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  1. #1
    Senior Member Lightfoot21's Avatar
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    Chainring bottom bracket compatibility advice please

    I have a Shimano 105 chainring on a Schwinn Tempo bike from the early 80's. It is a 52/42 biopace and I would like to replace it with another compact but with a 34 tooth small chainring (for easierIMG_1756.jpgIMG_1757.jpg climbing). I see them on ebay quite often and plan to buy one when I get a decent deal, but I wanted to be sure of compatibility. As long as it is a compact will it work with no modifications of my rear cluster? And my bonus question is --Can I put a triple chainring on the bike with little trouble? Any suggestions would be greatly appreciated.

  2. #2
    GONE~
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    Are you buying a new compact crankset or do you just want to buy a 34t chainring to fit on your current cranks? I'm afraid you can't as your BCD (bolt circle diameter) will not fit anything smaller than a 38t.

    If you're switching crankset, then you could just buy a new crankset but you might need to buy a different bottom bracket.

    You could use a taller chainring bolt with a washer to squeeze in a 39 or 38 chainring on it but it seems redundant.

  3. #3
    DOS
    DOS is offline
    Senior Member DOS's Avatar
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    I think your options are:

    --Keep crankset but get 39t or 38t inner ring to give you a bit lower gearing

    -- Get a compact crankset, which will also require a new BB but allow you to keep current derailleurs (with some adjustment to the front).

    --Get triple, but that will require new BB as well as probably two new derailleurs (hard to tell from pics).
    My Opinions > My Knowledge

  4. #4
    coprolite fietsbob's Avatar
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    Crank arm design is what determines the BB length required.
    Want a 50-34 compact?, buy the package deal. Crank and BB.

    remove the parts you have and bag them as a set..

    the one you have is a 130, you need a 110 bolt circle crank to get below 38t

  5. #5
    Senior Member Lightfoot21's Avatar
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    thanks for the replies. It really helped. I had an old compact that I put on tonight, (which only took me from a 42 inner to a 40). I'll start looking for a 50/34 combo and buy the crank and bb.

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