Advertise on Bikeforums.net



User Tag List

Results 1 to 5 of 5
  1. #1
    Senior Member ekimyel's Avatar
    Join Date
    Mar 2006
    Location
    Western PA
    Posts
    383
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    ? about changing chainrings

    I just ordered a Motobecan Fantom Cross Pro. It comes with a TruVativ GXP External Outboard crank with 48-38 chainrings. I would like to make it a compact 50-34. Can I just change the chainrings? It comes with a Shimano Ultegra 6600 front deraileur. Will I need to get a compact deraileur? I'm using this bike as my bad weather road bike more or less. Will use it on some rails to trails, and possibly a little bit of single track. I live in an extremely hilly area, and I have a compact on my road bike. I'd like to have the same gearing on this bike, but don't really want to buy an new compact crank unless I have to. Thanks for any advice....
    "Life ain't about how many breaths you take. It's what you do with those breaths"... (John Challis)

    2007 Specialized Roubaix Pro
    2008 Electra Straight 8

  2. #2
    Soma Lover
    Join Date
    Dec 2004
    Location
    Logan, UT
    My Bikes
    one bike for every day of the week
    Posts
    765
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    The 50-34 chainrings have a different bolt circle diameter than the 48-38 crankarms. IMHO, I think a mid-cage mountain rear derailleur and an 11-32 cassette would be cheaper and work better than a new crankset and bottom bracket. Shifting across span of 31 total teeth (50-34+27-12) with a road double rear derailleur generally works but it can be temperamental. The Shimano CYA maximum is only 29 teeth. I don't know how well a standard 20-speed front derailleur works with a compact crank but my 18-speed setup works just fine.

  3. #3
    Senior Member ekimyel's Avatar
    Join Date
    Mar 2006
    Location
    Western PA
    Posts
    383
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Good suggestion, except I want to be able to swap these wheels over onto my road bike in case I have a problem with the wheels on that bike. Ahh, maybe I don't, I guess if i have a problem that bad, i can just swap the cassettes.

    What derailleur \ cassette do you recommend? I don't have much experience at all with mountain bike components. Thanks for your help...
    "Life ain't about how many breaths you take. It's what you do with those breaths"... (John Challis)

    2007 Specialized Roubaix Pro
    2008 Electra Straight 8

  4. #4
    Senior Member
    Join Date
    Apr 2005
    Location
    MD
    Posts
    773
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Aren't mountain derailleur's 9sp and the current Ultegra's 10sp?

    Road cranks are 130BCD or 110BCD. 130 is the standard and 110 is the compact. 130 can go as small as 38T. So it depends on what BCD your cranks are. I suspect your stock cranks are 130, so unless you swap them out for a 110, you won't be able to install a 34T. Seems like an expensive upgrade. I find a 38T chainring and a 27T cog to be plenty of gearing. If I can't get up that hill, then I should be running.

    As for a compact FD. I think it's mostly marketing. The only place I can think of where it would help is if you're running a small big ring (say 42/44) and the derailleur cage is too long to clear the chain stay while being low enough on the seat tube to be just a penny's width above the big ring.

  5. #5
    Soma Lover
    Join Date
    Dec 2004
    Location
    Logan, UT
    My Bikes
    one bike for every day of the week
    Posts
    765
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by bitterken
    Aren't mountain derailleur's 9sp and the current Ultegra's 10sp?
    http://store.interlocracing.com/10elcas.html

    They don't shift as well as the high-end Shimano cassettes but I'm happy with their 9-speed 11-28 cassette. It shifts about as well as a Shimano HG50 or HG70. I'd be inclined to believe that if you're careful to stay within the Shimano CYA 33-tooth limit (48-38+32-11=31 teeth) that a 9-speed mid-cage mountain derailleur would shift it just fine.

Posting Permissions

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