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Questions about Bottom Bracket spindle length, clearance, and chainline. (vlemmy.net)

submitted 1 year ago by [email protected] to c/bikewrench

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Building a parts bin bike, on an old Gary Fisher Advance frame. The only thing I need to buy is a bottom bracket. The current square taper 73/113 is toast. I checked the 94 Fisher catalog and that was what it originally came with stock along with a triple crank set, the largest ring being 42t. The only crank sets I have lying around have biopace rings both have 48t on the largest ring. Out of curiosity I installed the drive side on the 113 mm spindle, and the largest 48t ring of the triple just hits the chain stay twice each rotation because of the ovalized ring. Where it's not hitting there still wouldn't be enough clearance for a 7/8sp chain. I think if I went up to a 118mm it could work. Do i just measure the current chain line and add 5mm? How much offset until it badly affects shifting? I've also heard it can mess up the taper on the crank arms. Not overly concerned about the components being a parts bin bike. Just concerned about clearance and good shifting. Should I just the go 113mm spindle and then buy new smaller round chain rings? Thanks

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[–] j4k3 2 points 1 year ago

It shouldn't make much of a difference going from 113 to 118 so long as you have room inside your H/L FD stops. You need to have the cable pull to move the bracketing the extra further distance out. It is very likely that you have this flexibility. The only time I have personally had issues with FD and crank spacing is on aero road with odd internal routing paired with a high leverage ratio FD.

So long as you can place more starting tension on the FD and it an pull the distance you should be fine. I mean, I don't recall anyone making FD's for specific BB width setups. I think the one FD should always work with all of the common widths.

[–] tacocat 2 points 1 year ago

Current chainline +2.5mm: (118-113)/2 (assuming both spindles are symmetrical)

Based on experience above 5mm chainline difference is when shifting suffers

Length doesn't damage tapers. Mismatched taper types will, all of this sounds like JIS tapers. ISO tapers not common so just check the specs on the new spindle, shimano uses JIS so folks trying to be compatible will follow suit.

Smaller round chain rings, pretty sure the oval 48T will work. I would measure the front/back chainlines and make a judgement call.

[–] [email protected] 1 points 1 year ago

Finally got around to assembling everything yesterday. 118mm did the trick, Thanks again!