Sachs 2 speed torpedo overdrive ratio

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

1. meb New Member

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Aug 21, 2003
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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. Ryan Cousineau Guest

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 New Member

Joined:
Aug 21, 2003
Messages:
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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. Michael Press Guest

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
> 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