Triple bb for double crank?
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  1. #1
    PT
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    Triple bb for double crank?

    I have a Rohloff hub on a bike that has a 54mm chainline. I would like to use a Ritchey compact road crank (single ring, 34 or 36 tooth) and in order to get to a 54 mm chainline I'll need to use an octalink 118 mm triple bb instead of the usual 109.5 mm bb.

    Question #1: will the crank be properly centered on the triple bb spindle? My concern is that the "pitch" of the crank arms are different for doubles and triples.

    Question #2: If the 118 mm bb works with a double, I'll still be a few mm under 54 for the crank. On some triple cranks spacers are often used to correctly position the middle chainring, so I thought I might be able to use spacers/washers to move the chainring out a bit. Does anyone have experience with or an opinion on this strategy?

    Thanks!
    Hmm... Can I think about that?

  2. #2

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    Are you sure?

    I'm a long way from conversant with internal hubs, but I was under the impression they tended to be narrower than a multi-geared hub. If the hub is centered relative to the frame it doesn't sound logical to me to have to space your crankset out wide like that.

  3. #3
    PT
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    Positive...

    Quote Originally Posted by Spoke Wrench
    Are you sure?

    I'm a long way from conversant with internal hubs, but I was under the impression they tended to be narrower than a multi-geared hub. If the hub is centered relative to the frame it doesn't sound logical to me to have to space your crankset out wide like that.
    I already have it set up with a mtb crank and the chainline is exactly 54mm. The Rohloff cog is way outboard, the better to build a really strong wheel (spoke flanges are far apart and symmetrically positioned). A straight running chain is one of the advantages of singlespeeds or internal hubs. My goal here is to get as narrow a Q-factor crank as possible, and in my hands a Shimano-based road triple has a Q-factor of about 155 mm as compared to a road double at 145mm and most mountain triples at 165 -- 170 mm.

    Check http://www.rohloffusa.com/frame.htm for details on the hub.
    Hmm... Can I think about that?

  4. #4
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    define chainline...

    There is more than one way to measure "chainline". For some odd reason, a double chainline is most often defined as 43.5mm to the center of the space between the two chainrings. How the heck do you measure a point out in space? The answer is that you can't, so you you really have to know the chainring spacing (about 7.5mm) and add half that amount to get a chainline of 47.25mm to the tip of a tooth on the big ring.

    If you use an 8.5mm longer spindle with a double crank, the Q factor increases by 8.5mm and the chainline moves 4.25mm to about 51.5mm. A spacer could be used on the right side BB cup to move the chainline another 2mm to the right. I'm not sure if the resulting 53.5mm chainline (to the center of the big ring) is defined the same way as what you need.

  5. #5

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    Spacers

    My Aluminum frame was out to the driveside a couple of milimeters, causing some chain rub on the big ring when I was in the middle of the cassette. I added a two mm spacer between the BB and the frame and problem solved. So, I know this worked for me, but if you really want to be precise about this, take your bike to a shop with a park chainline gauge. It somehow bolts or slides on to the chainring to show how straight the chainline is, so from there you'll know where to go. Hope that helps!

  6. #6
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    Quote Originally Posted by tfkbiker
    My Aluminum frame was out to the driveside a couple of milimeters, causing some chain rub on the big ring when I was in the middle of the cassette. I added a two mm spacer between the BB and the frame and problem solved. So, I know this worked for me, but if you really want to be precise about this, take your bike to a shop with a park chainline gauge. It somehow bolts or slides on to the chainring to show how straight the chainline is, so from there you'll know where to go. Hope that helps!
    I have a Park chainline gauge and it can easily give more error than measuring. - TF
    "Don't those guys know they're old?!!"
    Me, off the back, at my first 50+ road race.

  7. #7
    PT
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    For this, chainline is defined by the cog on the hub...

    Quote Originally Posted by C-40
    There is more than one way to measure "chainline".
    The chainline on the hub is spec'd from the center of the frame to center of the cog -- 54mm. To have the chain run straight and true, I need to have a crank that gets a 34 tooth chainring close to that position.

    Quote Originally Posted by C-40
    If you use an 8.5mm longer spindle with a double crank, the Q factor increases by 8.5mm and the chainline moves 4.25mm to about 51.5mm. A spacer could be used on the right side BB cup to move the chainline another 2mm to the right.
    While the spacer on the right side BB cup would pretty much get the ring where it needs to be, I'd just as soon have the pedals centered relative to the frame, not displaced to one side or the other. How about 2mm of spacers at the chainring, pushing the chainring out towards the crankarm a bit? There should be more than adequate clearance between the crank arm and the chainring/chain. Has anyone tried such a fix?
    Hmm... Can I think about that?

  8. #8
    RoadBikeReview Member
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    clarify..

    Are you just using one chainring? The chainline I described was 51.5mm to the BIG ring, not the middle/inner ring. If you'r eonly using one, then chainring spacers would be no problem.

  9. #9
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    Yes, a single ring...

    Quote Originally Posted by C-40
    Are you just using one chainring? The chainline I described was 51.5mm to the BIG ring, not the middle/inner ring. If you'r eonly using one, then chainring spacers would be no problem.
    Yup, a single ring in the most outboard position on the crank. So thanks, I'll look into the appropriate chainring spacers to get me where I need to be.
    Hmm... Can I think about that?

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