A possibility for the “sprung” uni discussed earlier would be to use two chains
and gear assemblies. Each chain and assembly would have its own tensioner, one
on the front side and one on the back side. (So that one chain would always be
tight regardless of the direction being pedaled.)
The problem here would be that until a pedal-reversal, one of the chains would
stop the seat from rising. (This would also be a problem with any tensioner
mechanism. Unfortunately, this would also be felt as a turning force on the
pedal, possibly with bad effects on balance as the wheel and/or pedal turn to
relieve chain tension.)
The suggestion to move an idler wheel outwards as the seat compresses has merit
but would likely be difficult in practice. The idler wheel’s size should’t be
critical, but the total chain length would. A shaped cam of some sort, driven by
the seat’s compression, would probably be required to get the needed precision
of idler extension.
Another possibility would be to use a scissors type extension mechanism, so:
--- ---
| |
+ \ / \ \ / \
+ or + / \ / / \ /
+
| |
O O
[wheel not to scale]
Three chains would be used – one from the pedal/top joint to the middle bend,
one from the middle bend to the bottom joint, and one from the bottom joint to
the wheel. The chains thereby all have constant length and need no tensioner
mechanism. Sprockets would be required at each joint, and the uni joints would
have to use the same axis as the sprockets for the frame’s flex. Additional
guides and support cables would also be necessary to make sure that the upper
and lower frame sections remain colinear (for the model pictured on the left)
and provide for control of the degree of flex.
Some side effects of this construction would be changing rotational inertia
(about the vertical axis) as the uni height is changed, and increased frame bulk
which might impede mount/dismount when not extended.
This mechanism could be used for shock absorption, or just for fun as a variable
height unicycle.
/r