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Bicycle computer installed

Discussion in 'Modifications and Projects' started by bobcat, Jul 24, 2012.

  1. It all started when my gs500 doesn't have a clock - originally i was only gonna put an old digital watch around the handle bar but after goggling a bit and notice some people putting bike computers on their bike, i decided to give it a go as well.

    99bikes had Sigma 1009 computer on special at the chermside store for $34 http://99bikes.com.au/shop/accessories/sigma-bc-1009-sts-wireless-computer

    installation is pretty straight forward, similar to a bicycle http://www.sigmasport.com/en/produkte/bikecomputer/topline_2009_wired/bc1009/produktvideo/temp.html

    the unit was mounted on the handlebar

    and wire down to the front brakes. The hardest thing is the find the right place to mount the magnet.


    I've ordered some rare earth magnet from ebay as suggested by other installer online http://www.ebay.com.au/itm/10PCS-8mm-x-1mm-Super-Strong-Round-Rare-Earth-Neodymium-Magnets-Magnet-Kid-Toy-/170855552573?pt=AU_Hardware&hash=item27c7c8d63d
    so when they come I'll probably epoxy the magnet onto the rotor

    somehow the images hosted by servimg made them upside down for some reason
  2. So much fail.
    • Like Like x 1
  3. this is a joke right?????????
  4. I am confused
    Isnt it like a speedo
    What does it do ?
  5. I know a bloke who did that on his vintage car, coz it was so old it didn't have a speedo or at least not one that was sufficiently accurate. He too, used a rare earth magnet to attach the sensor. Worked a treat, and he was very proud of his "whizgiggery".

    Interesting chap, if you started him on the subject, he could go on for hours about rare earth magnets. Tragically, now in the advanced stages of dementia and can't tell you what he had for breakfast let alone discuss rare earth magnets.
  6. I use it mainly as a clock but yes it also doubles as a speedo too, and a trip computer
  7. My wireless bicycle computer (on my bicycle) gets confused when I go near my swipe card machine to open my garage. Starts thinking I'm going 76.6 km/hr. Quite annoying because I like to try and crack my top speed and if I forget to check what it was before I open the garage I lose it.
  8. Not 68 kmph then?
  9. 68 kph? On a bicycle? Inconceivable!
  10. Seen this done on a lot vintage bikes.
  11. Does the unit have a backlight?
  12. the more expensive 1609 model does, you can get time on ebay for about $50
  13. Is that magnet going to stay on the brake rotor with just those zip ties?

    I used a wired Sixma 1609 (if you want to buy it, it's all yours minus the magnet) and epoxied the magnet as close to the inside of the brake rotor as possible to minimise the chances of the epoxy overheating and weakening and the actual magnet flinging off due to sheer force on a spinning rotor.

    Worked a real treat with no issues. Was accurate right up until some very high speeds (I left the original speedo on to calibrate initially).
  14. TLDR; Should be fine.

    The GS500 has a redline of 11,000 RPM, top gear ratio of 0.851 (top speed of 175km/h) so in a theoretical world you could get the front wheel doing approximately 9360RPM.

    A rough approximation from the photos would suggest that the magnet is ~60mm from the axle. Neodymium magnets have a density of around 7.5g/cm^3, let's assume it's a cubic centimeter. Most magnets I've seen on bicycles aren't neodymium, but are around that volume when you include the spokes clip and stuff. So, let's say it weighs 7.5g.

    The compressive strength of a neodymium magnet is 1100 Newtons/mm^2, ignoring some loss for a non-perfect "magnetic grip". Perfect world, remember. Assuming we have a single cubic centimeter of it, we're talking 1100Nm of compressive strength (1100N/mm^2 * 1cm^3).

    Let's also ignore other factors, such as gravity, wind resistance, inertia, apples and acts of God.

    I'm also assuming the magnet sticks to the rotor. I've just realised it may not, thus invalidating all this below. Read on, for science. :p

    Maths time!

    The acceleration required to keep an object on a circular path is w^2 r, where w is the angular speed. This centripetal acceleration is directed towards the center of rotation, and the corresponding force is given by f=ma. However, we need to convert the RPM of the wheel to a frequency, so we divide the RPM by 60.

    a = w^2 * r
    F = ma
    w = 2 pi * freq

    ... F = M (w^2 * r)
    ... F = M ( 2 pi (rpm/60)^2 * r)

    Convert to SI, and substitute:
    ... F = 0.0075kg (2 pi ( 9360 / 60 )^2 * 0.006m
    ... F = ~432N
    ... F = 432 kg m/s^2

    So, the instantaneous "centrifugal force" wanting to fling the magnet off of the disk is 432N. The magnet is gripping to the disk of it's own strength at 1100N, given all above assumptions.

    You've also then got to consider the friction of the magnet sliding across the rotor, and the force of the zip ties holding it there. The zip ties are applying a downwards force on the magnet (and pushing the rotor back into the magnet), increasing the relativistic weight of the magnet on the rotor. With some quick googling, it looks like 10lbs is an appropriate tensile strength of small zipties.

    If the front wheel of his GS500 starts travelling > 15000RPM, then that magnet is going to fly with around 12kg of force for it's cubic centimeter, and put a really nice hole in his radiator.


    Please be aware, it's just gone 2am, there's probably a error or two in the above. Don't be angry at me if you all lose your magnets :p
    • Like Like x 1

  15. =D> ORZ
    Awesome physics, the last time I did f=ma was too many years ago. This is way too hard core for my morning read with only 1 dose of caffeine!

    Clift notes - so if I don't go over 15000rpm then i'll be fine? and the closer to the centre the magnet is mounted, the less likely it's gonna fly off due to less force?

    atm the magnet is ziptied and glued on.

    I'm just amazed at how far out my speedo is - no wonder in my first week of riding when I was sticking to the speed limit on the road, all the cars behind me seems to be a bit annoyed :-k
  16. With all that math you forgot that there is a final drive ratio, usually about 3:1, depending on front/rear sprocket sizes.

    I suck at arithmetic, let alone mathematics, so I'd guess that the front wheel speed, which will be similar to that of the rear wheel, should be a third of that 9,360 RPM figure quoted above.

    Say, at max speed it's then about 3,000 rpm, at legal speeds it will be a fraction of that.
  17. close ..

    but the 1100 you referencing to is the force normal to the contact surface between rotor and magnet? which then you add the tension in the zip ties.

    then the centripetal force is actually that normal force multiplied by the friction co-efficient based on the surfaces of the magnet and rotor. if magnet is in a plastic cover this will be quite low. so the holding force is probably a magnitude lower than what you calculated?

    anyway ...

    it is a tad silly to attach a setup in this way me thinks. :)
  18. Why do you ask this?

    I installed a Simga Sport cycle computer onto my CBR1000F about 8 years ago because of the same reason - no clock. I carried it over to the Blackbird, even though it does have a clock. Did this because it's cheaper than a Speedo Healer and it offers me a basic trip computer into the bargain.

    I use a rare earth magnet for the pickup. It pretty well "glues itself" to one of the disc's retaining bolts, but a dab of Locktite industrial strength adhesive has ensured that it's permanency for the past 60,000 km.

    I calibrated the CC against a Garmin Etrex Legend GPS on a local road which is flat, straight and fairly quiet and is about 4km long before it ends.

    I did a number of runs, each at a steady 100 km/h indicated, each time stopping to adjust the wheel size setting until both speed and distance traveled equaled what the GPS was telling me.

    And I can say that it's spot on up to $2.60...
  19. agreed .. best way to work out wheel rpm vs speed is to calculate wheel circumference then can get rpm from speed. going of engine speed is a pain. which is why all the ecu driven speedos of today instantly become wrong with a sprocket change

    sbb .. shutting up now .. my work should not become attached to riding ;)
  20. I would am not sure about the connection with the cable ties on the brake rotor, why not just silcone the magnet it into one of the many holes on the brake rotor. Mine has never come out after 30 thou km's