Bottom bracket size query
04-10-24, 05:09 AM
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Bottom bracket size query
Hi everyone. My first time here so forgive me if this has been answered a million times before. I have removed the crankset from a MuddyFox MTB and want to use it on another frame. The MuddyFox frame had a 68mm BB shell and the new one in 73mm. The original bottom bracket has a spindle length of 122 so I did a bit of maths subtracting the shell width from the axle length to get the amount of exposed spindle and then added that on to the new BB shell and I got 127mm. I can't find many bottom brackets with a size of 73x127 so I suspect I am going about it the wrong way. As you can tell I am new to this. Any advice gratefully received.
04-10-24, 06:19 AM
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You just need to match the 73 mm bb and the spindle length of the crankset, 122, no changing those numbers. I believe your crank set is square taper, which means a 73mm square taper bb for 122 length. Such as a Shimano UN-300
04-10-24, 07:48 AM
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Muddy Fox makes 13 bikes they classify as mountain bikes. Most have the same chainset on them but some don't. Knowing what chainset you have might make a difference for the suggestions.
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04-10-24, 08:56 AM
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Many thanks. The chainset is cpi 22-32-42. I was under the impression that the wider the bb shell, the longer the spindle had to be, to compensate. Seems I was wrong
04-10-24, 09:24 AM
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Understand that there are probably different types of BB's for some of the cranks offered on their mountain bikes. Some might be for 2 piece cranks that don't use a cartridge BB. And for those that do use a cartridge BB, there are square taper, Octalinkv2 or ISIS and others. All of those BB's can fit in the same BB shell, but only one of those types is appropriate for your cranks.
04-10-24, 12:11 PM
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The OP should have all the info they need. If the original bike had a 68/122 bottom bracket, and the new bike with 73mm shell will be using the same crankset, then a 73/122 BB is what is needed. I’d recommend something other than the bb-un300, as these have tiny bearings and tend to wear out quickly.
04-10-24, 04:53 PM
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The bbkt spindle needs to be the right length to align the chainset with the rear sprockets. You should be able to check by sighting along the middle chainring to the middle sprocket that alignment is correct. You can measure/calculate this "chainline" which is the distance from the midplane of the bike in mm to mid-chainring/sprocket.
