Tiny cycling power meter mounts on a bike's valve stem
Ordinarily, cycling power meters are mounted on the crank, pedal, or sometimes even the rider's shoe. The AroFly, however, takes a different approach – it threads onto the rear tire's Schrader or Presta valve stem. It's also claimed to be the world's smallest power meter.
Using a pitot tube-based sensor, the AroFly detects repetitive spikes in the tire's air pressure that correspond to the rider's pedalling cadence – random spikes caused by uneven road surfaces are filtered out, plus the device compensates for different tire sizes. The AroFly also utilizes an integrated accelerometer to measure the wheel's rate of rotation, from which it determines the bike's current speed.
Data from both sensors is transmitted by Bluetooth Low Energy to the handlebar-mounted AroFly A-Plus Meter cycling computer, or to an iOS/Android app on the rider's smartphone. In either case, custom software analyzes the cadence and speed data in order to determine the rider's real-time power output – the reading is reportedly accurate to +/- 2 percent, as compared to readings from a Kisler force plate.
All three figures are displayed onscreen, along with the user's heart rate if the optional AroFly A-Pulse heart rate-monitoring wristband (or a compatible third-party monitor) is being used. Additionally, utilizing the computer or phone's GPS and other sensors, information such as cycling route, distance travelled, altitude, gradient, and ambient temperature can be accessed.
The AroFly itself is powered by a user-replaceable lithium coin cell battery (that should be good for 80 hours of use), it weighs 10 grams, and is IP67 waterproof – that means it can be submerged up to 1 meter (3.3 ft) for 30 minutes.
It's aimed at novice to intermediate-level riders, and is currently available via the link below. Prices range from US$269 for a tire-sensor/cycling-computer combo, to $319 for a package that also includes the heart rate monitor.
And for another example of a power meter that goes in an unconventional place, check out the handlebar-mounted PowerPod.