except that you are only factoring in the rider's weight and not the actual sum of forces being applied to the the crank. If you do a free body diagram of the system, you have the weight of the rider, the force applied by the rider pushing the down stroke, and gravity for simplicity all summed at one point on the pedal down stroke. So assuming your 140 pound rider is a pro cyclist who can most like single leg press over 400 pounds on each down stroke (probably a modest number) You get the sum of the forces in Nm as 109 + 311.5 (probably modest) + gravity (negligible) so you are looking at roughly 420 Nm of torque