Crankset based bicycle power measurement

An input torque measuring device for a drive train of a bicycle, the drive train including a first crank arm and a second crank arm configured to be engaged by a rider of the bicycle at an outboard end, an inboard end of each crank arm rotatably mounted to the bicycle at a bottom bracket of the bicycle, at least one generally planar chain ring oriented generally perpendicular to the bottom bracket and configured to rotate about the bottom bracket and to rotate a driven wheel of the bicycle, the device comprising:

• a spider connected to a first crank arm adjacent the bottom bracket and extending out to and directly supporting the at least one chain ring, the spider configured to transmit force applied to the crank arms directly to the at least one chain ring;
• the at least one chain ring mounted to the spider by a removable fastener extending through an opening in the spider and a corresponding opening in the chain ring, with a bushing fitted within the opening of the spider so that the fastener exerts clamping force against the chain ring and the bushing;
• a plurality of sensors configured to respond to the force applied to the pedals and transmitted to the at least one chain ring and produce an electronic signal relative to the force transmitted to the at least one chain ring, each sensor oriented to be generally parallel to a plane of the at least one chain ring;
• an electronic circuit configured to receive the electronic signals from each sensor, the electrical circuit included as part of a flexible circuit board;
• each of the plurality of sensors mounted within a recess and encapsulated in a protective material and the flexible circuit board positioned within a recess and encapsulated in protective material.