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 easierhttp://bikeforums.net/attachment.php...hmentid=227325http://bikeforums.net/attachment.php...hmentid=227324 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.

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.

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).

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

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.