MTB drop conversion bottom bracket & crankset woes
09-07-20, 10:38 AM
#1
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MTB drop conversion bottom bracket & crankset woes
So, i have this converted mtb... https://www.bikeforums.net/20949423-post153.html
I built it up with a BB386evo BSA bottom bracket and FSA Gossamer Pro ABS 386evo crankset. This setup was very tight at the chainstays for the crank arms and chainrings. VERY tight with only a couple mm clearance, so close that my cadence sensor magnet which sticks to the backside of the left pedal spindle was too thick, and gunk from the 36t chainring was building up on the chainstay. I had to put a spacer on each side to allow that little bit of clearance, and now the bb bearings are shot, at least on the non-drive side, grinding and crunching.
For now I'll throw on a square taper bb and cheapo crankset so I can finish the season. But, I might be interested in a more modern solution to provide appropriate crank arm and chainring clearance.
Any suggestions? I'm open to just about anything, and I'm not tied to any particular bb standard as long as it will work on my BSA frame. Thanks!
I built it up with a BB386evo BSA bottom bracket and FSA Gossamer Pro ABS 386evo crankset. This setup was very tight at the chainstays for the crank arms and chainrings. VERY tight with only a couple mm clearance, so close that my cadence sensor magnet which sticks to the backside of the left pedal spindle was too thick, and gunk from the 36t chainring was building up on the chainstay. I had to put a spacer on each side to allow that little bit of clearance, and now the bb bearings are shot, at least on the non-drive side, grinding and crunching.
For now I'll throw on a square taper bb and cheapo crankset so I can finish the season. But, I might be interested in a more modern solution to provide appropriate crank arm and chainring clearance.
Any suggestions? I'm open to just about anything, and I'm not tied to any particular bb standard as long as it will work on my BSA frame. Thanks!
09-07-20, 12:34 PM
#2
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I never saw any reason to get away from square taper BBs in the first place. "More modern" does not always mean "better", often it means "cheaper for the manufacturer".
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09-07-20, 12:45 PM
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The correct chainline for that bike is 47.5 - 50.0 mm. A Shimano UN55 square taper is an excellent choice and you can get whatever size you need to get your chainline correct. You need to choose your crankset first and then buy the BB that gives you the right chainline for that crankset. I like Sugino XD600 cranksets for my vintage MTBs. You might have to go one size larger BB from what was the original length.
Despite "newer" systems, square taper is still my preferred crankset.
Despite "newer" systems, square taper is still my preferred crankset.
Last edited by Jeff Neese; 09-07-20 at 12:48 PM.
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09-07-20, 02:39 PM
#4
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As it happens I have a new Shimano un55 bb hanging around. Before i converted this bike it had the original Shimano 21 series cartridge bb... which worked problem free for 20+ years. Yea, maybe square taper is not such a bad idea. ;^)
