The Arofly screws onto the rear tire valve. It measures speed and, from changes in tire pressure, it calculates cadence and power! Huh?
The 10-gram unit is made in Taiwan by TBS Group Corp. To set it up, you simply unscrew your rear Presta valve nut to the top and install a Presta adaptor (it also works on Schrader valves without an adaptor) and screw the tiny Arofly sensor/transmitter — it’s just 20mm in diameter — onto the valve stem. You also sync it via Bluetooth to the accompanying A-Plus head unit or to a smartphone with the Arofly app running.
In addition to a sensitive pressure-sensing mechanism, inside the Arofly valve-mounted unit is also a gravity sensor, which determines speed from rotational frequency and tire diameter. When the rider pedals, tire pressure varies (and bumps are averaged out). Based on that variation, cadence can be determined.
The wild part is that Arofly claims to also measure power to an accuracy of plus/minus two percent measured on a Kistler force plate, using the speed, cadence, and pressure variation, along with GPS data from the smartphone or head unit, and user-inputted rider weight and tire diameter (which the unit is claimed to correct with GPS and speed data).
North American marketing director Otto Brown claims that Arofly distinguishes easily between, say, a rider spinning easily at 10mph and 90rpm vs. the same rider pedaling as hard as he can against a strong headwind on the same course at 10mph and 90rpm, because when the rider is pedaling harder, the pressure variations in the rear tire will be greater. He also claims that it can still measure power on a bumpy road like the ones featured in Paris-Roubaix, but that there would be a longer delay in obtaining the measurement due to averaging out all of the bump inputs.
The Arofly unit sells for $129, while the Arofly with the A-Plus head unit (required, and movable from bike to bike), sells for $249. Its battery is claimed to last two months.