Author Topic: MTB bottom bracket and chainset problem  (Read 284 times)

MTB bottom bracket and chainset problem
« on: June 21, 2020, 01:38:50 pm »
I have a Trek 4500 MTB from about 2003 that I've been servicing to use as a commuter bike. Changed all the cables, brake blocks and chain. Chainset was bent so bought a SHIMANO FC TY501 Square 6/7/8 Speed Chainset and replaced the original 73/113 square taper bottom bracket with a new Shimano UN55 73/113.
When I fitted the chainset to the BB, the chainset fouled the BB shell - turned out to be 4 protrusions on the inside face of the chainset, presumably designed to stop the chain falling between chainset and BB shell and jamming. So I flattened them with a hammer  :o and all is well.
Except.... the chainset now sits so far inboard that I can't get the (new) derailleur to go far enough in to stop the chain rubbing on the inner plate of the mech when in low/low gear. Seems that the BB spindle isn't long enough?
So - is it that the chainset isn't designed for use on a MTB? (turns out to be a "trekking" chainset) or is there some difference in the BB despite the measurements being the same as the one I took off? (I've checked the existing and it has the same 73/113 marked on it).
Do I need a new BB - 73/122.5 maybe - or do I replace the chainset with a MTB specific one?
Many thanks for any help!

LittleWheelsandBig

  • Whimsy Rider
Re: MTB bottom bracket and chainset problem
« Reply #1 on: June 21, 2020, 01:46:26 pm »
BB lengths should match the crankset, not the frame. Different cranksets need different length BBs. You need a longer BB. Check what Shimano recommends for that particular model crank.

https://www.treefortbikes.com/Shimano-Tourney-TY501-6-7-8-Speed-Crankset-28-38-48t suggests 122.5mm

I do not know if the chainring sizes are the same for your new and old cranksets. If there is a big difference in the big chainring sizes, then the existing front derailleur may not work even if the chainrings were in the correct position laterally.
Wheel meet again, don't know where, don't know when...

Re: MTB bottom bracket and chainset problem
« Reply #2 on: June 21, 2020, 04:11:50 pm »
I occasionally fit a mountain bike shimano hollotech chainset to my cx bike (to give me 36/22 chain rings for occasional silly stuff). I have to fit a couple of spacers (essentials large plastic washers) to get the alignment correct and take up the slack. This only applies to hollow tech style of bottom brackets. I was skeptical at first. It seems fine.


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Re: MTB bottom bracket and chainset problem
« Reply #3 on: June 21, 2020, 04:30:09 pm »
I occasionally fit a mountain bike shimano hollotech chainset to my cx bike (to give me 36/22 chain rings for occasional silly stuff). I have to fit a couple of spacers (essentials large plastic washers) to get the alignment correct and take up the slack. This only applies to hollow tech style of bottom brackets. I was skeptical at first. It seems fine.


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Different type to square taper and inserting washers not relevant.

Re: MTB bottom bracket and chainset problem
« Reply #4 on: June 21, 2020, 04:47:58 pm »
Is it definitely a 73mm shell?  They are not all that common on 21st century bikes.
And Darkness and Decay and the Coronavirus held illimitable dominion over all.

Re: MTB bottom bracket and chainset problem
« Reply #5 on: June 21, 2020, 09:59:23 pm »
I would measure the chainline with the original chainset fitted and work out what length of BB is required to achieve the same chainline with the new chainset fitted.
Most of the stuff I say is true because I saw it in a dream and I don't have the presence of mind to make up lies when I'm asleep.   Bryan Andreas

Torslanda

  • Professional Gobshite
  • Just a tart for retro kit . . .
    • John's Bikes
Re: MTB bottom bracket and chainset problem
« Reply #6 on: June 22, 2020, 08:07:31 pm »
Or simply take it on trust, I fit A LOT of TY chainsets.

You need a 122 BB.
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.