<https://forum.bikehub.co.za/topic/1927-powertap/#comment-32455>
<http://schema.org/text>
"It can't be used on the front wheel. The principle is that the \"force\" that you apply on the pedals, through the chain and onto the hub is measured and multiplied by the cadence to give you a power value.    The SRM system measures this force on the spider    The Ergo measures the force on the bottom bracket but the slight difference is that this force is derived only from the left leg as there is no force on the BB from the right pedal stroke."
<https://forum.bikehub.co.za/topic/1927-powertap/>