The present invention relates to a sensor apparatus (1) for determining a crank cadence and a bicycle speed. The sensor apparatus (1) comprises a housing (2) with an accelerometer (4) with first and second perpendicular measurement axes (z, y), capable of providing first and second signals dependent on the two measurement axes (z, y), a wireless transmitter (6), and a power source (8). The housing (2) comprises an attachment means (3) for attaching the housing (2) to a wheel of the bicycle. The sensor apparatus (1) further comprises a first means for determining 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, and a second means for determining the speed from the rate at which consecutive maximum and minimum extreme values occur in the second signal.