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  1. #1
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    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.

    Any thoughts?

  2. #2
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    For the magnet/sensor problem: Try a tiny magnet that you can get at radio Shack and glue it to stoker's cranks. It should clear the chain and be seen by the sensor

  3. #3
    Double Secret Probation R900's Avatar
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    Quote Originally Posted by cornucopia72
    For the magnet/sensor problem: Try a tiny magnet that you can get at radio Shack and glue it to stoker's cranks. It should clear the chain and be seen by the sensor
    +1 worked for us...
    Time to Ride...

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    I talked to Garmin and they suggested abandoning the rear wheel magnet and mounting the cadence sensor adjacent to the captain's crank. I think I might try the tiny radio shack magnet approach first before giving up on the rear wheel sensor. For what it's worth, the support guy at Garmin adjusted my understanding of the role of the rear wheel magnet. His position was that the only time that the rear wheel sensor would come into play was when there was no satellite reception. I had believed that the two measures would be used simultaneously to increase the accuracy of instantaneous data. Ya learn something new every day, huh?

  5. #5
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    Quote Originally Posted by Neill
    I talked to Garmin and they suggested abandoning the rear wheel magnet and mounting the cadence sensor adjacent to the captain's crank. I think I might try the tiny radio shack magnet approach first before giving up on the rear wheel sensor. For what it's worth, the support guy at Garmin adjusted my understanding of the role of the rear wheel magnet. His position was that the only time that the rear wheel sensor would come into play was when there was no satellite reception. I had believed that the two measures would be used simultaneously to increase the accuracy of instantaneous data. Ya learn something new every day, huh?
    I do not think that is entirely accurate. We have a Garmin edge with cadence and HRM. We also have a Cateye computer with HRM. I wear one HRM and my wife wears the other one. While the cadence/wheel sensor was working, both computers were in almost perfect unisone. Then the battery in the Garmin transmitter died and the readings were off by 2 to 3 tenths. No big deal... but the total distance for a long ride can be off by as much as .25 to .5 miles depending on the length of the ride. Our guess is that the garmin GPS does not account for small deviations from smooth lines.

  6. #6
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    Quote Originally Posted by Neill
    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.
    I have a Garmin 205 installed on a Santana with the drive chain on the right and the timing chain on the left. My sensor is located on the left chain stay and the magnet is on the left Stoker crank arm.

    Quote Originally Posted by Neill
    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.
    You must have a very loose timing chain or very long crank arms. I've not experienced this problem.

  7. #7
    Captain - 2nd in Command djsincla's Avatar
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    Quote Originally Posted by Neill
    An explanation of why a GPS unit needs a wheel sensor is another whole discussion but suffice it to say that it does.
    It does not require the wheel sensor. Commonly used on Tadpoles with no issue.

    From the manual on Page 27:

    "The speed data from the GSC10 is only recorded and used for distance calculation when the GPS signal is weak or GPS is turned off."

    I have found the GPS technology used in the 305 is very sensitive and will easily receive GPS signals indoors.

  8. #8
    Cycling Anarchist Trsnrtr's Avatar
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    I use a 305 on four different bikes and don't use the speed sensor at all. Never had a problem.
    Dennis T

  9. #9
    Double Secret Probation R900's Avatar
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    Here is a pic of our setup using a Radio Shack magnet:

    Time to Ride...

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