Garmin GPS sensor issue on tandem
I'm wondering who has thought about this problem and how you dealt with it.
I started moving some accessories from older bikes to our newly acquired tandem. One of the items I was particularly interested in getting installed was my Garmin Edge 305 GPS. This unit has a single sensor unit which picks up both cadence and wheel speed. An explanation of why a GPS unit needs a wheel sensor is another whole discussion but suffice it to say that it does. Cadence seems to be one of our favorite topics, and so having some data to analyze both during and post-ride seemed as though it would be of significant value.
The real problem is the cadence portion of the sensor. The sensor is designed to zip-tie to a chain stay so that it can "see" both the rear wheel and the pedal crank. It "sees" the wheel rotation by virtue of a magnet attached to a rear wheel spoke and similarly a magnet on a pedal crank allows the sensor unit to read RPMs at the crank. The problem is with the magnet attached to the crank arm. Since the stoker crank has the drive chain on the right and the timing chain on the left, it isn't possible to put the sensor on the non-chain side of the bike. Consequently, when the crank arm passes close to the timing chain, the attached magnet attracts the upper and lower runs of the chain as it passes by and causes the chain to click against the magnet twice with each revolution.
While it might be possible to mount the sensor so that it picks up the captain's crank (from the non-chain side) but that would preclude picking up the rear wheel magnet. Likewise, picking up the rear wheel speed seems to preclude getting captain's crank rotation. Since the GPS device captures this data along with the other data that it is collecting I am resisting the obvious solution of mounting a secondary bike computer for the purposes of measuring cadence because merging the data from the two devices would be impossible or at best a technological nightmare.
I guess similarly, this model of GPS won't be able handle two heart rate monitors recording at the same time, will it.