Sachs 2 speed torpedo overdrive ratio

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.


--
meb

In article ,
meb 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 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

Ryan Cousineau Wrote:
In article ,
meb 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 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?

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.


--
meb

In article ,
meb wrote:

Ryan Cousineau Wrote:
In article ,
meb 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 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 pressu

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