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Cycling computer magnet placement??

MontyCCMontyCC Posts: 46
Hi Folks,

Just picked up my Tricross today and decided to fit the computer on there and then for a true/proper odometer reading.

After placing the magnet close to the hub, and tightening all the cable ties, one of the guys in the bike shop mentioned it's better to have the magnet and sensor closer to the rims than the hub because the magnet "travels slower" and therefore gives a more accurate reading. This makes no sense to me atall and after staring long a nd hard on the spokes, I can't figure out how the placement of the magnet and sensor makes any diff where it is on the spoke as the whole spoke will pass the sensor at the same time

Someone please explain this to me

Thanks

Posts

  • whyamiherewhyamihere Posts: 7,337
    Doesn't make a difference.
  • DanEvsDanEvs Posts: 640
    It makes no difference, one rotation of the wheel is exactly that and the magnet will pass the sensor at the same time whether it's at the hub end or rim end.
  • bigmatbigmat Posts: 5,108
    Good tip is to place magnet opposite the inner tube valve, helps keep the wheek nicely balanced.
  • BigDarbsBigDarbs Posts: 132
    I know I am going to regret replying to this!

    It really doesn't matter where the magnet is placed, it will give the same mph reading for the bike.

    However he is correct in that if placed at the outer edge, it will be travelling at a different rotational (not ground) speed, but he has got it the wrong way around. Nearer the hub the magnet will spend longer in front of the sensor, but it will obviously pass the sensor exactly the same number of times, giving the same speed read out on your computer.

    The outer edge of the rim is travelling at a much greater rotational speed than the edge of the hub. (This is why you get a much greater sensation of speed and flung off the edge of a merry-go-round, but at the centre there is little force to throw you off, and also why you can read the sticker on the hub while the wheel is rotating but not the writing on the tyre!)

    If you mount the magnet at the outer edge of the wheel it is a much smaller proportion of the circumference of the wheel than if it is mounted close to the hub. If you assume that the magnet is 10mm across, on a 700 c wheel the circumference is 2.2 metres. So the magnet is only a very small proportion of the circumference, hence the magnet will pass the sensor very quickly for a given ground speed, but closer to the hub it will spend proportionally longer in front of the sensor, but it will pass it exactly the same number of times for the given ground speed. At the hub the circumference may only be 18 cm so the 10mm magnet is a much greater proportion on the circumference, hence spends longer in front of the sensor. Another way of looking at it is that if it takes 1 second for the wheel to do one rotation, the outer edge of the rim is travelling 2.2 meters in 1 second, but the edge of the hub is travelling only 18 cm in one second.

    A 700c wheel rotating at 100 times a minute has an outer edge speed of about 3.7 metres per second, but the flange of the hub is rotating at about 0.6 metres per second. (for info the base of a 45 metre wind turbine blade rotating at 15 rpm will be travelling at about 6.5 mph but the tip of the blade will be travelling at 157 mph!!)

    Your mechanic should have said if the magnet is mounted close to the hub it spends proportionally more time in front of the sensor, hence should give a more accurate reading. At the outer edge the magnet is flashing past the sensor so may be more prone to mis-reading.

    Having said all of that, it really doesn’t matter, as bike computers seem very reliable in picking up sensor outputs! I am now going out on my bike!
  • MontyCCMontyCC Posts: 46
    Nice!! Thanks for the education! I did think the LBS guy got it the other way round but as I've learnt, never get into a technical squabble with them as you'll be fighting a loosing battle. (Much like the missus :roll: )
  • whyamiherewhyamihere Posts: 7,337
    BigDarbs wrote:
    Your mechanic should have said if the magnet is mounted close to the hub it spends proportionally more time in front of the sensor, hence should give a more accurate reading. At the outer edge the magnet is flashing past the sensor so may be more prone to mis-reading.
    Nope, doesn't make a difference. All the sensor does is detects a change in the magnetic field. It's going to detect that no matter how long the magnet's in front of it for.
  • BigDarbsBigDarbs Posts: 132
    whyamihere wrote:
    BigDarbs wrote:
    Your mechanic should have said if the magnet is mounted close to the hub it spends proportionally more time in front of the sensor, hence should give a more accurate reading. At the outer edge the magnet is flashing past the sensor so may be more prone to mis-reading.
    Nope, doesn't make a difference. All the sensor does is detects a change in the magnetic field. It's going to detect that no matter how long the magnet's in front of it for.

    It’s all about likelihood. Any instrumentation system (which a cycle computer is) will increase the chance of a reliable reading if it can guarantee a clear signal. Anything that increases the chance of a clear signal is worth pursuing.

    Even though cycle computer sensor is a simple inductive coil that senses the magnet as it passes, inducing a current in the coil and hence reading on your computer, the system will be less prone to error if the magnet is alongside the coil for longer. This will reduce any error caused by the magnet being too far away from the sensor, set at an angle, incorrect polarity etc. The current in the coil takes finite time to form and collapse, the longer it has to form the stronger the current is likely to be, and hence increases the likelihood of a clear reading.

    So in the real world the closer the magnet is to your hub, the less sensitive it will be to being slightly out of line with the sensor.
  • So placing the magnet anywhere will indicate the same speed
    But what if your computer is like mine and it logs distance? ?
    The reason for entering the wheel size is to get distance.
    If it didn't matter where you put the magnet then you would never need to enter wheel size
    As someone already posted
    The outside of a 700 rim will travel 13 meters while the inside will only travel 1/2 a meter essentially.
    This would make for huge miscalculations in actual distance traveled wouldn't it??
    Or am I missing something??
  • cooldadcooldad Posts: 32,904
    Apart from resurrecting a 9 year old thread?

    The computer calculates distance from wheel size and revolutions, so it makes no difference where the magnet is. It measure revs, multiplies by the wheel size you entered, and calculates the distance.

    Tyre size may make a small difference.
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  • imposter2.0imposter2.0 Posts: 11,003
      So placing the magnet anywhere will indicate the same speed
      But what if your computer is like mine and it logs distance? ?
      The reason for entering the wheel size is to get distance.
      If it didn't matter where you put the magnet then you would never need to enter wheel size
      As someone already posted
      The outside of a 700 rim will travel 13 meters while the inside will only travel 1/2 a meter essentially.
      This would make for huge miscalculations in actual distance traveled wouldn't it??
      Or am I missing something??

      Can't believe you dug up a 9yo thread to post something so absurd. The distance the magnet travels is irrelevant. A magnet placed near the hub will still pass the sensor at exactly the same time as a magnet placed at the rim edge. Tyre circumference dictates the distance travelled, nothing else.
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