CityCyclingEdinburgh Forum » Questions/Support/Help

Which BB axel length for correct Chainline?

(11 posts)
  • Started 10 years ago by DaveC
  • Latest reply from algo
  • This topic is not resolved

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  1. DaveC
    Member

    I would like to change from double (Shimano 105) to triple, on my Dolan.

    I'm not buynig a new 105 as its nearly 100 quid.

    I have a spare Driveline triple with very little wear from my bent, that the seller threw in as well as the BB and front triple mech. He swapped out the original Driveline (170 crank arm length) triple (30,42,52) for an FSA (150) triple (22,34,44) and gave me the origainal crank, BB and front mech.

    I didn't know which size BB to buy, so I emailed Dolan who say the BB is 68mm English Thread and Axel width somewhere around 115mm but that I should ask the crankset manufacturer. I don't think this is possible as I think the Ice B2 is circa 2009 and Driveline being obsqure and non British?? might not be willing to help, especailly as I can't be sure which model it is, let alone now which one this is.

    So am I right in thinking the 68mm is the BB opening diameter in the frame? And that the 115mm is the width of the axel (end points of the square taper shaft)?

    I've been advised to take care, to get the chainline right. After googling this I know what it is but Dolan advise contacting the crankset manufacturer about what they recomend, suggested around 115mm. See above for Driveline doubts.

    Looking at the BB (square taper = ST) in my spares box I see one end is part of the BB body and threaded. The other end is a threaded cap which slides over the BB. I assume it should sit all the way in so the width of the BB in the frame is a standard length?

    Am I right in my understanding, that the larger (axel) figure is the determining factor in the chainline setting? and not the width of the threaded BB part of the frame?

    Posted 10 years ago #

  2. algo
    Member

    Dave,

    I've just done exactly this - yes the 68mm is nothing to do with chainline, just the diameter of the BB - 115mm is the width which determines the chainline.

    For the front derailleur it should say what chainline it expects - 45 or 50 (for example), and I'd measure that and try and determine the width of the BB. I have an old 68mm English Bottom Bracket (112 I think) which you could try just to measure it if that would help.

    Posted 10 years ago #

  3. Uberuce
    Member

    Is chainline that critical on geared bikes? If you think how far the chain moves laterally as it goes from the big to little cog on the back block, that's hugely more than the difference in BB axle length.

    As long as it's not so short that the pedals strike the frame when on the granny, I doubt you can buy a BB that'll get it wrong. Not without going radically offpiste and getting one for tandems or fat bikes or something. Not even sure if they have crazy BB sizes, mind.

    Posted 10 years ago #

  4. kaputnik
    Moderator

    68mm = BB shell width. Think how big your BB would be if it had a diameter of nearly 7cm!

    Other things to consider are if you have a short-arm rear derailleur it might not have enough reach in it to keep the chain taught across the full range of gears you might need to go up to a long arm.

    Posted 10 years ago #

  5. Cyclops
    Member

    All Shimano road triples have a chainline of 45mm (measured to the middle chainring). 122.5mm is the axle length you need if you are using a Shimano BB with a Shimano triple chainset and I strongly suspect the figure is possibly the same for a Driveline chainset.

    Posted 10 years ago #

  6. algo
    Member

    @kaputnik - sorry that's not what I meant - thanks for clarifying :-) Good point on rear derailleur.

    Dave - if you do happen to have a 9 speed short arm and need a long arm I have a new acera 9 speed long cage you can have if you want it

    Posted 10 years ago #

  7. wingpig
    Member

    "...122.5mm is the axle length you need if you are using a Shimano BB with a Shimano triple chainset..."

    Not on my Shimano square taper BB and Shimano triple crankset combination, it's not. BB axle length is 113mm, IIRC.

    Posted 10 years ago #

  8. DaveC
    Member

    @algo. Nope, I'm still confussed. Where do I measure 45 - 50 from? I have two front triple deraillurs but they only say FD-4703 for the Tiagra and FD-6603 for the Ultegra? Other than that the tube size is inside the clamp? Plus its 10 speed.

    I'm removing the BB (not sure whether its Hollowtech, Octalink or otherwise) and using a Square Taper as that is what the Driveline crankset is.

    I spoke to a fellow cyclists here, and I am going to fit my existing spare (ST) BB in (from the Bent), and then have a look at where the chain (chainline?) lies in the middle ring(s) on both front and rear cassette.

    I'm not sure about the rear cage length. Its a Shimano 105 and came with a cassette 11-28 but now has a 32 happily on the wheel, and the front derailleur is a double 105 FC-5750 crankset. The current spec off Dolan's website states 105 RD-4700 (rear derailleur) but this could have changed due to stock or avaliability?

    Posted 10 years ago #

  9. algo
    Member

    They'll both be 45mm I'm pretty sure as they're from the road sets - mountain bike tend to be 50 I think....

    (are you sure you mean 4703 btw? I can't find any info on that, just 4503 (9spd) and 4603 (10spd))

    Posted 10 years ago #

  10. DaveC
    Member

    So to recap. My plan is to fit the existing ST BB, and then look at the chainline. Then guesstimate/measure the in/de/crease and buy a BB based on that.

    Is there a more foolproof measurement method?

    Posted 10 years ago #

  11. algo
    Member

    Dave - indeed that sounds sensible. Measure from the middle of the seat post to the middle of the middle ring - that should be 45mm... I'm afraid my rear derailleur is no good to you sorry (and anyway it's much lower quality)

    Posted 10 years ago #

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