Bicycle Mechanics - Adjusting Chainline for new chainset possible?
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I spotted a chainset on the internet i would like to buy. The shop says when used with a 110mm square bottom bracket the chainline is 43.5mm which i think is a road chainline. I have a mtb groupset so can i just add the 6.5mm by increasing the bottom bracket spindle length by 13mm? I would have to buy a 122.5mm bb thats only 0.25mm off.
Would this work? The chainrings included are the right size and difference for a mtb front derailleur.
08-04-10, 06:19 PM
First off figure out if you have a 68 or 72 mm BB shell on the frame. Also the chain stay mount at the front is often different between MTB's and road bikes thanks to the widely different width of tires. So it's quite possible that when you mount the road crankset to your present BB that the clearance between the crank arms and the chain stays will be just right. I know that switching road for MTB cranksets to get to what I want I've only had to switch out my BB sometimes due to the shift in the chainstay clearance situation. So it's worth getting the CRANKSET, not "chainset" first and try it with your present BB. If it doesn't work out then BB's with different axle lengths are pretty easy to get from your LBS's so the time frame penalty isn't that big a deal.
Also due to the larger chainring sizes of a road crankset and the wider positioning of the chainstays on a MTB you may NEED the longer stock BB to ensure that the rings do not interfere with the stays. Again a very good reason to not jump into buying a new BB to go with your new crankset until you know you need one.
08-04-10, 06:29 PM
A single speed chainline is important to get straight, since you have only one and it works best that way.
but with derailleurs , especially a triple , lets say a 9 speed cassette,
only the middle chainring will be in line with the 5th cog in back, the one in the center of the cluster.
so the middle chainring will line up with 4th or 6th chainring, It's not an issue . or as said above ,
no big deal get a different BB theyre only $20 these days whole cartridge kit.
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