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  1. #1
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    Speed sensor magnet location on wheel

    I have seen different people place their magnets at a different radius on their front wheel from each other. I bouhgt a Rox 9 computer, and the instructions do not tell you exactly where on the wheel to place the magnet. I have made the logical assumbtiong that because the sensor works based on the DIA of the wheel, that the magnet and sensor would be the closest to the outside DIA of the wheel.

    Is this correct or does it not matter?


    Does the same apply for a Cadence Magnet? you would want it as close to the pedal axis as posible.

  2. #2
    Senior Member DanteB's Avatar
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    It doesn't matter.
    Make mine a double!

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    Senior Member Ziemas's Avatar
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    Quote Originally Posted by DanteB View Post
    It doesn't matter.
    +1

    One rotation is one rotation, no matter where the magnet is.

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    The sensors don't measure speed directly. They just count the number of times the magnet goes past. For cadence, that's all you need. For speed, that's the reason you need to program in the tire size.

    The main requirement is that you can get the right gap between the sensor and magnet, without anything getting in the way of anything else. Usually this means going part way down the fork in order to get them close. For the cadence, it may depend on the shape of your chain stays, but typically you need to go towards the back. The secondary requirement, with a wireless, is getting closer to the computer. So go as far as necessary for the gap, but no further.

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    okay sounds good thank you ! =)

  6. #6
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by GaryBy View Post
    The sensors don't measure speed directly. They just count the number of times the magnet goes past. For cadence, that's all you need. For speed, that's the reason you need to program in the tire size.

    The main requirement is that you can get the right gap between the sensor and magnet, without anything getting in the way of anything else. Usually this means going part way down the fork in order to get them close. For the cadence, it may depend on the shape of your chain stays, but typically you need to go towards the back. The secondary requirement, with a wireless, is getting closer to the computer. So go as far as necessary for the gap, but no further.
    And if you really want to mess with your buddy's head, put another magnet...or 2... on his wheel. He'll be going real fast
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  7. #7
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    The sensor attached to your fork, is a coil of wire. When the magnet goes past it, it "feels" a change in the magnetic field, which gets stronger as the magnet approaches and then weakens as the magnet goes past and away.

    This change in the magnetic field, creates a voltage in the coil. The faster the magnetic field changes, the higher the voltage. So, the faster the magnet moves past the wheel, the sharper a voltage pulse the sensor creates.

    (also, the closer the magnet comes to the sensor, the stronger the pulse. But once you've got the sensor and magnet mounted, this closeness doesn't change, it's the same for every revolution no matter how fast or slow).)

    Conceivably, if the magnet moves verrrrrrry slllloowwwwly past the sensor, it might not "feel" the magnet strongly enough - it might create only a very low voltage pulse, that the computer doesn't see as a legit pulse.

    So IMHO, mounting the magnet and sensor closer to the rim, is good. The magnet will whip past the sensor more quickly and create a higher-voltage pulse. This means that, even if you are going slowly (walking the bike?), you will get a reading on your speedometer.

    That said, I have one bike with the magnet and sensor mounted about 6" from the axle, and I get a good reading even at 2mph. Also get good readings at 30mph, which is as fast as I've pushed that bike recently (fat people go down hills really fast ) So I guess that particular computer can handle even pretty-low-voltage pulses.

    The rest of my bikes with computers, have the magnet and sensor mounted about 6" from the rim. They also work great.

    YMMV.

    In general, I'd mount it fairly close to the rim if I were you.
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    Senior Member Pig_Chaser's Avatar
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    Quote Originally Posted by Little-Acorn View Post
    The sensor attached to your fork, is a coil of wire. When the magnet goes past it, it "feels" a change in the magnetic field, which gets stronger as the magnet approaches and then weakens as the magnet goes past and away.

    This change in the magnetic field, creates a voltage in the coil. The faster the magnetic field changes, the higher the voltage. So, the faster the magnet moves past the wheel, the sharper a voltage pulse the sensor creates.

    (also, the closer the magnet comes to the sensor, the stronger the pulse. But once you've got the sensor and magnet mounted, this closeness doesn't change, it's the same for every revolution no matter how fast or slow).)

    Conceivably, if the magnet moves verrrrrrry slllloowwwwly past the sensor, it might not "feel" the magnet strongly enough - it might create only a very low voltage pulse, that the computer doesn't see as a legit pulse.

    So IMHO, mounting the magnet and sensor closer to the rim, is good. The magnet will whip past the sensor more quickly and create a higher-voltage pulse. This means that, even if you are going slowly (walking the bike?), you will get a reading on your speedometer.

    That said, I have one bike with the magnet and sensor mounted about 6" from the axle, and I get a good reading even at 2mph. Also get good readings at 30mph, which is as fast as I've pushed that bike recently (fat people go down hills really fast ) So I guess that particular computer can handle even pretty-low-voltage pulses.

    The rest of my bikes with computers, have the magnet and sensor mounted about 6" from the rim. They also work great.

    YMMV.

    In general, I'd mount it fairly close to the rim if I were you.
    The wired bicycle computers surely use a coil as described above. I've been wondering, however, if the wireless ones use a hall effect switch... You have to have a battery in the sensor/sender anyway.

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    In general, I'd mount it fairly close to the rim if I were you.
    What do you call "the rim". The outter most of the wheel?

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    This is where it was originally:


    And this is where I have moved it based on the information on the thread... did I understand right?




    Neither of these is more accurate than the other, but the closer to the center hub the slower speeds I can detect correct?

    BTW, can I use another magnet for the sensor, The sigma doesnt seem to be very secure and damages the paint of the spokes.

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    I would be amazed if there were any coils in the detectors. Hall effect switch perhaps, reed relay most definitely. As stated by others, it doesn't matter. The computer simply counts how many times it pulses in a given time and does the calculation for the speed based on a wheel size. Accuracy at slow speeds is problematic and has more to do with the time base for the calculation.

    torgrot

  12. #12
    made in italy bicycletothesun's Avatar
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    I keep my speed magnet near the hub. Has worked thus far...and I've even rode alongside a friends car (with an electronic speedometer) and compared my readings to theirs. It seems accurate enough for my uses

    (I'm using a Cateye cadence strada wired. And the magnet has to go near the hub because there would be too much cord along the chainstays)

  13. #13
    Zoom zoom zoom zoom bonk znomit's Avatar
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    The sensor will work at very very low magnet speeds. You can test this by slowly rotating the wheel back and forth.

    Near the hub is best for the following reasons.

    -Rotational inertia is smaller by a wee bit.
    -When the sensor gets bumped into the spokes there is less chance of damage as the collision speed is lower.

    These are offset by having to have longer wires.
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