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  1. #1
    Senior Member surfengine's Avatar
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    Front Derailleur size

    I have a Trek2200 frame that has a clamp and I want to buy a front derailleur but not sure how to determine what size.
    I see talk of 31.8 and 34...what do these numbers mean?
    Do those numbers matter since i already have a clip?

  2. #2
    Low car diet JiveTurkey's Avatar
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    Do you have something like this: http://www.nashbar.com/bikes/Product...category=10000 already on your frame? Or maybe the part that attaches to the front derailleur is directly brazed onto the frame?

    If either is so, you need a front derailleur meant for a braze-on, like: http://www.nashbar.com/bikes/Product..._200276_200379

    If the braze-on clamp can be removed, you can either get the braze-on clamp and braze-on FD, or just buy a traditional FD with the clamp built-in. In which case you need to determine the clamp size.

    The clamp size is the diameter of your seat tube where it would clamp. For aluminum frames, that is typically 31.8mm (1-1/4") or 34.9mm (1-3/8"). You can measure with calipers. If you don't have calipers, wrap a string around the seat tube to get the Circumference (it's easiest to work in mm). Diameter = Circumference / 3.14. If the string is about 100mm, you need 31.8mm clamp; if the string is about 110mm, you need 34.9mm clamp.
    Last edited by JiveTurkey; 05-14-09 at 08:39 PM.
    Quote Originally Posted by slopvehicle View Post
    Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.

  3. #3
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    The numbers are the diameter of the frame's seat tube and the required diameter of the front derailleur's clamp. Round seat tubes are almost all 28.6 mm (1-1/8"), 31.8 mm (1-1/4") or 34.9 mm (1-3/8") in diameter and these are the size choices for clamp-on front derailleurs or the clamp-on adapters used with braze-on front derailleurs.

    If by "clip" you mean you already have the clamp-on adapter, all you need is a braze-on style front derailleur.

  4. #4
    Senior Member surfengine's Avatar
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    thanks guys...good info

    see pictures for the type of clamp I have.
    It looks similar to the one you pointed out, but not exactly the same.

    I measured 4 3/8" which came to 111.125mm. So I guess that means I have the 34.9mm.

    What do you guys think? Will my clamp support the braze on of a Shimano 6600 front derailleur?
    Attached Images Attached Images

  5. #5
    Low car diet JiveTurkey's Avatar
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    Hmm, I'm not super familiar with braze-on derailleurs, but though it doesn't look exactly like the one I linked, it appears to be of similar design. The FD attaches to the back of the clamp (visible in the first picture).

    You can get a braze-on FD, or a 34.9mm FD. If you go with the latter, be certain you measured correctly (the clamp you have may actually have the size stamped on the inside of it).
    Quote Originally Posted by slopvehicle View Post
    Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.

  6. #6
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    Your Trek is almost certainly 34.9 mm as all of their similar Al and Carbon/Al frames were. The adapter you show is a Shimano braze-on adaptes so any Shimano braze-on fd should fit fine. As JiveTurkey noted, the derailleur mounts to the back of the clamp.

  7. #7
    Senior Member Grand Bois's Avatar
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    I'd take that ugly clamp off and buy the appropriate size clamp on derailer.

  8. #8
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    Quote Originally Posted by Dirtdrop View Post
    I'd take that ugly clamp off and buy the appropriate size clamp on derailer.
    Until very recently, Shimano didn't make any 34.9 mm clamp-on front derailleurs, at least not for OTC sales (they did make some specifically for Trek). They made 28.6 and 32.8 mm clamp-ons but 34.9 seat tubes required an adapter clamp and use of a braze-on fd.

    There is nothing wrong with the OP's adapter except it's badly discolored, possibly from salt exposure. A new one would look nicer.

  9. #9
    Great State of Varmint Panthers007's Avatar
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    Yep ^^^. A little cleaning and some metal-polish would make it shine like new - or better. I've found that recently - if you order the largest clamp-on size - 34.9mm - FD, Shimano will often include a clamp also for 31.8mm and 28.6mm seat-tubes. This means that many bike-shops likely have a barrel of them somewhere. Call around if anyone needs one.
    Quote Originally Posted by Cateye View Post
    Only panthers007 is stupid enough to believe that this is a good idea.

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