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  1. #1
    you guys ridin'? MTBaddict's Avatar
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    Square Taper Cranksets: Road vs. Mountain

    Is there a difference in chainline between road cranksets and mountain bike cranksets? I am specifically concern about triple cranksets. If I replace my mountain triple with a road triple, will the chainline be the same?

    I have measured the centerline (chainline) of my cassette and found it to be 42.5mm (using the centerline of the frame as the reference datum). So I would like to install a crankset with same centerline. The triple mountain crankset that I have currently installed (FSA) measures 50mm from the center ring to the frame center.

    If I switch to a road crankset (likely a Sugino XD600) will the chainline change? Sugino actually recommends a bottom bracket axle of 118mm to get a chainline of 50mm. So it seems to me that these two (Sugino road & FSA Mountain) cranksets are different.

    Is this the standard way that tapered square cranksets fitu, i.e., do all road cranks fit the same and all mountain cranks fit the same but the two are different?
    Santa Cruz Heckler, Gary Fisher Hoo Koo E Koo, Schwinn Le Tour III, Giant Perigee, Motobecane Le Champion Ti SL, Jamis Nova, Salsa Vaya

  2. #2
    Senior member Dan Burkhart's Avatar
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    There are more issues at play here. Chainstay clearance for one. Even at a 50mm chainline, standard for most XC MTBs, a road triple's middle chainring would not clear the chain stays on most MTBs even if the crankarms do which is another problem.
    You might be able to get a long enough BB spindle to get clearance, but you will sacrifice chainline to do it.
    I'm speaking in generalities here. Your particular bike might work, but I wouldn't bet the farm on it.

  3. #3
    me have long head tube TallRider's Avatar
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    square-taper cranksets were not standardized to always work with a certain length bottom bracket, so to get the proper chainline for your application, you need to know two things:
    * your desired chainline
    * the crank you're using

    chainline for triple cranks is measured from the bike's centerline to the the centerline of the middle chainring.
    standard chainline for road triple (assuming 130mm-spaced rear wheel) is 45mm, and for mtb (assuming 135mm-spaced rear wheel) is 47.5-50mm.

    Sheldon Brown has a database of bottom bracket sizes for various cranks. The page talks about chainline as well.
    http://www.sheldonbrown.com/bbsize.html

    In your case, I assume you're talking about a road bike (I don't think you ever actually say) because you mention 42.5mm chainline on the rear wheel.
    I am using the Sugino XD2 (which I think is the same forging as the XD600) and have achieved proper chainline as well as equal spacing (left pedal and right pedal are equidistant from bike's centerline) with a Shimano 110mm bottom bracket. I would recommend that you do the same.
    Note that standard chainline for road double is 43.5mm, but 45mm is used for road triples because there are more problems cross-chaining from granny gear across the cassette if the crank is moved too far inward, and because the granny gear can rub against the chainstay on some frames.
    If your frame is an mtb frame, you may well need to move the crank further out so you don't bump the chainstay with your granny gear.

  4. #4
    you guys ridin'? MTBaddict's Avatar
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    Quote Originally Posted by timcupery View Post

    In your case, I assume you're talking about a road bike (I don't think you ever actually say) because you mention 42.5mm chainline on the rear wheel.
    I am using the Sugino XD2 (which I think is the same forging as the XD600) and have achieved proper chainline as well as equal spacing (left pedal and right pedal are equidistant from bike's centerline) with a Shimano 110mm bottom bracket. I would recommend that you do the same.
    Note that standard chainline for road double is 43.5mm, but 45mm is used for road triples.
    YES, in fact it is a cyclo-cross (road) frame.

    Based on what you are telling me, I have the right bottom bracket for the Sugino crankset, even though the manufacturer seems to recommend a different length.
    Santa Cruz Heckler, Gary Fisher Hoo Koo E Koo, Schwinn Le Tour III, Giant Perigee, Motobecane Le Champion Ti SL, Jamis Nova, Salsa Vaya

  5. #5
    me have long head tube TallRider's Avatar
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    you said that the manufacturer recommended a 118mm BB for 50mm chainline. so I don't think that Sugino is wrong in this case.
    since you don't want 50mm chainline (assuming your frame and/or rear hub is 130mm spacing), you should use a 110mm BB.

  6. #6
    you guys ridin'? MTBaddict's Avatar
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    Quote Originally Posted by timcupery View Post
    assuming your frame and/or rear hub is 130mm spacing, you should use a 110mm BB.
    That's it, rear dropout spacing (over lock nut dimension) is 130mm.

    Now if I can just find someone that has the Sugino XD600 with 175mm crank length in stock...
    Santa Cruz Heckler, Gary Fisher Hoo Koo E Koo, Schwinn Le Tour III, Giant Perigee, Motobecane Le Champion Ti SL, Jamis Nova, Salsa Vaya

  7. #7
    me have long head tube TallRider's Avatar
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    I bought my XD (also 175mm) on eBay for $40 a few years back. You'd have to be really lucky to find it that cheap now, I think. Good luck.

  8. #8
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    what about a sugino xd on a mountain bike frame? what bb length would be optimal? from what I've read a 113mm bb is recommended for a 47.5 chainline...would this be correct?
    Last edited by bionnaki; 12-18-09 at 02:53 AM.

  9. #9
    me have long head tube TallRider's Avatar
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    For a mountain bike frame with 135mm rear spacing: 113mm or 115mm, but depending on how much the chainstays are flared to make room for fatter tires, you may need to go with 50mm chainline and use the 118mm bottom bracket as Sugino recommends.

  10. #10
    you guys ridin'? MTBaddict's Avatar
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    OK, I got the Sugino XD600 triple crankset and a Shimano UN54 BB with a 110mm spindle. The chainline at the middle ring is still ~50mm! Looks like I could have stuck with the MTB crankset as it was no different.

    I have order an IRD bottom bracket (QB-75) with a spindle length of 103mm to bring the chainline in as much as possible. I will then decide which crankset to use.

    IMHO, Sugino seems to have placed a higher value on form rather than function by the way in which they clocked (oriented) the chainring bolt pattern relative to the crank arm. They placed one of the chainring bolts directly behind the crank arm thereby severely (and unnecessarily) limiting wrench access. There are nice chainring bolts available that use hex tools on both sides for installation/disassembly and there are nice chainring bolts available that use Torx tools on both sides. The Sugino uses chainring bolts with the traditional nut that requires a spanner wrench. With this crank, direct approach is blocked by the crank arm, so only a 90 degree angle spanner can be used. Access is further limited by the pin on big ring that prevents the chain from dropping between the crank arm and the big ring. Bottom line: this arrangement could be a real problem when changing rings if the fastener locks up due to corrosion or general dirt. If I keep the Sugino crankset, I will remove the chainring bolts and re-assemble them up using a thick anti-seize compound to help removal when the time comes.
    Santa Cruz Heckler, Gary Fisher Hoo Koo E Koo, Schwinn Le Tour III, Giant Perigee, Motobecane Le Champion Ti SL, Jamis Nova, Salsa Vaya

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