The RPM Cadence Sensor is small, light and can be paired with an iOS app over Bluetooth to transmit cadence data on your rides. You get an LED flash when the two are paired, so you’ll know ahead of time that your data will actually be waiting for you on the app once you’re done. It’s powered by a replaceable coin battery, but you shouldn’t need to worry about that for a while – the battery should last for about two years.
The other nice thing about RPM, and Wahoo devices in general, is that you’re not just locked into the Wahoo Fitness app. Wahoo makes their devices compatible with other fitness apps, too – RPM, for example, works with Strava and MapMyRide, two fitness apps made just for cyclists.
You can get RPM now from Wahoo for about $50 on its own, or with their RFLKT Smart Bike Computer for $150.