how old are the motorbikes?

the older bikes apparently use a diffent way of sensing the wheels moving ..
magents and coils, (passive sensor below)
and the newer ones hall effect semi conductores

cribbed
"Types of ABS Sensors

TRW ABS Sensors have undergone a huge amount of change and development since introduction to the market, which means they are available in a variety of designs depending on specific car requirements. The two basic types are active and passive ABS Sensors.

Active sensors need an external energy source to operate and are designed for toothed/magnetic encoder wheels. There are also two types of active sensors, Hall sensors and Magneto Resistive sensors.

• The Hall sensor reacts to changes in magnetic fields with a voltage difference which is transmitted to the control unit as a clearly defined square wave signal. Hall sensors record the wheel speed via a segmented disc that rotates with the wheel or a ring with alternating magnetic fields (magnetic encoder). Hall sensors are highly precise but require a precise installation position.
• The magneto-resistive AMR sensors are currently regarded as the most advanced but also the most expensive ABS sensors. Their great advantage over the other designs is their ability to detect the rotational direction of the wheels. The signal is extremely precise and AMR sensors are practically tolerant to their installation position. The encoder disc comprises magnetic arc segments which cause a clear change in resistance when passing the sensor. For the control unit, this change acts as a parameter for the wheel’s rotational speed and direction.

Passive sensors require a toothed wheel and only start to operate when the wheel reaches a certain rotational speed. They are larger than active sensors and less precise, which means their presence in modern vehicles is becoming scarcer, but given the simplicity of the design they are easier to check and often more robust. TRW passive sensors are built with a coil around a magnetic core, this permanent magnet creates a magnetic field. As the toothed wheel turns, it changes the field, which then induces an electrical current depending on whether there is a tooth or not."

Liam