BB replacement problem

Cousin Nick
Cousin Nick Posts: 27
edited April 2013 in Workshop
I've just replaced the bottom bracket on my 2010 Specialized Secteur (Triple). The old BB was a cheap 'n nasty FSA with what appeared to be completely unsealed bearings. Size was 68mm shell (marked) but the spindle length was not shown. I measured the spindle with calipers and it was 115mm. Its a square taper spindle. I've bought a Shimano UN55 replacement 68/115mm.
However, now that everything is reassembled, front mech alignment is all to cock (bearing in mind that I've only replaced the BB, nothing else has been moved). When I shift the chain to the inner ring of the triple (and the inner rear) the chain rubs the inner plate of the front mech. Adjusting the L-limit screw will not prevent this, since it is the inner bar of the front mech parallelogram touching the frame i.e. the chainset must be closer to the frame on the new BB than it was on the old one! The only explanation that I can come up with is that the old FSA unit was slightly 'off centre' compared with the new Shimano unit. Unfortunately I'd binned the old one so am unable to check this idea. (Yes, I have checked the new unit and it is indeed 68/115).
Anyone else had a similar experience?
The only solution I can see at the moment is to get a 68/118 BB, which would give me an extra 1.5mm clearance on each side, and that would be sufficient to prevent the rubbing.
All comments/suggestions welcome.

Nick

Comments

  • maddog 2
    maddog 2 Posts: 8,114
    you shouldn't run small-small anyway, so it's not a 'real' problem in that sense.

    As long as the gear combos that you run in normal riding work then just set it up and ride it. A narrower chainline is actually preferable on a triple anyway, as triples are w i d e and running on the big ring can put the chain under tension on the bigger sprockets.
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • Cousin Nick
    Cousin Nick Posts: 27
    Its not small/small, its small front, large rear i.e. lowest gear (in fact it still rubs on second-to-largest rear / small front.
    As I said above, ONLY the BB has been changed, nothing else, and it was fine before.
    Nick
  • nicklouse
    nicklouse Posts: 50,673
    let some tension off the cable and then reset the front mech.

    all sounds normal. you do not need a 118 BB.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Cousin Nick
    Cousin Nick Posts: 27
    Nick, its a bottom pull, bottom swing mech. If I release the cable entirely so that the mech swings in as far as it will go (touches the seat tube) the inner face of the mech still rubs the chain. The mech physically cannot move any further in. This is what is so weird. The only reason that I can think of was that the old BB spindle 'stuck out' slightly more on the drive side.
  • cycleclinic
    cycleclinic Posts: 6,865
    I wonder if the amount of axle on the drive side is different on the FSA campared to the Shimano BB?
    http://www.thecycleclinic.co.uk -wheel building and other stuff.
  • Cousin Nick
    Cousin Nick Posts: 27
    My thoughts exactly!
    Unfortunately I'd binned the FSA, so I'm unable to check. However 68/118 mm BB should give me the extra 1.5 mm that I need.
    N
  • slowbike
    slowbike Posts: 8,498
    Can you twist the mech at all - I take it it is the trailing edge that rubs on the chain - so if you can twist this in a little it should clear it?
  • Cousin Nick
    Cousin Nick Posts: 27
    Tried twisting the mech, but the amount needed is excessive and far beyond 'Blue book' recommendations.
    Again, the point is that everything was running perfectly before changing the BB. My suspicion is that not all 68/115mm BBs are the same and I suspect the original FSA BB had more spindle protruding on the drive side than the Shimano replacement.
    N
  • zoltansocrates
    zoltansocrates Posts: 118
    does the shimano one have a slight offset? have you fitted it 'the wrong way round'? sounds a mystery really
  • keef66
    keef66 Posts: 13,123
    zoltansocrates wrote:
    does the shimano one have a slight offset? have you fitted it 'the wrong way round'? sounds a mystery really

    I've never known an assymetric Shimano BB. (but my experience with these things is limited)

    AFAIK it's impossible to fit a standard 68mm BS bottom bracket the wrong way round due to the threading.

    I'm thinking the original must've been somehow offset. See if you can deduce anything useful from the oracle here:

    http://sheldonbrown.com/bbsize.html
  • keef66
    keef66 Posts: 13,123
    Sheldon appears to suggest 118mm for some square taper Shimano triple cranksets, 113mm for Sora and Tiagra. I still think you'd reached the correct conclusion!
  • Cousin Nick
    Cousin Nick Posts: 27
    You gotta love Sheldon - a two-wheeled nerd! But a great one at that.
    I think the answer may lie here:
    http://sheldonbrown.com/bbtaper.html

    As suspected, not all square taper spindles are equal.
    I really wish I'd not chucked ot the old one, just so I could make comparisons.
    N
  • keef66
    keef66 Posts: 13,123
    If you have a Shimano crankset on a Shimano BB they'll both be JIS. Question is, what was the original...
  • Monty Dog
    Monty Dog Posts: 20,614
    FSA square taper chainset will be JIS and all Shimano BBs are symmetrical. You could get a 1mm spacer from your LBS (try Shimano 10 speed cassette spacer) and fit on the driveside - it should move your chainline a touch.
    Make mine an Italian, with Campagnolo on the side..
  • Cousin Nick
    Cousin Nick Posts: 27
    Shimano 68/118mm BB now fitted and everything working as it should.

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