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I just replaced my cassette and my crank with one from the LBS that the guy said would work. I've had no problems with the cassette, but the crank is too wide so that my front derailleur does not reach the 3rd chainring. So here's my question:

Is there a specification on cranks that show the length each chainring is away from each other, and for that matter how far it sits from the frame as well? This way I can look at the one I used to have on and find one with similar measurements.

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2 Answers 2

Good question.

Bottom bracket axles come in a variety of widths as do bottom bracket shells. Typically these measurements are marked on the axle or cartridge and you need these measurements to get a replacement. Needless to say you need to take a fair amount of parts off the bike to get to read the axle/cartridge, however that is the best way to get the required numbers.

In the olden days when steel was king it normally came down to where the bike was from to get the bottom bracket width but a lot has changed since then and there are no convenient rules of thumb. That said, with a micrometer or even a ruler you can measure the bottom bracket shell width.

In these simpler times a given bottom bracket shell size would come in up to three axle lengths for 1/2/3 chainrings - the longer axle needed for the 3 chainrings.

You can work out what you need for a given bike thanks to the internet - Shimano provide their specifications online as do the less common brands.

As for chainring spacing that does not really of concern here - your LBS got it slightly wrong but you can forgive them because you have to 'get a few wrong' before replacement bottom bracket axle/shell lengths are understood.

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It's possible that you just need to adjust your front derailleur a bit to accommodate your new setup. There are many threads here and videos online about how to do this.

The crank may not be the exact same distance from the frame as your old one, but that doesn't mean it doesn't work.

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