New BB, chain-set but front mech won't work - HELP!

[Ian Lewis]

Hello,

I have a 2009 Trek 7.3fx Hybrid. Recently I changed the BB, Front Mech, chainset and cassette because they were all a bit knackered after a few years of hard riding.

Everything was fine until I put the front mech on. Even if I pull it as far as possible from the bike, the outer edge only ever comes parallel with the chain, and not slightly beyond it. Therefore I can't change into the top chainring.

I've tried changing the end stop screw, but this doesn't make enough difference. Now I made sure that the new Shimano BB was exactly the same length as the old one (quite long at 122mm). Is it possible or likely that the new chainset might be a bit thicker than the old one, and so I need to get a shorter BB, rather than replicating the old one?

The old front mech was a Shimano C102, new one is a Shimano Alivio FDM412, new chain set is Shimano FC-M411 Alivio 48/38/28T, instead of original M361.

If anyone can help or advise I would be extremely grateful!!

 

[Lee_M]

sounds to me like the wrong width BB

 

[Ian Lewis]

Thanks Lee - however will it now be a case of trial and error with getting a new one? At least before I thought I could just copy what I had.

 

[rusty bearing]

According to the spec you should be using BB-UN26(-K) the spindle length being 123mm whereas the original was a BB-ES25(-K) spindle length 126mm so your chainset would be 3mm further out than it should be. Link here:
http://techdocs.shimano.com/media/t...4A/SI-0094A-001-ENG_v1_m56577569830667768.PDF

So if you had a 122 then you may need to fit a 118 which I think from memory is UN-54 something

 

[RecordAceFromNew]

Hello,

I have a 2009 Trek 7.3fx Hybrid. Recently I changed the BB, Front Mech, chainset and cassette because they were all a bit knackered after a few years of hard riding.

Everything was fine until I put the front mech on. Even if I pull it as far as possible from the bike, the outer edge only ever comes parallel with the chain, and not slightly beyond it. Therefore I can't change into the top chainring.

I've tried changing the end stop screw, but this doesn't make enough difference. Now I made sure that the new Shimano BB was exactly the same length as the old one (quite long at 122mm). Is it possible or likely that the new chainset might be a bit thicker than the old one, and so I need to get a shorter BB, rather than replicating the old one?

The old front mech was a Shimano C102, new one is a Shimano Alivio FDM412, new chain set is Shimano FC-M411 Alivio 48/38/28T, instead of original M361.

If anyone can help or advise I would be extremely grateful!!

According to this you have the correct new BB for the M411 chainset (albeit the 50mm chainline in that link is not consistent with the 47.5mm one indicated here despite both being Shimano tech docs... ). But I expect either chainline would work fine but worth checking (it is the centre of the seat tube to centre of middle ring) that it is not something outside the two for whatever reason.

Because the new FD-M412 works for both 47.5mm or 50mm chainlines according to spec, it is actually surprising that you are encountering reach problem.

Assuming the H limit screw has been back-out completely, the second thing I would therefore check is front mech cable tension, so that it is not slack while on the small ring? While the L and H limit screws serve to confine the mech movement within a range, sufficient cable tension is necessary to move the front mech on to the big ring.