The Bluetooth Special Interest Group has a standardized GATT (Generitt ATTribute Profile) for cycling speed and cadence sensors called the Cycling Speed and Cadence (CSC) profile. (You can see a list of all the GATT services here.)

This defines the commands and data that can be exchanged between bike sensor devices and the client device such as a phone, tablet, or BLE capable microcontroller (like we'll use in our project).

If you want to see how the Bluetooth SIG defines a GATT, such as the Cycling Speed and Cadence Profile, you can look at the official XML file here.

Even better, run that URL through a code beautifier, such as codebeautify.org for a more human-readable version.

Cycling Speed & Cadence Characteristics

The CSC service defines characteristics that can be served from the sensors to a connected device.

CSC Measurement

The most important for most needs is the CSC Measurement characteristic which serves up the following information:

Wheel revolutions - used to calculate speed of a known wheel diameter
Last wheel event time
Crank revolutions -- used to indicate pedaling cadence
Last crank event time

Not all Cycling sensor support all of the above characteristics. Some individual sensors will report just speed or just cadence.

Sensor Location

CSC sensors will also include a characteristic for the intended location of the cadence sensor, typically the right crank. This is built into the sensor firmware, not something that the device is determining on the fly!

SC Control Point

The speed/cadence (SC) control point characteristic is used to allow the client device to write control points to the CSC in order to initiate sensor calibration.