Calling all mechanical wizards

It's nowhere near constant, lossen the axle and measure the torque with the door up, and with the door down. (use a pipe wrench to grip the axle)

The garage door thing has to have a force-distance curve that matches the varying load. But a similar mechanism, a torsion spring driving a tapered shaft driving a cable, could be made to have a constant force, or practically any reasonable force-payout curve. A torsion spring driving a drum feeding out a cable could be made nearly constant over some decent range.

John

Look at a tape measure; it is curved along its length; combine that with another built-in curl to make naturally it wind up. I understand that the unwinding force is constant (due to that first curve).

Another dirty trick is to use a lead weight and a pulley..

A spring alone won't do it. The clever clockmaker's invention was the "fusee" - coupling the spring drum via a chain wound around a conical spiral pulley. As the spring unwound and the torque decreased, the chain pulled on an increasing diameter, resulting in near constant torque. Usually only found in high-end chronometers, but I have an antique pocket watch that has a fusee.