crank replacement help
Ok so I have a ss conversion, and I did not do the conversion. I am very happy with the bike, and I do not have any chainline issues.
The bottom bracket has died and needs to be replaced. I would also like to go ahead and replace the crank because it is the old non replaceable chainring model, and I would like to drop down a bit in gearing. It is at 52/17 right now and I use this as a commuter. Its fast, and while I do live in flat florida, it would be nice not to be sweaty even on cool days when I get where I am going.
So basically I need some help replacing these parts without messing anything up. So what measurements do I need to take to maintain chainline? I am thinking about going to a 46 or 48 in the front with the 17 in the back, which might make using a mtn crank the easiest option. If I were keeping the crank I would just buy the same bb, but with changing it things get a little more complicated. Also, this is all to be done as cheap as possible. Thanks.
chainline measurement is crank dependent, track cranks usually use a 109 or 109.5 mm BB spindle, others vary, sometimes by alot
sheldon brown or harriscyclery.com has a crank chart with BB spindle lengths but it doesnt get all of them
im running a 115mm with 2 different cranksets and its pretty close, one is a sugino 75 the other a dotek BMX/mtb crank I think.
You get like 3-6mm of leeway cause you can always put the chainring on the inside or outside of the spider and/or use spacers, it doesnt have to be absolutely perfect. Not all cogs have the exact same chainline either, but a mm here or there isnt much.
Figure out what cranks your gonna use and if you cant find the number you need ask around again I guess.........
jack of one or two trades
If your chainline is spot on, and you don't want to change that, you can keep your existing cranks and just get a bigger cog to gear down. Measure your spindle length before you remove the BB, and make sure you get the same length. Install new BB, cog. Intall same (old) cranks. Bing! Only real downside is you might have to add a link to your chain if you are already very forward in your dropouts. Big deal.
Originally Posted by Dr Irwin Goldstein