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  1. #1
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    "stupid question regarding front derailers"

    I'm buying a new front derailer.........what does it mean or what is the difference regarding "28.6mm", "31.8mm " or "34.9mm" Top/Down? Is that the travel?

    FYI; I'm changing the cassette to a 9 speed and upgrading my wheels, cranks, both derailers and shifters

  2. #2
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    Numbers refers to the clamp size for the seat tube.

    Top/ Down refers to how the cable runs to actuate the derailleur.

  3. #3
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    Good question
    The numbers you are referring to are the seat tube diameters . If it says 31.6mm the FD will fit that tube. Shims are needed for anything smaller.
    The "top" and Bottom" refer to two different things -

    Top/Bottom Pull
    Bottom Pulls - the cable comes up from under the Bottom Bracket to the Derailleur. This setup is usually found on road bikes.
    Top Pulls -The cable runs down the seat tube. This design is better for MTB's because it gets the cable out of harms way.

    Top/Bottom Swing
    This is determined from which side of the clamp the cage swings from (top or bottom)
    Bottom swings didn't used to be a problem until rear suspensions got in the way - the solution... drop the clamp below the cage closer to the BB - Top Swing.

    There are also brazed mounts (welded tabs) and Bottom Bracket mounts too
    Honestly... ahh I give up

  4. #4
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    Thanks for education. I'm certain my seat tube is 34.9 or 31.8mm, so there shouldn't be any problems.

  5. #5
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    Quote Originally Posted by SoCalGuero
    Thanks for education. I'm certain my seat tube is 34.9 or 31.8mm, so there shouldn't be any problems.
    If you are unsure though, take off your seat post And clamp and measure the OUTSIDE diameter w/ a metric ruler. Nothin worse than gettin something and finding it's off by half a centimeter
    Honestly... ahh I give up

  6. #6
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    Quote Originally Posted by highdelll
    If you are unsure though, take off your seat post And clamp and measure the OUTSIDE diameter w/ a metric ruler. Nothin worse than gettin something and finding it's off by half a centimeter
    I'm confused. He'd want to measure the outside diameter of the seat tube, no? No need to take out/off the seat post and clamp. Or am I misunderstanding your post? Edit: I see what you're saying. He should measure the OD of the seat tube after removing the post and clamp. That makes much more sense than the way I initially read your answer. Just make sure the seat tube is not tapered.

  7. #7
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    Quote Originally Posted by tvrbob86
    I'm confused. He'd want to measure the outside diameter of the seat tube, no? No need to take out/off the seat post and clamp. Or am I misunderstanding your post? Edit: I see what you're saying. He should measure the OD of the seat tube after removing the post and clamp. That makes much more sense than the way I initially read your answer. Just make sure the seat tube is not tapered.
    Ahhh yes! I should've more clear.
    tvrbob is correct (so was i - haha -just not clear)
    *OD of the seat TUBE
    if it's tapered measure it w/ calipers at the clamp location - mechanics will have a set (auto or bicycle)
    Honestly... ahh I give up

  8. #8
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    I had actually ordered the 34.9mm and may need to use shims, is that alright?

  9. #9
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    [SIZE="3"]I'm fairly certain it's the 34.9 .......I guess I'll recheck when I get home tonight.[/SIZE]

  10. #10
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    Quote Originally Posted by SoCalGuero
    I had actually ordered the 34.9mm and may need to use shims, is that alright?
    yes. hopefully you wont need 'em - just more pieces to mess with is all
    if you do need 'em, your new FD should come w/ a variety
    Honestly... ahh I give up

  11. #11
    ~Disc~Golf~
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    a side note...
    i don't know if this applies to you, but it did affect me.
    I got a new frame, the seat tube was wider, which pushed out the FD cage with regards to BB spindle width. Consequently, I had to get a longer spindle to accommodate my 3-ring setup. With the FD 'fully-relaxed', the cage could not move 'inline' enough to catch the smallest chain-ring.
    The wider spindle did affect my Q-factor, but i like it...feels more stable to me
    Just a thought
    Honestly... ahh I give up

  12. #12
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    Quote Originally Posted by highdelll
    Ahhh yes! I should've more clear.
    tvrbob is correct (so was i - haha -just not clear)
    *OD of the seat TUBE
    if it's tapered measure it w/ calipers at the clamp location - mechanics will have a set (auto or bicycle)
    OP--My first question is why replace the front derailleur at all? Was the old one not working? Your old one was probably fine. Hopefully you ordered the right one in all respects...

    highdell, auto or bicycle calipers? How about metric vs English measure? I use my calipers for all kinds of stuff aside from bike or auto...
    "...the people get the government they deserve..."
    suum quique

  13. #13
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    Quote Originally Posted by Bikinfoolferlife
    OP--My first question is why replace the front derailleur at all? Was the old one not working? Your old one was probably fine. Hopefully you ordered the right one in all respects...

    highdell, auto or bicycle calipers? How about metric vs English measure? I use my calipers for all kinds of stuff aside from bike or auto...
    i thought it would be understood to use metric - what do you mean by 'english'?? SAE?
    auto or bicycle was referring to the type of mechanic. I know in many areas an auto mechanic might be closer than a bike shop, so it may be quicker to go an get it measured if you didn't have the tools.
    man, i gotta start being alot more explicit
    Honestly... ahh I give up

  14. #14
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    Quote Originally Posted by highdelll
    i thought it would be understood to use metric - what do you mean by 'english'?? SAE?
    auto or bicycle was referring to the type of mechanic. I know in many areas an auto mechanic might be closer than a bike shop, so it may be quicker to go an get it measured if you didn't have the tools.
    man, i gotta start being alot more explicit
    By English measure I meant inches/feet, what I learned to call it growing up. Then I looked at this http://en.wikipedia.org/wiki/English_unit and now don't know if I should continue calling it that, but lots shorter than US Customary Units. Don't think SAE works, though. I don't know if metric is a good assumption, many seat tube diameters are actually conversions from inches (34.9 mm I'm pretty sure is just a conversion of 1-3/8 inches, 31.8 mm from 1-1/4 inch, for example). I bought a set of USCU type dial calipers a while back and have found them useful for a variety of measuring needs.
    "...the people get the government they deserve..."
    suum quique

  15. #15
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    Quote Originally Posted by Bikinfoolferlife
    By English measure I meant inches/feet, what I learned to call it growing up. Then I looked at this http://en.wikipedia.org/wiki/English_unit and now don't know if I should continue calling it that, but lots shorter than US Customary Units. Don't think SAE works, though. I don't know if metric is a good assumption, many seat tube diameters are actually conversions from inches (34.9 mm I'm pretty sure is just a conversion of 1-3/8 inches, 31.8 mm from 1-1/4 inch, for example). I bought a set of USCU type dial calipers a while back and have found them useful for a variety of measuring needs.
    Yeah, SAE isn't technically correct either. US Standard is more appropriate. And you are correct that it probably is a conversion from say: 1-3/8". But most sizes aren't listed that way - and yet some are -
    I say we drop US Standard all-together and go metric - It just makes more sense. Besides the US, the only countries IN THE WORLD that still use US standard are Liberia and Myanmar.
    Honestly... ahh I give up

  16. #16
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    Quote Originally Posted by highdelll
    Yeah, SAE isn't technically correct either. US Standard is more appropriate. And you are correct that it probably is a conversion from say: 1-3/8". But most sizes aren't listed that way - and yet some are -
    I say we drop US Standard all-together and go metric - It just makes more sense. Besides the US, the only countries IN THE WORLD that still use US standard are Liberia and Myanmar.
    I didn't even know about Liberia or Myanmar! I can't believe we still haven't at least started to make some sort of switch...but at least a lot of kids are getting more exposure than I got...
    "...the people get the government they deserve..."
    suum quique

  17. #17
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    Quote Originally Posted by Bikinfoolferlife
    I didn't even know about Liberia or Myanmar! I can't believe we still haven't at least started to make some sort of switch...but at least a lot of kids are getting more exposure than I got...
    yeah...
    we tried to switch before, (Carter in the '70's??) but i guess people were scurrd of change
    ahhh ..makes much more sence/easier with a base-10 system!
    Honestly... ahh I give up

  18. #18
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    Quote Originally Posted by Bikinfoolferlife
    OP--My first question is why replace the front derailleur at all? Was the old one not working? Your old one was probably fine. Hopefully you ordered the right one in all respects...

    highdell, auto or bicycle calipers? How about metric vs English measure? I use my calipers for all kinds of stuff aside from bike or auto...
    Long story..........I snapped my old shifters and replaced them with rapid fires. My local shop said, "dude...you should convert your 8 speed cassette to a 9 speed". Why not? since he gave me a killer deal on the cassette, new chain and shifters. I tried to change them and I realized the chain was smaller and required new cranks and back derailleur (it kept jumping off the spockets). Being alittle anal, I ordered the matching front derailleur and it gave me these "sizes" that are in question. I ordered a "28.6-34.9 Dual Pull" so I'm certain there shouldn't be any problems.

    Instead of buying a new bike, I essentially upgraded all my components and cranks (and saved $$$$$).

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