New chainset incompatible - what to do?

rob190
rob190 Posts: 16
in MTB workshop & tech
This is a follow-up to a previous question.
I have a Specialized Rockhopper from the mid 90s. The existing 'Strongarm' chainset was badly worn so I replaced it, together with the chain and cassette. The new components are:
SHIMANO Acera Fc-m361, Shimano CS-HG200 cassette, SHIMANO HG40 chain.

The problem is I can't select the lowest front gear. The square taper on the M361 is recessed so the gears are closer to the frame than before, I estimate by about 5mm.




Ther front derailleur is right up against the frame so clearly no more adjustment is possible.

What do I need to do? The following come to mind:

  1. New bottom bracket? If so, what type exactly? The existing one is 110mm end-to-end.
  2. Someone suggested rotating the derailleur. Not sure I like that and I thought the arm had to be parallel to the chain.
  3. A new front derailleur? Again, what type should I be looking for?
  4. (Another) new chainset. Seems a waste but I guess they are not so expensive these days.
  5. A spacer between the bottom bracket and inner gear? I can't see how this would work but maybe there is something you can buy that would be ok.


I'm really confused by the compatibility issues here. Last time I overhauled a bike (about 10 years ago) I went down to the local hardware store, bought a bunch of things that looked about right and it all just worked.



Comments

  • whyamihere
    whyamihere Posts: 7,715
    You need a new BB. The spec for that chainset is a 122.5mm square taper BB.

    Measure the BB shell (the part of the frame the BB goes into). It will either be 68 or 73mm wide. You're then looking for a BB with that shell width, a 122.5mm axle (or thereabouts, a 122 or 123mm will work fine) and a JIS taper. If it helps the search, you can make a 73mm BB work in a 68mm shell with a 2.5mm spacer on the drive-side, but you can't make a 68mm BB work in a 73mm shell.
  • rob190
    rob190 Posts: 16
    edited March 2021
    Thanks. That makes sense. The BB shell is 73mm.

    Unfortunately it appears impossible to buy a 73mm/122.5mm BB in the UK. All the companies I've used before (Evans, Wiggle, plus a couple of local shops) and all the usual Ebay/Amazon etc. either don't list them or they're out of stock. Haven't given up yet.
  • whyamihere
    whyamihere Posts: 7,715
    Bikester claim to have stock:

    https://www.bikester.co.uk/shimano-bb-un300-square-taper-bottom-bracket-bsa-73mm-incl-crank-bolts-1276716.html

    If you wanted a bit more assurance about future availability though, it might be worth looking at a Hollowtech 2 style crankset, where the axle is integrated with the cranks. It's probably a little more up front, but the BBs are cheap and plentiful.
  • rob190
    rob190 Posts: 16
    Thanks again.

    I did search Bikester and found this page https://www.bikester.co.uk/shimano-bb-un300-square-taper-bottom-bracket-bsa-73mm-1276713.html which says they're not in stock. (Different SKU - doesn't have the bolts.) The joy of online shopping :-)
  • david37
    david37 Posts: 1,313
    rob190 said:

    Thanks again.

    I did search Bikester and found this page https://www.bikester.co.uk/shimano-bb-un300-square-taper-bottom-bracket-bsa-73mm-1276713.html which says they're not in stock. (Different SKU - doesn't have the bolts.) The joy of online shopping :-)

    just seen this, im not sure if you're still looking but ive got one sitting on the shelf

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