Advertise on Bikeforums.net



User Tag List

Results 1 to 6 of 6
  1. #1
    Senior Member
    Join Date
    Oct 2008
    Location
    Chicago, IL
    My Bikes
    S-Works Roubaix SL2^H4, Secteur Sport, TriCross, Kaffenback, Lurcher 29er
    Posts
    2,404
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Ultegra 6700 FD with 46T Chainring too close to chainstay?

    I'm helping a friend build up an S-Works Crux (essentially the old S-Works TriCross frame) from the module (frame/crank) up. We're using Ultegra 6700 with the chainrings that came on the frame from Specialized -- 46T/34T.

    When I adjust the position of the front derailleur so it's close enough to the chainring the back end of the derailleur cage is very close to the chainstay (see pictures -- the clearance is less than 1 mm). He and I have discussed it and think we have two options -- move the FD up a little bit and maybe compromise shift performance, or replace the chainrings with a 50/34 set, which would require moving the derailleur up a bit.

    I'm trying to decide if this is going to be a problem that will need to be addressed before the bike is ever ridden. Obviously it'll be far enough away when he's riding on the 46T ring but I'm concerned if the clearance is enough for any movement/deflection of the chainstay or derailleur.
    Attached Images Attached Images

  2. #2
    AEO
    AEO is offline
    Senior Member AEO's Avatar
    Join Date
    Jun 2007
    Location
    A Coffin Called Earth. or Toronto, ON
    My Bikes
    Bianchi, Miyata, Dahon, Rossin
    Posts
    12,255
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    either solution would work.
    seems like, true to the frame's purpose, the designers never thought about a CX type chainring setup.
    Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
    http://sanfrancisco.ibtimes.com/arti...ger-photos.htm

  3. #3
    just pokin' along desertdork's Avatar
    Join Date
    Aug 2008
    Location
    the desert
    Posts
    1,071
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I have a somewhat different bike (steel frame, triple crank) that presented me with a similar situation. Due to the low bottom bracket, the FD cage comes very close to the chainstay when on the smallest ring. There's enough space for a credit card to fit...and just a smidge more. If it's true that a credit card measures 0.7mm (based on another member's statement), then I might have 1.0 - 1.25mm clearance. That's just a reasonable guess, though. The cage has never marked the frame, and I have no reason to believe it has touched it.

    Looking at your pictures, it appears that (a) you have wiggle room to raise the dérailleur a tad, and (b) the edge of the cage just overlaps the edge of the chainstay. Given that the rings only have a 12t difference, am I wrong in thinking that shifting will not be noticeably affected if you raise the dérailleur 1 - 2mm? And if this still failed to provide adequate clearance, perhaps a spacer under the drive side bb cup would help, though then chainline may not be as precise.

    The first thing I'd want to do is be certain the cage is angled properly. This requires the chain on the bike, in addition to the rest of the drivetrain.

    I suspect that the area near the bb is stiff enough that it would be rideable with the minimal clearance you state, but the cautious side of me would still want to bump the FD up as long as shifting remained good. Then again, if the owner would really rather be happier with a 50/34, then none of this is to bother with.

  4. #4
    bike whisperer Kimmo's Avatar
    Join Date
    Dec 2009
    Location
    Melbourne, Oz
    My Bikes
    copy/paste links: http://velospace.org/node/36949 http://velospace.org/node/47746 http://velospace.org/node/47747
    Posts
    6,842
    Mentioned
    9 Post(s)
    Tagged
    0 Thread(s)
    IMO that's almost certainly enough of a gap.

    If the derailleur is adjusted to shift optimally in that position, I'd just stick a little blob of plasticine in the chainstay and take it for a hard ride, then inspect the blob to see how thin it got squashed. I'm betting the gap wouldn't even be halved.

  5. #5
    Gear Combo Guru Chris_W's Avatar
    Join Date
    May 2007
    Location
    Switzerland
    My Bikes
    Spec' Tarmac (road), Spec' Secteur Disc (commuter & tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder), a train pass, and NO car :)
    Posts
    2,090
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I would go with the option of moving the FD up by 1 mm, the effect on shifting may not even be noticeable.

  6. #6
    Senior Member
    Join Date
    Oct 2008
    Location
    Chicago, IL
    My Bikes
    S-Works Roubaix SL2^H4, Secteur Sport, TriCross, Kaffenback, Lurcher 29er
    Posts
    2,404
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Thanks all -- I'll give moving the FD up a smidge a go.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •