MTB bottom bracket and chainset problem

[johnshutt58]

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 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!

 

[Cycleops]

Hello and welcome to the forum John.

When you say it’s so far inboard the chain rubs on the inner plate of the derailleur I presume you mean the front one? Have you tried adjusting the hi=lo screws or are they at their limit? If they are you do need a longer spindle bb.

 

[C R]

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 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!

The chainset spec will tell you which bb spindle width you need. In this particular case you need a bb with a 122.5 spindle.

 

[Rickshaw Phil]

Hi and John

It does sound like your new chainset is designed to fit a longer bottom bracket axle.

Looking up the technical specs (here: https://productinfo.shimano.com/#/spec/MTB/Crankset) shows a code (D-NL) rather than an axle length. A bit more searching found that D-NL relates to a 122.5mm spindle length as shown here: https://bike.shimano.com/en-EU/product/component/tourney-a070/BB-UN26.html

They don't make it easy but, yes, it appears the 122.5mm is the one you need.