Force on a pedal really doesn't correlate well with the rotational torque on the crank shaft.
Since the torque on the crank shaft is the product of that force and its distance from the shaft, the force correlates exactly with the torque.
QuoteSince the torque on the crank shaft is the product of that force and its distance from the shaft, the force correlates exactly with the torque.Having to kniow the rotational position of the crank shaft to get your distance from the crank shaft centerline just adds complexity to the project. The OP said "and too expensive to put a rotative sensor."
Torque is just perpendicular force x distance from the axisThere was talk about measuring against something "resistive". I can only assume you mean the opposing forces on a bike, namely acceleration and aerodynamic drag, which accounts for 90% of a cyclists power output at a constant speed.