Its a rotating frame on the ground, so not upright.
Okay I understand. So somehting like this (side view):
Where | = piezo element and O = steelball with eg. 10gramms
And then measure the resistance of the piezo elemnent and calculate the force with the weight that presses against the piezo accoring to its weight. The radius and RPM (measured separately) are known.
One question though: how do I get the wires from the wheel to the Arduino that is not on the wheel?! O.o
F = kg * (ms^2/r) ?
F = Newton
kg = mass
ms = Meters per Second
r = radius
Should that be enough? I think I can calculate the force but I would like actual readings because the load of the wheel can vary.