i think i need a spring energy-storage mechanism. Maybe a very large
wind-up clock motor?

the scenario is: a slightly-valuable commodity originates at the top
of a mountain. More or less a bushel at a time. there is an existing
paved road. the commodity fetches a price, but not enough to pay for
very much transport.

the concept is to use a cargo bicycle, maybe with trailer, to carry
the items down to the bottom of the mountain, "winding up the clock"
on the spring-storage.

at the bottom, the load is transferred to buyers. the now very-much-
lighter bicycle is to be driven UP the mountain, using the stored
energy.

Any cat which will catch the mice is fine with me. I'm just thinking
that anything involving electricity, hydraulic fluids, etc, will be
too involved, and losses throguh two conversions. Capacitor storage
might be feasible, if there's high quality ones being sold. The charge
doesn't need to held for long amounts of time. Down the hill, then
back up.

Ideally, this will be a ready-to-go or mostly ready-to-go
commercial item. I am rather mechanically/electrically handy. But
this is business, not pleasure. Time spent should be cost-effective.