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BB for compact

bicebice Posts: 772
edited September 2009 in Workshop
Can I improve this: I am running a goodish quality no-name Shimano compact on a steel frame Ciocc. I replaced the square tapered BB for a shorter one (I can't now remember the size, but suspect 110mm or 107mm).

On the larger chainring, when I am in 4th highest gear the chain is already coming from the chainring to the cassett at an inward angle.

I have an 8 spd cassette, but can only use 7 gears as I have an 8spd Miche cassette with indexed Shimano downshifters. (I have to use an 8 spd cassette as I have Campag wheels.) With my chainring/BB set-up I would not want to try to use all 8 gears.

The chain alignment on my compact is far more extreme than with my all Campag double 10spd.

Should I be using a shorter BB? How short do Shimano compatible square tapered ones go?

Posts

  • Monty DogMonty Dog Posts: 20,614
    Check your chainline - it should be 46mm for the large chainring i.e. centreline (front-on) of the big chainring to the centreline of the frame seattube or downtube. If it is out, then you need a BB longer or shorter by twice the difference i.e. if 44.5mm, then you need a new BB 3mm longer.
    Make mine an Italian, with Campagnolo on the side..
  • bicebice Posts: 772
    I looked into this, but wondered how you can get these precise measurements. 0.5mm measured to the centre of the tubing? I haven't got any tool that can do that.

    I have just assumed that if the chain line is straight to the centre of the cassette, with the wheel correctly centred in the frame, you are more or less there so far as it matters. With this bike, which I commute on, the chain is at an appreciable angle to the centr eof the cassette. That can't be right, surely?
  • whyamiherewhyamihere Posts: 7,681
    Measure to the edge of the seat tube and add half the diameter. Simple.
  • John.TJohn.T Posts: 3,698
    The chainset is usually offset a little towards the outside. This is to give a bit more clearance between the chain and the big ring when on the small one and the smaller sprockets. It means you can use more of the cassette on the small ring. On a 9sp the small ring (double) will be about lined up with the centre sprocket.
    107 seems to be the shortest that Shimano do now but a bit of time on't web should find some alternatives.
    My Fleet.
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