Optimum Speed Sensor Position

jammykam
jammykam Posts: 10
edited June 2008 in Road beginners
I've just bought myself a Polar CS200 bike computer, I wanted something with heart rate monitor to help with training - needed a bike computer anyway and was pretty cheap!

Where should the front wheel sensor sit? It doesn't say in the instructions how far in from the hub the optimal position is. I would have thought that in an incorrect position the readings would be incorrect?

cheers
Kam

Comments

  • Beeblebrox
    Beeblebrox Posts: 145
    Wherever it is placed on the wheel it'll pass the sensor at the same rate per second, just at different speeds. If it doesn't seem to work reliably at the very outside (where it is fastest moving) it might be it is just going too fast to be picked up.

    That said, mine is a cheapy one from Halfords, and has to be at the very edge or the sensor would hit the spokes - so that can be another limiting factor.

    But in short, it doesn't matter where.

    Edit - Looks like a cool computer, I especially like the 'Water resistant 10m' bit. When that'd be useful on a bike, I don't know.
  • Garry H
    Garry H Posts: 6,639
    jammykam wrote:
    I've just bought myself a Polar CS200 bike computer, I wanted something with heart rate monitor to help with training - needed a bike computer anyway and was pretty cheap!

    Where should the front wheel sensor sit? It doesn't say in the instructions how far in from the hub the optimal position is. I would have thought that in an incorrect position the readings would be incorrect?

    cheers
    Kam

    I don't think it matters, as long as the sensor picks up the magnet. The bike still moves the same distance per rotation. I would think that any difference based on the placing of the sensor would be extremely minimal.
  • Marko1962
    Marko1962 Posts: 320
    What Beeblebox said..

    On my Cateye V3 I have placed my magnet as close to the hub as is possible, I think it says so somewhere in my instructions to do this and I also found out from experience that this is something I need to do as when on a run I was getting erratic speed readouts. Moving the sensor and magnet more towards the hub sorted that out, the magnet on the V3 is fairly small and weak compared to others that I've had.

    Centrifugal force will come more into play and could make the wheel feel unbalanced the further out the magnet is from the hub esp if it's a heavy magnet and the more chance of it flying off when your screaming along weeeeee...
  • jammykam
    jammykam Posts: 10
    Thanks for the replies both of you. Yes, makes perfect sense since the magnet will pass the same number of times regardless of position and the wheel size is programmed as well - I was thinking about when you change car wheels to bigger size the speedo gets messed up a little.

    At the moment i got it on the outer edge, simple because the sensor looked more "tucked away" under the brake caliper. I may move it if the wheel feels unbalanced, haven't been out on a long enough ride to notice yet.

    Yeah, the computer is really cool, I got a cheapo wired one on my MTB but I love the fact that on this I can keep track of my progress cos it stores heart rate/speed/cadence reading that I can download after my ride. Not sure I'll be doing any underwater riding though... but good to know the option is there :P

    Great value for money imo, only paid £80 for it inc cadence sensor :D
  • sibx
    sibx Posts: 102
    Where was it that you purchased it from, jammykam?

    I already have a C200 but after a crash last year both the handlebar mount and the casing of the computer itself broke, so I have to used blu-tac at the minute to ensure it doesn't come flying off at speed haha!
  • woody-som
    woody-som Posts: 1,001
    There is one thing that will help balance the wheel when you mount the magnet near the rim, as all mine are. Fix the bike off the ground level, and then spin the front wheel, let it stop on it's own, but the heaviest part of the wheel will be at the bottom, then put the magnet on the top of the wheel.
  • 3leggeddog
    3leggeddog Posts: 150
    Marko1962 wrote:
    What Beeblebox said..

    On my Cateye V3 I have placed my magnet as close to the hub as is possible, I think it says so somewhere in my instructions to do this and I also found out from experience that this is something I need to do as when on a run I was getting erratic speed readouts. Moving the sensor and magnet more towards the hub sorted that out, the magnet on the V3 is fairly small and weak compared to others that I've had.

    Centrifugal force will come more into play and could make the wheel feel unbalanced the further out the magnet is from the hub esp if it's a heavy magnet and the more chance of it flying off when your screaming along weeeeee...

    Aarghhh

    CENTRIPETAL FORCE decreases with radius F=mv2/r, or rw2 if using arngular velocity. Much more chance of losing your magnet close to the hub
  • FCE2007
    FCE2007 Posts: 925
    Oh!
    Powered by Haribo. Zwift convert P.aul Laws [x]
  • jammykam
    jammykam Posts: 10
    sibx wrote:
    Where was it that you purchased it from, jammykam?

    I already have a C200 but after a crash last year both the handlebar mount and the casing of the computer itself broke, so I have to used blu-tac at the minute to ensure it doesn't come flying off at speed haha!

    Bought it from Fitness Megastore, excellent service from them. Looks like they have a cycle mount at least: http://polar.fitnessmegastore.co.uk/polar-297/accessories-303/polar-cs-series-bike-12787.htm. Might be worth contacting them to see if they can get hold of a case for you as well.
  • jammykam
    jammykam Posts: 10
    woody-som wrote:
    There is one thing that will help balance the wheel when you mount the magnet near the rim, as all mine are. Fix the bike off the ground level, and then spin the front wheel, let it stop on it's own, but the heaviest part of the wheel will be at the bottom, then put the magnet on the top of the wheel.

    This makes sense, kinda like balancing the wheels on a car. Good thinking, will give this go...

    cheers
  • Marko1962
    Marko1962 Posts: 320
    edited June 2008
    3leggeddog wrote:
    Aarghhh

    CENTRIPETAL FORCE decreases with radius F=mv2/r, or rw2 if using arngular velocity. Much more chance of losing your magnet close to the hub

    Are you trying to say there is more rotational force exerted on a magnet closer to the hub than on one farther away and closer to the rim? If this is so which stone will be slung the furthest from a slingshot the one from a 1 ft or the one from a 2 ft slingshot both spun at the same rpm? dont think it would be the 1ft for sure...
  • 3leggeddog
    3leggeddog Posts: 150
    Yes

    the force experienced closer to the hub is greater than that closer to the rim, as the acceleration is greater. This works the same way as cornering, it is easier to take a wide line than a narrow line.

    The longer sling shot provides a faster linear velocity and so shoots the stone further.

    I take it you must have had girlfriends when you were at school, you certainly didnt study physics!
  • Marko1962
    Marko1962 Posts: 320
    No need for personal attacks.

    All I know from real world experience is that there is more chance of a magnet to go flying off the spokes the farther away from the hub it is compared to one closer the hub, but if you feel the need to argue the toss then feel free to do so, I don't particularly care...
  • Jez mon
    Jez mon Posts: 3,809
    I'm pretty sure that saying you weren't a socially challenged physicist at school doesn't constitute a personal attack, last time I looked, circular motion was only taught at A-level anyway. Back OT, IME it doesn't make a blind bit of difference where you put a spoke magnet and i've never seen one fly off, ever.
    You live and learn. At any rate, you live
  • 3leggeddog
    3leggeddog Posts: 150
    Marko1962 wrote:
    No need for personal attacks.

    All I know from real world experience is that there is more chance of a magnet to go flying off the spokes the farther away from the hub it is compared to one closer the hub, but if you feel the need to argue the toss then feel free to do so, I don't particularly care...

    Please re read my post!

    apologies for any offence caused, its called self depreciation by the way.

    are you american?
  • 3leggeddog wrote:
    Yes

    the force experienced closer to the hub is greater than that closer to the rim, as the acceleration is greater. This works the same way as cornering, it is easier to take a wide line than a narrow line.

    The longer sling shot provides a faster linear velocity and so shoots the stone further.

    I take it you must have had girlfriends when you were at school, you certainly didnt study physics!

    I think you could have done with more time at the textbooks and less time chasing the girls :)

    Centrifugal forces increase with radius, for a particular rate of rotation.

    Someone quoted the standard formula

    F=mv2/r

    But v, the linear velocity, is itself proportional to the radius. That is, the further you are from the hub, the greater the effective straight-line velocity. So for a fixed angular velocity (that is, rate of wheel rotation) w

    F =mw2r

    That is, the centrifugal force increases in proportion to the distance from the hub.

    Your cornering example is incorrect because when we drive, or ride, we don't do so at a particular rotational velocity that is appropriate to the corner. We corner at a particular linear velocity. If you took a really sharp bend at, say, 2mph, you'd find the centrifugal forces quite manageable.

    Having said all this, I can think of no reason why a properly-fastened spoke magnet should ever come lose and fly off. I've never known it to happen.
  • fluff.
    fluff. Posts: 771
    In the spirit of the thread, of course centrifugal isn't a force atall :)
  • fluff. wrote:
    In the spirit of the thread, of course centrifugal isn't a force atall :)

    That depends on your frame of reference and how fussy your definition of `force' is. From the frame of reference of and observer on a rotating platform it's a force. I appreciate that it's fashionable to refer to centrifugal forces and coriolis forces and what-nots are `fictitious forces', but that's not particular helpful to people who design things that spin :)
  • Marko1962
    Marko1962 Posts: 320
    3leggeddog wrote:
    Please re read my post!

    apologies for any offence caused, its called self depreciation by the way.

    are you american?

    An emoticon would have gone a long way to expressing your intent, but I'll let you off if you let me off ;) and no I'm not a Yank, are you a no humour Swede by any chance ;)