Wahoo RPM Speed Sensor.

The Wahoo RPM speed sensor is the sleek, magnet-less solution for capturing cycling speed data. It easily mounts to your bike's front wheel hub, and the Bluetooth Smart and ANT+ dual band technology allows you to connect to and display data on both smartphones and bike computers. Tracking speed data has never been this easy.

Specs:

• Part Number: WFRPMSPD
• Physical Dimensions (LxWxH): 1.25" x.75" x 1"
• Weight: 7 grams (without mount)
• Battery: CR2032
• Battery Life: Up to 12 months
• Water Rating: IPX7 (waterproof up to 5 ft)
• Wireless Signal Strength Up to 10 feet
• *Note: Speed Sensor is NOT compatible with spin bikes