Sachs 2 speed torpedo overdrive ratio

Discussion in 'Cycling Equipment' started by meb, Aug 27, 2006.

  1. meb

    meb New Member

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    I have a folding bike with a Sachs 2 speed overdrive hub. The ratio doesn't appear on Sheldon's site for the 2 speed, although the 3 speed is shown there with a 1.36:1 ratio. I've taped the sprocket and hub and counted revolutions and it appears to be 1.36:1 as in the 3 speed. Can anyone confirm this 1.36:1 ratio for the 2 speed?

    FWIW: the bike has 203-62 tires and a 100T beltring and 30T sprocket, so if that 1.36:1 ratio is correct, would put high gear at 59 gear inches over the 42 gear inch low.
     
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  2. In article <[email protected]>,
    meb <[email protected]> wrote:

    > I have a folding bike with a Sachs 2 speed overdrive hub. The ratio
    > doesn't appear on Sheldon's site for the 2 speed, although the 3 speed
    > is shown there with a 1.36:1 ratio. I've taped the sprocket and hub
    > and counted revolutions and it appears to be 1.36:1 as in the 3 speed.
    > Can anyone confirm this 1.36:1 ratio for the 2 speed?
    >
    > FWIW: the bike has 203-62 tires and a 100T beltring and 30T sprocket,
    > so if that 1.36:1 ratio is correct, would put high gear at 59 gear
    > inches over the 42 gear inch low.


    I don't know either (I have a Duomatic, the fun kickback-shifter), but
    your tape test sounds pretty decisive.

    You might get more accurate results with a rollout test. The easy way to
    measure the exact rolling circumference of the tire is to put a drip of
    paint on the ground and ride over it. The next paint-mark will give you
    the circumference.

    --
    Ryan Cousineau [email protected] http://www.wiredcola.com/
    "I don't want kids who are thinking about going into mathematics
    to think that they have to take drugs to succeed." -Paul Erdos
     
  3. meb

    meb New Member

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    I guess a rollout might be the way to get some more precision. However, wouldn't the tire, particularly a small diameter 203, be prone to significant dynamic changes in diameter due to loading, pressure changes, wear- more so than a 20/26/700C tire?

    This one is also a kickback shifting. More sensitive than my preference- a fraction of a cm on the pedals changes gears-so I get a lot of premature upshifts and downshifts. At least it's easy to correct the involuntary shifts.

    Thanks.
     
  4. In article <[email protected]>,
    meb <[email protected]> wrote:

    > Ryan Cousineau Wrote:
    > > In article <[email protected]>,
    > > meb <[email protected]> wrote:
    > >
    > > > I have a folding bike with a Sachs 2 speed overdrive hub. The ratio
    > > > doesn't appear on Sheldon's site for the 2 speed, although the 3

    > > speed
    > > > is shown there with a 1.36:1 ratio. I've taped the sprocket and hub
    > > > and counted revolutions and it appears to be 1.36:1 as in the 3

    > > speed.
    > > > Can anyone confirm this 1.36:1 ratio for the 2 speed?
    > > >
    > > > FWIW: the bike has 203-62 tires and a 100T beltring and 30T

    > > sprocket,
    > > > so if that 1.36:1 ratio is correct, would put high gear at 59 gear
    > > > inches over the 42 gear inch low.

    > >
    > > I don't know either (I have a Duomatic, the fun kickback-shifter), but
    > > your tape test sounds pretty decisive.
    > >
    > > You might get more accurate results with a rollout test. The easy way
    > > to
    > > measure the exact rolling circumference of the tire is to put a drip
    > > of
    > > paint on the ground and ride over it. The next paint-mark will give
    > > you
    > > the circumference.
    > >
    > > --
    > > Ryan Cousineau [email protected] http://www.wiredcola.com/
    > > "I don't want kids who are thinking about going into mathematics
    > > to think that they have to take drugs to succeed." -Paul Erdos

    >
    > I guess a rollout might be the way to get some more precision.
    > However, wouldn't the tire, particularly a small diameter 203, be prone
    > to significant dynamic changes in diameter due to loading, pressure
    > changes, wear- more so than a 20/26/700C tire?


    Roll-out distance depends on load and tire pressure:

    A 622x25 wheel, about 780 mm in diameter.

    Rider and bicycle mass: 80 kg
    Wheel circumference: 2125 mm
    Roll out, 120 psi: 2107 mm
    Roll out, 90 psi: 2099 mm

    --
    Michael Press
     
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