I've been trying to replace the drivetrain components on my brother's bike, which started life as a 2000 Specialized Rockhopper FSR. After a broken frame incident over 20 years ago, the bike shop swapped all the components over to a 2001 Specialized Enduro FSR frame (no idea if anything was changed in terms of the bottom bracket).
Initially I was going to replace the chainrings on the crank (Specialized Forearm Expert according to bicyclebluebook.com), but for some odd reason the smallest chainring is a 58bcd 4-bolt pattern, I could only find one listing in Italy…. so I ended up buying a Shimano FC-MT101 crankset since options for a 3 chainring square tapered crankset are pretty limited.
Turns out this crankset doesn't play well with the bottom bracket on the bike; any attempts to snug down the bolt causes the crank to bottom out on the frame resulting in total binding.
According to the Shimano site, the MT101 is compatible with a BB-UN101 bottom bracket. I measured the shell width on his bike, and surprise surprise it's not the standard 73mm width for MTBs, it comes out to 71mm.
Is there anything I can do here? I'm at a loss and not sure how to proceed. Will a 68mm bottom bracket fit? Thanks in advance for reading this long post! I've attached some pics of the right side of the bottom bracket, sorry for the filth covering everything..
by rjvader
4 Comments
You need to take spindle length of the BB into consideration when buying a crankset. Each crankset has a spindle length compatability.
Don’t change the shell size
Regular BB has size 68-73
That spindle looks… really, really, super duper short to me? Like, short enough (or shifted to the left) that you couldn’t get a crank arm seated on the taper without hitting the bearing/shell…
Did you want to keep square taper for retro-ness? I’d grab your choice of hollow spindle crank & outboard BB, and enjoy not dealing with square taper.
Watch out for your existing E-type front derailleur. Those things can be an entire headache in and of themselves. That might also be contributing to the 71 vs 73mm shell width confusion.
As another comment mentioned, each frame and crank combo has a specific BB spindle length but it’s a bit of a lost art to pick the right one. In the shop it was easy back in the day since we’d have a whole drawer full of them and we could guess and check.
I’m truly sorry you’re going through this, I’ve been there myself, when a simple worn chainring causes an unfathomable amount of work just to get riding again.
If I were to do it again, especially these days, it’s a great time to jump ship and go for a 1x setup if that appeals to you at all. Trust me…
Even if you get all the parts bolted up right and everything’s working, there’s a decent chance that e-type derailleur will have other plans in store for you, either spacing, or you’ll run out of adjustment, or the angle will be off, or your chain line will be goofed and it’ll be a nightmare to fix. For example, you can’t change the angle without loosening the BB, which means taking off the crank. … …… you might end up hating that bike by the end of the job.
Edit to add, cus I’m shocked I remember this… the spindle length usually comes in ~2mm increments, and it’s independent of shell width – you need to have both numbers right – so like we used to use 68×110 was a really common size for road bikes, 73×114 is sticking out in my head as a typical narrow mountain size, but some got long, like I think there was a 122 or even a 137?? There was a hack back in the day for some of them where you could swap the non-drive side cup to convert from a 68 to a 73 and vice versa.
Some manufacturers still make them and might have calculators – Phil Wood was the cream of the crop, they might have an online calculator or bulletin to show you how to measure chain line and then determine your BB spindle length.