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  1. #1
    mtbr member
    Reputation: litespeedchick's Avatar
    Join Date
    Jan 2004

    Need remedial drivetrain info: bb, cranks, rings, 8 spd etc.

    OK, let's work backwards:

    Shop says I need a new bb. I haven't been noticing any problems lately except chain suck, but whatever. i bought the pricey ti Race Face bb back in 2001 or so at the same time I bought the RF pretty powder coated cranks. So I guess it's time. I can't remember replacing it in between, but maybe I did. anyway...

    The shops says there aren't many square taper bb's left in the world. Question #1: recommend a light weight sq taper 68x107.

    So that gets me thinking...i wouldn't mind having a spiffy new set of light weight carbon cranks. Would that allow me a better selection of bb's? But... Question #2: can I get new cranks without getting new rings? And if so, does it matter that my current chainrings are 5 bolt, not four (like the sweet Race Face next SL 650 gm crankset/ bb) Honestly, I've always liked to keep Shimano rings.

    Which leads me to...I'm still 8 speed. Question #3: how does that fact affect this whole situation? If I changed cranks/bb/ AND rings....I would still have to change both derailuers, crankset, ....and shifters? I suppose that's why I've never switched away from 8 speed...'cause everything doesn't wear out at the same time.

    Sorry this question is so disjointed...I appreciate any light you can shed.

    BTW, this is a 1996 Litespeed Obed frame that gets ridden in Pisgah, it weighs a little over 22 pounds (real weight w/ pedals and knobbies, etc.) and I don't want her to pork up at all. I don't mind spending a few $$$.

  2. #2
    Fat-tired Roadie
    Reputation: AndrwSwitch's Avatar
    Join Date
    Nov 2007
    The simplest choice would be to replace the bottom bracket only. If you don't trust the shop's opinion, test it for yourself. There should be no play if you try to push or pull the crank arm toward or away from the frame. Try it pointing straight up or down and then turn it ninety degrees and try it again. If there is, and it's not between the crank arm and the bottom bracket (as in, it moves the arm on the other side too,) your bottom bracket is shot. Also try turning the crank arm with your fingers near where it bolts on and the chain off. If you feel some crunchiness, your bottom bracket is done. (I'm assuming it's a non-rebuildable type.)

    You need another 68x107 bottom bracket. Throwing the term into Google Shopping, there are a lot of Shimano UN54 BBs available in that size. That's a perfectly good bottom bracket. There's a UN73 kicking around online in that size; that's a little fancier. If you really want to throw money at it, Phil Wood also does square taper.

    Spiffy new carbon cranks won't get you a better selection of bottom brackets. The new external ones aren't standardized the way square taper and ISIS were. However, there are really nice Shimano-compatible bottom brackets available for all the cranks that fit that standard, which is all Shimano, some FSA, and some others.

    Getting new cranks to fit a five-bolt ring would be a little bit difficult. It's not common lately. However, there are still some companies doing it. You'd be looking at a boutique part from a smaller brand. I'm sure someone will correct me if I'm wrong - I don't think anyone's presently doing an external bottom bracket crankset that accepts a MTB five-bolt spider.

    You should be able to swap out your entire crankset without having to do more than some minor adjustments at the front derailleur. So you shouldn't need to replace the entire drivetrain. It's not a bad idea to do the cassette and chain at the same time as all the rings, though, since the rings are really the most expensive part to replace and that'll get you a little more wear life.

    It sounds like you're not unhappy with your current crankset. IMO, either get a new bottom bracket and be happy, or replace the entire crankset, rings included, rather than tying yourself to a somewhat uncommon bolt pattern. If you go the new crankset route, read up the bottom bracket for that crankset. You can tie yourself to one company's bottom brackets, depending on what you buy, so it should be a company whose bearings, and any other proprietary fittings, have a decent reputation.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  3. #3
    mtbr member
    Reputation: litespeedchick's Avatar
    Join Date
    Jan 2004
    thank you SO much for taking the time to enlighten me on all this stuff. much appreciated.

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