Re: Misc TA Crankset questions

Discussion in 'Cycling Equipment' started by Sheldon Brown, Oct 15, 2004.

  1. Retro Bob wrote:

    > On Sheldon's site, it says that you want to do a half step plus
    > granny" setup with this TA crank. It infers (if I'm reading it right)
    > that it will be difficult to shift the small to medium wheel if the
    > medium is not close in size to the large (outer) chainwheel.
    >
    > http://sheldonbrown.com/harris/french-cranks.html#cyclotouriste
    >
    > I'm having trouble grasping this, or I'm just reading it wrong.
    > Why would the medium to large size chainwheel relationship
    > impact the small to medium shift ? I would think that you'd want
    > the medium closer to the middle of the range for the smoothest
    > shifting from the small to medium.


    It has to do with front derailer design.

    With all derailer shifts, shifting is better if the derailer is working
    close to the sprocket involved, so there's only a short length of
    straight chain between the derailer and the sprocket. That way, you
    create a greater sideways chain angle for a given amount of sideways
    chain movement.

    Upshifts are accomplished by the inside plate of the front derailer
    cage. The lower down that inside plate hangs (as long as it doesn't get
    so low as to foul the chainring teeth) the better it will shift.

    Modern "triple" front derailers have inner plates that hang down much
    lower than the outer plate, and this lets them make smooth shifts from
    small to middle ring on evenly-spaced triples. These derailers are
    generally optimized for about a 10 tooth difference between middle and
    big ring.

    There's a catch, though. Those "triple" derailers also have very
    3-dimensional _outer_ plates, to let clueless newbies ride in their
    small-small gears without rubbing.

    The TA Cyclotouriste crank has unusually narrow tread ("Q factor") so it
    does not have sufficient clearance between the big ring and the arm to
    permit this type of derailer to work. If you get it adjusted for good
    shifting, the arm will bump into the bottom of the cage every revolution.

    The TA cyclotouriste crank _does_ work quite nicely with "double" type
    front derailers, but "double" front derailers don't have the low-hanging
    inner plate. This type of derailer does not handle an evenly spaced
    triple well, but works fine with a (yuck!) half-step-plus-granny.

    My favorite way to use the TA Cyclotouriste is as a super wide-range
    double. For instance, the 50-28 on my Hetchins:

    http://sheldonbrown.org/hetchins

    Sheldon "Always A Catch" Brown
    +-------------------------------------------------------------+
    | Want of care does us more damage than want of knowledge. |
    | -- Benjamin Franklin |
    +-------------------------------------------------------------+
    Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041
    http://harriscyclery.com
    Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
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  2. On 2004-10-15, Sheldon Brown <[email protected]> wrote:

    > The TA Cyclotouriste crank has unusually narrow tread ("Q factor") so it
    > does not have sufficient clearance between the big ring and the arm to
    > permit this type of derailer to work. If you get it adjusted for good
    > shifting, the arm will bump into the bottom of the cage every revolution.
    >
    > The TA cyclotouriste crank _does_ work quite nicely with "double" type
    > front derailers, but "double" front derailers don't have the low-hanging
    > inner plate. This type of derailer does not handle an evenly spaced
    > triple well, but works fine with a (yuck!) half-step-plus-granny.


    I seem to recall that some of the higher end Simplex front derailleurs had
    a plate that bolted behind the inner cage to add more drop when used on a
    triple setup.

    --

    -John ([email protected])
     
  3. I wrote:

    >>The TA Cyclotouriste crank has unusually narrow tread ("Q factor") so it
    >>does not have sufficient clearance between the big ring and the arm to
    >>permit this type of derailer to work. If you get it adjusted for good
    >>shifting, the arm will bump into the bottom of the cage every revolution.
    >>
    >>The TA cyclotouriste crank _does_ work quite nicely with "double" type
    >>front derailers, but "double" front derailers don't have the low-hanging
    >>inner plate. This type of derailer does not handle an evenly spaced
    >>triple well, but works fine with a (yuck!) half-step-plus-granny.

    >

    John Thompson wrote:
    >
    > I seem to recall that some of the higher end Simplex front derailleurs had
    > a plate that bolted behind the inner cage to add more drop when used on a
    > triple setup.


    I remember SunTours with that Disney rodent feature, Cyclone, I believe.

    However this is irrelevant to the TA Cyclotouriste issue. The purpose
    of the extension was to permit a wider range without causing the chain
    to rub on the bottom of the derailer's cage.

    It didn't address the issue of shifting from small to middle on an
    evenly-spaced triple chainring setup, only the overall tooth difference.

    Sheldon "Ancient History" Brown
    +--------------------------------+
    | One does not win at chess by |
    | seizing every opportune pawn |
    | -- Michael Flynn |
    +--------------------------------+
    Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041
    http://harriscyclery.com
    Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  4. >> I seem to recall that some of the higher end Simplex front
    >> derailleurs had a plate that bolted behind the inner cage
    >> to add more drop when used on a triple setup.

    >
    > I remember SunTours with that Disney rodent feature, Cyclone,
    > I believe.


    Yep. Long ago, when I converted my touring bike from a double to a
    triple, I continued to use the same Cyclone with the addition of that
    Disney feature. With a TA crank, the conversion was cheap -- new bolts &
    spacers, a granny chainwheel, and the Cyclone extension -- and it worked
    approximately "okay". Worst of all, those parts are very likely still in
    my garage, in a box somewhere.

    --
    "Bicycling is a healthy and manly pursuit with much
    to recommend it, and, unlike other foolish crazes,
    it has not died out." -- The Daily Telegraph (1877)
     
  5. A Muzi

    A Muzi Guest

    John Thompson wrote:
    > I seem to recall that some of the higher end Simplex front derailleurs had
    > a plate that bolted behind the inner cage to add more drop when used on a
    > triple setup.
    >

    Suntour Cyclone fronts had such a cage extender option

    --
    Andrew Muzi
    www.yellowjersey.org
    Open every day since 1 April, 1971
     
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