You bike with your phone anyway, so why not have it collect your speed and cadence data with the Wahoo™ Fitness Premium Speed and Cadence Bike Sensor.

• Store bike speed and cadence data during a bike workout.
    - Easily connects to most bikes.
    - Durable.
    - Speed and cadence sensors are independent so they can be placed in the best positions.
• Small profile: low drag.
    - Modifiable: can be used with recumbent bikes.
    - Tether is spliced and extended to fit.
• Wireless: ANT+ ultra low power technology license free.
    - 2.4 GHz ISM band operation.
• Waterproof up to 5 feet.
• Battery: 3 volt CR2032 coin cell.
    - Replaceable.
• Compatible with any ANT+ receiving device.
    - Signals: sends and receives up to 10 feet (unobstructed).
• Weight: 18 grams.
    - Accessories: 12 grams.
    - Total: 30 grams.
• Imported.