Ted said:Peter said:The tension on the typical tandem synch chain is entirely determined by
the force exerted on the pedals by the front rider and the relative
sizes of the front crank vs. timing chainring. What the rear rider does
and how well he/she is working together with the person in front doesn't
affect the synch chain.
Aren't the chainrings on the timing side always the same size? You are
implying that they can differ, but that would cause continually changing
phase.
No, I'm not implying that at all. The front rider pushes down on the
pedal with a lever arm equal to the crank length, say 170mm, while the
timing chain is attached at a lever arm distance equal to the radius of
its chainring, say that's 85mm. The tension in the timing chain will
then be the force exerted by the rider on the pedal multiplied by the
ratio of the lever arm lengths (in this case 2:1).
BTW, we did actually use different size timing rings (40 and 38) for
awhile with a kid-back arrangement on our tandem. It did mean that my
daughter and I had a slowly changing phase but with her lighter weight
and power it wasn't an issue.
Quoted message said:
If the stoker does not "work together" with the captain it certainly
does affect the synch chain, because there will be force variations
which even act in opposition at times.
Even if the stoker were to actively try to pedal backwards, the timing
chain tension would still be determined by how hard the front rider is
pushing down on his/her pedal. Note that if the front rider takes
his/her feet off the pedals there's no way for the stoker to create any
significant tension in the timing chain since the front crank will just
spin freely. [Assuming the usual tandem arrangement where the drive
chain is engages the stoker's chainring.]
Quoted message said:
It's also important to note that the timing rings are not really
coplanar when a tandem is ridden, so there is a little cross-chaining.
The bottom tube is bent along its axis under the application of power,
as is evident by observation of chain sag. In extreme cases, I have
seen a properly adjusted timing chain come off a chain ring.
It's also not uncommon for there to be a slight misalignment between the
two chainrings, but it's much smaller than seen in most gears on the
multi-speed drive chain.
But my point was that the difference in chain life between the timing
chain and the drive chain can be explained by the facts that the driving
chain is transmitting the power from both riders whereas the timing
chain only does so from the front rider and the driving chain is acting
on a small cog and therefore bends much more under load than the timing
chain which is acting on a chainring that's typically considerably
larger. So the longer life is not a good indication that it's the
result of a better chainline.
