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  1. #1
    Senior Member triplebutted's Avatar
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    Why the XTRA large jockey wheels on some rear derailleurs?

    May not be Vintage related but I may need to put a long cage on my Centurion so I can ride it with a 32 or 34t freewheel (doing a ride that has a 20% grade). I stripped a mtb bike of parts and there was a Shimano Acera or something with one of the pulley wheels that was twice the size! What's up with that?
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  2. #2
    Bianchi Goddess Bianchigirll's Avatar
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    I think it has something to do with taking up more chain for the wide spaced cassettes and compact cranks
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  3. #3
    Senior Member sauze's Avatar
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    Like this right?

    I had always assumed it was a way to improve capacity for RD's without lengthening the cage more , but you should take everything I say with a fairly giant grain of salt.

  4. #4
    Senior Member triplebutted's Avatar
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    Yah, kinda looks like that. I guess I'll try it. Can't hurt right?
    197? Mercier
    197? Gios Torino
    1984 Centurion Comp TA
    1984 Lotus Legend Compe (year corrected by Snydermann, thanks!!)
    1996 Lemond Zurich
    2001 Trek 8000 MTB
    2006 Fuji Tourer
    2009 Voodoo Rada

  5. #5
    Iconoclast rat fink's Avatar
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    +1 For chain-wrap.
    "Winning is the best deodorant. Someone can look at your bike and say it stinks, but if you win with it, suddenly it's okay." - Jim Busby

  6. #6
    Curmudgeon in Training 20grit's Avatar
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    For more drillium room on metal wheels.

  7. #7
    Wrench Savant balindamood's Avatar
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    I have seen some directions on 80's Suntour derailluers that swapped both the upper an lower jockey wheels sizes about based upon total teeth and cog sizes. I have never worried about it personnally, but apparently Suntour thought about it and tried to fine-tune it.
    "Where you come from is gone;
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  8. #8
    Holy Spokes it's Batsman! Glennfordx4's Avatar
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    Shimano uses the larger jockey pulleys on all their Mega Range designed RD's that run the 34 tooth Mega Range cog. That's the only place I have seen them in use and have a few new ones in my bins, these are only used on low end RD's.


    So Many Bikes Too Little Space

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  9. #9
    Port Rocket-Sauce's Avatar
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    Oversized derailer pulleys were the must-have accessory in the pro peloton in 2010.

    By then end of the summer, they were everywhere. Developed by Berner and then copied by everyone else, they are said to noticeably reduce drive-train friction by allowing less severe chain bends

    David Millars TT bike 2010:



    Frändy Schleck's Specialized Tarmac 2010:



    And Lance's TT Bike:

  10. #10
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    FYI, only the top pulley is a jockey pulley. The proper name of the bottom pulley is a tensioner or toggle pulley. Increasing the diameter of the toggle pulley wraps wraps about 50 -66% more chain than increasing the length of the cage by the same amount. The actual percentage will vary depending on the cage geometry. Thus you can increase wrap without decreasing ground clearance as much (an issue with ATBs in off-road conditions). A dirty chain is also less likely to bind and skip on a bigger pulley. It also reduces the deflection of a longer cage. Finally, it looks different and this must not be underestimated.

  11. #11
    bike whisperer Kimmo's Avatar
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    I dig the larger pulleys cause they're quieter.

  12. #12
    old and fixed... clubman's Avatar
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    Quote Originally Posted by balindamood View Post
    I have seen some directions on 80's Suntour derailluers that swapped both the upper an lower jockey wheels sizes about based upon total teeth and cog sizes. I have never worried about it personnally, but apparently Suntour thought about it and tried to fine-tune it.
    The 3 pulley Suntours had a mixed bag, 2 big and one small.


  13. #13
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    Oversize pulleys are also becoming the rage on current high end racing derailleurs on the claim that the looser chain wrap increases efficiency. I wonder if it will herald a reversal to 14T small cogs and correspondingly larger chainrings? With carbon fibre frame technology being what it is, they already have to ballast some frames to meet UCI minimum weight requirments, so marginally heavier components are no longer a big concern.

  14. #14
    Ride, Wrench, Swap, Race dddd's Avatar
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    When the first Dura-Ace triple gruppo debutted in the 9-speed era, they used oversized pulleys with a cage that was considerably shorter than the comparable Ultegra or 105 triple derailers had.

    One thing about the pulley friction, besides how sharp that the chain bends, is that there is a ratio of diameters between the sleeve diameter (where rubbing occurs) and the working diameter (where the chain passes).

    The greater is this ratio, the lower the energy losses from friction, as the frictional force at the sleeve is leveraged by the greater diameter of the pulley vs. the diameter of the sleeve.
    And, with a resulting lower rotation speed, the increase in that diameter ratio further improves efficiency, now to the second power (or "x-squared").

    This ratio also argues for smaller diameter sleeves or for rolling-element bearings, one reason that I somewhat dis-like most Suntour and SRAM derailers.
    The Suntour pulleys I can replace with (6mm bolt-hole diameter) ball-bearing pullies (from Huret or the aftermarket), but SRAM's design uses a huge-diameter sleeve in the top pulley position that is elemental to the derailer's design.
    Last edited by dddd; 12-13-12 at 08:50 PM.

  15. #15
    Senior Member RobbieTunes's Avatar
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    Quote Originally Posted by clubman View Post
    The 3 pulley Suntours had a mixed bag, 2 big and one small.

    I just came across one of those today in the parts box of a local bike shop....pretty cool.

    Robbie ♪♫♪...☻

    Perhaps you didn't really hear what you thought I said...
    ...or maybe you did, and that's why you're so mad.


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