Chain length stuff

I use old Torpedo hubs on my bikes, which these days have rather
short rear forks. It is always a problem when I change to a different
rear sprocket, to find the right chain length so that the rear axle
is still within the fork length. With these hubs, the chain has to
be tight.

Is there a formula for calculating the length between the bottom
bracket and the rear axle for a given chain length and number of
teeth on the sprocket? Or, better still, the change in that length
when I change to a sprocket with a different number of teeth? That
would tell me how many chain links I have to add or subtract when I
change to a different sprocket, so as to still be within the fork
travel.

I thought along the lines, if I add one tooth, I must add one chain
link (pair), but that doesn't work, probably because an extra tooth
also changes the sprocket's diameter.

As an aside, I have a small collection of these hubs and they are now
about 70 years old. When I get one, it is usually rusty, but when I
open it up, it is perfect inside. They don't make them like that any
more.
--
Dieter Britz

On Sat, 20 Mar 2021 08:07:52 +0000, Dieter Britz wrote:

I use old Torpedo hubs on my bikes, which these days have rather short
rear forks. It is always a problem when I change to a different rear
sprocket, to find the right chain length so that the rear axle is still
within the fork length. With these hubs, the chain has to be tight.
[...]

Sorry, I found a good site that tells me all I need to know.

--
Dieter Britz

On 3/20/2021 3:07 AM, Dieter Britz wrote:
I use old Torpedo hubs on my bikes, which these days have rather
short rear forks. It is always a problem when I change to a different
rear sprocket, to find the right chain length so that the rear axle
is still within the fork length. With these hubs, the chain has to
be tight.

Is there a formula for calculating the length between the bottom
bracket and the rear axle for a given chain length and number of
teeth on the sprocket? Or, better still, the change in that length
when I change to a sprocket with a different number of teeth? That
would tell me how many chain links I have to add or subtract when I
change to a different sprocket, so as to still be within the fork
travel.

I thought along the lines, if I add one tooth, I must add one chain
link (pair), but that doesn't work, probably because an extra tooth
also changes the sprocket's diameter.

As an aside, I have a small collection of these hubs and they are now
about 70 years old. When I get one, it is usually rusty, but when I
open it up, it is perfect inside. They don't make them like that any
more.


There's no simple rule. Fit the chain on your bike.

Also, and this is important, don't over tighten your chain:
http://www.yellowjersey.org/chainchk.html

All the various components are nearly but not exactly round
and concentric, such that chain tension varies as the system
turns. A chain tight spot will greatly increase bearing
wear at hub and crank.

With minimally one inch of frame slot adjustment, you can
achieve proper chain tension. There are half links for 1/8"
chain if you feel one would be helpful:
http://www.yellowjersey.org/photosfr...t/HALFLINK.JPG

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