Magnet too far away from sensor on my cycle computer

Discussion in 'Bicycle Mechanics and Repairs' started by GraeX, 4 Aug 2012.

  1. GraeX

    GraeX Regular

    I recently got a Cateye Velo 5 cycle computer, managed to set it up fine but the problem is the magnet is really small and I can't get the sensor to pick up the magnet, the instructions say 5 mm space between them but the closest I can get them is a lot wider than that, what can I do to make it work?
  2. If I was to use the standard magnet that came with my computer I'd have the same problem.

    Luckily I had one of THESE that came with some wheels, and it's possible to rotate this type magnet on the spoke to bridge the gap.

    Maybe a different magnet would solve your problem too :thumbsup:
  3. OP

    GraeX Regular

    Thanks, going to see if I can pick one of the up from a local shop now, thank you :smile:
  4. OP

    GraeX Regular

  5. john xyz

    john xyz Active Member

    East Kent
    I've just installed a 'cateye' and seems to be working ok. To get the magnet near the sensor, i had to install it all quite a way down the fork, to a point where the spokes come closer to the forks, but still just within the 70cm limit.

    One thing i cant understand........... surely the point of positioning down the spoke/fork is critical to get the computer accurate? I mean, ther magnet passes the sensor more or less times, depending how far up teh spoke it is. How can it NOT matter where the positioning is, as long as its not more than 70 cm from computer??

    Sure i'm being dense but can anyone explain?!
  6. OP

    GraeX Regular

    If you put 2 magnets on the same spoke, one near the hub and one near the rim, they would pass by the inside of the fork at the same time, no matter how fast the wheel went. the one near the rim would travel faster because it has more distance to go.

    The sensor uses the size of your wheel combined with the time between passes of the magnet to calculate your speed and distance.

    there is a theory that it is best to put the sensor and magnet as close to the rim as you possibly can, because the slower speed of the magnet passing by would give a more accurate reading, but tests show this makes no difference.
  7. Rob3rt

    Rob3rt Man or Moose!

    Position does not matter because your odd conclusion is incorrect, it wont pass the sensor more or less times depending on how far up the spoke it is, the magnet will pass the sensor one time per wheel rotation. Given that the wheel diameter is known,by knowing the number of rotations per a given time frame and the wheel diameter, the distance traveled and speed can be calculated.
  8. john xyz

    john xyz Active Member

    East Kent
    Yes, well done guys! I knew it was easily explained...
  9. Night Train

    Night Train Maker of Things

    Greater Manchester
    Also positioning it too close to the rim means a smaller pulse relative to the gap in each revolution.

    Moving the magnet closer to the hub results in a longer pulse relative to the gap and an easier signal for the sensor to pick up.
  10. screenman

    screenman Legendary Member

    To double your speed and impress your mates put two magnets on the wheel.
    wealthysoup and john xyz like this.
  11. jaynana

    jaynana Well-Known Member

    NW London
    simple solution is move the magnet as much as necessary to teh centre of the wheel, ie, towards the hub, away from the rim, so that you can achieve 5mm between the magnet and the sensor.

    the theory that the speed is calculated based on the speed the magnet passes the sensor is all nonsense.

    the speed is calculated based on the frequency the magnet passes the sensor.

    which is why the only setting the speedo needs to figure out the speed is the wheel size and the tire size (to calculate the circumference of the wheel; thereafter speed = distance / time)

    so it doesn't matter what radius the sensor and magnet have on the wheel, which is why it can be moved closer to the centre of teh wheel to meet the 5mm requirement.
  12. slowmotion

    slowmotion Quite dreadful

    lost somewhere
    Leaving aside the magnet position on the spokes, you can usually bend the fork-mounted pick-up closer to the magnet as well.
  13. oilyormo

    oilyormo gettin warmer??

    i had a similar problem with my MTB where the forks are too wide to get a small enough gap so i spaced the sensor using a small peice of rubber hose, now works a treat.

    I have also found that periodically the magnet loses some magnetism ( dont ask me how or why) and wont work so i recharge it by sticking it onto another magnet for a couple of hrs and it works fine again.
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