Bicycle Mechanics - Requirements for 1 pc to 3 pc cranks conversion
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06-12-06, 12:04 PM
I am attempting to drag a 1980 Schwinn Collegiate 3 into the next millenium. I've laced some aluminum rims to the old hubs, slapped on a Shimano sidepull brake and now I'm interested in going 3 piece. I've been inside the BB to repack the bearings, so I've got a basic understanding of what's in there. What would you all recommend for converting to 3 piece and what cranks/BB/chainring would you use?  I did a search of the forums and will use the truvativ American-to-English adapter. I'd like to use a Shimano BB, but my skills are exceeded with the task of choosing lengths and cranks to recreated the chainline. 
Thanks for your responses. It's been a great project so far. It's lighter with every change of equipment, and Holy Cow! it actually stops now.
06-13-06, 11:26 AM
Sheldon Brown's site got me most of the way, but where to go next?
06-13-06, 12:42 PM
I understand that at least one part of your objective is to decrease weight, but is there anything else?
Do you plan to stay with a single chainring? If so, I would guess that the shortest spindle length available for the BB width and the crank you want to use would be what you want. The choice of BB type (square taper, octalink, ...) would be one issue.
My guess is that if you plan to order from Harris that they would be more than happy to help you know which BB and/or crank you should buy... or possibly tell you to save your money because you won't really gain enough to make it worth the cost and effort. :D
06-13-06, 01:00 PM
Since you already have the adapter taken care of, all you need is the BB and cranks. Most cranks will tell you what chainline they will give you for a given BB length. If you need to change it, mess with the BB size by twice as much as the move you want to make, and make sure you are getting a symmetrical BB. Since most bottom brackets are sized in 3mm intervals, you may have to use 1mm or 2mm bottom bracket spacers to get things right on. However, remember that a spacer moves the chainline by its entire width, since it moved the whole BB over.
Say you decide to use bulletproof cranks, which are a good, cheap option. As per Sheldon's measurements, the "inner" position would be 39.7mm with a 107mm BB, and the outer would be at 47.1. You're probably going to need to use the inner position for clearance reasons. So ideally you need to move the chainline 2.3 mm out . You could get pretty close by going up to a 109, which moves it out 1mm, and then putting on a 1mm spacer. .3mm ain't nothin' to worry about.
Disclaimer: old 1 piece crank bb shells are the standard 68mm wide, and I'm just assuming here that the converter rig maintains the shell width. It looks like it does, but I've never actually seen one in the wild.
06-13-06, 02:17 PM
Both replies appreciated. As to the reason: who can answer a question such as that? Why is the sky blue? Really I'm just cutting my teeth with some new projects I want to learn how to do. It's a bike I got for free, I like the aestetic, it is a great commuter/cargo bike (folding Wald baskets out back, Wald handlebar basket), the three speed SA AW hub fascinates me, etc. It has been the launch point for all sorts of new skills like wheel lacing, repacking of hubs and bb, new brake installation, all sorts of stuff. Sure it's heavy but every new part makes it lighter, cooler and more Luke-ified. Not that y'all needed the info, but you know how the bike awakens the soul.
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