A roller shade is disclosed comprising a motor pretensioned counterbalancing spring that lowers the torque load on the motor of the roller shade throughout the rolling up or rolling down cycles. The roller shade comprises a motor drive unit at least partially disposed within a roller tube. The motor drive unit comprises a motor adapted to drive a drive wheel through a clutch. The motor drive unit further comprises a first spring carrier adapted to be stationary during operation of the motor drive unit. The drive wheel is operably connected to the roller tube and comprises a second spring carrier. The motor drive unit further comprises a counterbalancing spring connected at its first end to the first spring carrier and at its second end to the second spring carrier. The counterbalancing spring is pretensioned using the motor prior to inserting the motor drive unit into the roller tube by driving the drive wheel and thereby rotating the second end of the counterbalancing spring with respect to the first end of the counterbalancing spring. The clutch translates rotational motion from the motor to the drive wheel, but locks rotational motion from the drive wheel thereby locking the pretension in the counterbalancing spring.