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