, measure & then re-machine. What's the most you'll be off, 10mm? For that matter, if you needed a different BB, you're out another $20, big deal. You can sell what you don't use.

Of course life could get interesting if your frame BB threads are bass-ackwards, then you've got to get into the high dollar stuff.
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#5
(06-30-2012, 04:05 PM)1FJEF Wrote:  
(06-25-2012, 05:17 PM)capner2112 Wrote:  Yes I'll be machining them CNC. I got some help figuring it out which was simpler than expected. Now it's getting the design down.

Why not design them with a removable or extra thick center section, you could then CNC different offsets to bolt into that section or simply re-machine after you discover the correction required. So say you started with a shimano 68mm UN-55 113mm (or whatever) BB, fit it, install you cranks, measure & then re-machine. What's the most you'll be off, 10mm? For that matter, if you needed a different BB, you're out another $20, big deal. You can sell what you don't use.

Of course life could get interesting if your frame BB threads are bass-ackwards, then you've got to get into the high dollar stuff.

Thanks for the design tips. A guy I work has made cranks and chainrings before, which I'm doing both, so he'll have some design ideas.

The design I'm developing connects the chainring at the crankarm. Like in BMX, I may use spacers to adjust the chainline. My coworker has previously used a straight square versus a taper, which is easier to machine. I'll update once in a while.
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#6
You are, of course, gonna provide us with progress pics. Ain't ya?
Wheelies don't pop themselves. (from a QBP fortune cookie)
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#7
(07-07-2012, 08:26 PM)RobAR Wrote:  You are, of course, gonna provide us with progress pics. Ain't ya?

For sure.
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