The PowerTap is a complete rear hub for the back wheel of a bicycle that houses a power meter. The hub contains a “torque tube” with strain gauges similar to those used by the SRM. These strain gauges measure the torsion inside the hub as it twists from the load that is applied to the pedals by the rider. The bicycle chain wraps around the cogs on the hub and, as it moves, causes small twists in the hub itself. This torque is measured and then converted into wattage at the PowerTap computer. The wattage that is measured in the PowerTap is the wattage that is actually getting to the road, as it has to “go through” the drivetrain from the crank. This causes the wattage to be about 5–10 watts lower than what would be measured by the SRM at the crank.

A major disadvantage to the PowerTap is that you are stuck into the wheel that the hub is built to. If you want to use your super trick wheels for racing, you’ll need to get an additional hub for racing and then keep your standard wheel for training which increases cost significantly.