A sensor (1) for determining both a cadence of a crank and a travelling speed of a bicycle includes an accelerometer (4) providing a first signal dependent on a first acceleration along a first measurement axis (z) and a second signal dependent on a second acceleration along a second measurement axis (y) perpendicular to the first measurement axis. The accelerometer (4), a wireless transmitter (6) and a power source (8) are contained in a housing (2) that can be attached to a wheel of the bicycle. The sensor (1) determines the cadence from the rate at which consecutive changes of polarity occur in the first signal, or from the rate at which consecutive positive and negative extreme values occur in the first signal. The sensor also determines the travelling speed from the rate at which consecutive maximum and minimum extreme values occur in the second signal.