Advertise on Bikeforums.net



User Tag List

Page 1 of 2 12 LastLast
Results 1 to 25 of 33
  1. #1
    Senior Member
    Join Date
    May 2004
    Posts
    1,398
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Double the magnets ...

    Has anyone tried using two magnets instead of one on their cyclocomputer. I'm curious if this would give more accurate readings at lower speeds (you would just have the circumference to keep things right).

  2. #2
    neptune diner bennyk's Avatar
    Join Date
    Feb 2005
    Location
    NYC
    My Bikes
    KHS Flite 300
    Posts
    477
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    wouldn't this mean that your measurement of the wheel's circumference would have to be more accurate?

    edited to say, yes it would make the low-speed readings more precise, but it could throw off all of the readings.

  3. #3
    Tour de World SteveFox's Avatar
    Join Date
    Oct 2005
    Location
    Hinton, AB, canada
    My Bikes
    Trek 520, Giant Iguana disc
    Posts
    185
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    are you saying 2 magnets at opposite ends of the wheel, or right next to eachother? if they are opposite sides, they would give you twice the speed reading as the magnet is passing past the sensor twice in one revolution of the wheel, making everything X2. if you spin your magnet across the sensor back and forth it does the same thing, gives a really high speed...i got 97 km/h by doing that with my hand.

    Steve
    5 Days Till my Bike Trip
    Steve's World Bike Trip

  4. #4
    Senior Member DannoXYZ's Avatar
    Join Date
    Jul 2005
    Location
    Saratoga, CA
    Posts
    11,495
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Two separate magnets wouldn't make a difference. If the field is too weak with one, you'll end up with two weak pulses per revolution. What you can do, is to stack two magnets on top of each other oriented such that their fields are reinforced. In which case, yes, you'll have a stronger signal.

    What's the problem anyway?

  5. #5
    Conservative Hippie
    Join Date
    Mar 2005
    Location
    Wakulla Co. FL
    Posts
    4,271
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    One magnet using the diameter of the wheel.
    Two magnets, 180 out, using half the diameter of the wheel.

    I don't think it would make any difference for practical purposes.

    Of course you could always try it and see.
    Last edited by CommuterRun; 01-17-06 at 03:44 AM.

  6. #6
    Senior Member Retro Grouch's Avatar
    Join Date
    Feb 2004
    Location
    St Peters, Missouri
    My Bikes
    Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
    Posts
    23,591
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Don't tell but putting two magnets on your wheel will make you go twice as fast. It's the only way that I can keep up with the guys that I ride with.

  7. #7
    Dog is my co-pilot 2manybikes's Avatar
    Join Date
    Jun 2003
    My Bikes
    2 many
    Posts
    13,638
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Retro Grouch
    Don't tell but putting two magnets on your wheel will make you go twice as fast. It's the only way that I can keep up with the guys that I ride with.
    Then I'm going for three !!
    [SIGPIC][/SIGPIC]

  8. #8
    Announcer EventServices's Avatar
    Join Date
    Jan 2005
    Location
    In the drops.
    My Bikes
    Too many and not enough
    Posts
    4,994
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    How accurate do you NEED it to be???

    I'd say this would be an excellent practical joke to play on other riders.

  9. #9
    Dog is my co-pilot 2manybikes's Avatar
    Join Date
    Jun 2003
    My Bikes
    2 many
    Posts
    13,638
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by EventServices
    How accurate do you NEED it to be???

    I'd say this would be an excellent practical joke to play on other riders.

    It certainly was !
    [SIGPIC][/SIGPIC]

  10. #10
    Tour de World SteveFox's Avatar
    Join Date
    Oct 2005
    Location
    Hinton, AB, canada
    My Bikes
    Trek 520, Giant Iguana disc
    Posts
    185
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    aww, i wish people ride thier bikes here...id do that to someone. dang

    steve
    5 Days Till my Bike Trip
    Steve's World Bike Trip

  11. #11
    Former Member
    Join Date
    Feb 2005
    Location
    dropmachine.com
    Posts
    4,061
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Getting two spoke locations on exactly opposite sides of the centre of the hub would be required, and I'm not sure you can find them with a conventional 32H 3-cross. Otherwise your speed will appear to oscillate within a single revolution of the wheel.

    File this under "dumb things I tried at university". Nowadays it's all about the GPS anyways...

  12. #12
    Announcer EventServices's Avatar
    Join Date
    Jan 2005
    Location
    In the drops.
    My Bikes
    Too many and not enough
    Posts
    4,994
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    But if you attach TWO gps units to your friend's front wheel ....

    ...now you've got a practical joke!

  13. #13
    Senior Member Retro Grouch's Avatar
    Join Date
    Feb 2004
    Location
    St Peters, Missouri
    My Bikes
    Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
    Posts
    23,591
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by 2manybikes
    Then I'm going for three !!
    I thought about that but then I'd get ahead of them, wouldn't I?

  14. #14
    Senior Member
    Join Date
    Jan 2006
    Location
    Highlands Ranch, Colordao
    My Bikes
    '84 Peugeot PSV-10, '00 Schwinn Moab 2, '01 Airborne Valkyrie, '04 Surly Cross-Check
    Posts
    137
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Just think how fast you could go by putting magnets on all 32 spokes! Of course, I might have to remove the playing cards...
    "Somewhere, something incredible is waiting to be known."

  15. #15
    Mad bike riding scientist cyccommute's Avatar
    Join Date
    Nov 2004
    Location
    Denver, CO
    My Bikes
    Some silver ones, a black one, a red one, an orange one and a couple of titanium ones
    Posts
    15,312
    Mentioned
    4 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by CommuterRun
    One magnet using the diameter of the wheel.
    Two magnets, 180 out, using half the diameter of the wheel.

    I don't think it would make any difference for practical purposes.

    Of course you could always try it and see.
    I placed second magnet on my daughters bike by accident. As we were riding along, she stated that we were going really fast. My computer said we were doing about 12 mph. Her's said we were doing around 20 mph. It took a while but I found the second magnet. Now I'm going to put one on my buddies bike without his knowledge and listen to him brag about how fast he is. I wish I had a way of putting one that would cut the speed in half. I could have lots of fun with this
    Stuart Black
    Solo Without Pie. The search for pie in the Midwest.
    Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
    Days of Wineless Roads. Bed and Breakfasting along the KATY
    Twisting Down the Alley. Misadventures in tornado alley.
    An Good Ol' Fashion Appalachian Butt Whoopin'.

  16. #16
    The Red Lantern Rev.Chuck's Avatar
    Join Date
    Apr 2003
    Location
    Raleigh NC
    Posts
    5,967
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    They don't even need to be 180 off. The computer would recieve two signals per revolution regardless of position.
    Are you a registered member? Why not? click here to register. Its free, and only takes 27 seconds!
    Help out the forums, abide by our community guidelines.

    I am in the woods and I have gone crazy.

  17. #17
    Da Big Kahuna
    Join Date
    May 2003
    Location
    Oahu, Hawaii
    Posts
    814
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Nah, the magnets add weight and wind resistance - just change the computer calibration to a really high number!

  18. #18
    Da Big Kahuna
    Join Date
    May 2003
    Location
    Oahu, Hawaii
    Posts
    814
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I'm not exactly sure if the OP had the same thing in mind as I have wondered about or not, but here is my question:

    I have noticed that the actual speed tends to jump certain numbers. For example, mine might show 16.9 and even if I try to just barely speed up, it will then go to 17.2 - same problem if I slow back down.

    I'm not sure it can't or never has shown the inbetween speeds, but it sure doesn't do it much if it does at all.

    I figure it has to do with how the computer software works in relating seconds to rpm (rounding things off basically).

    So I wondered if I put a second magnet across from the first and cut my calibration number in half, would that cause me to get more accurate numbers? Sure, it isn't a big deal, but certainly more accuracy is better than less.

  19. #19
    Former Member
    Join Date
    Feb 2005
    Location
    dropmachine.com
    Posts
    4,061
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    If the magnets were not 180 off, the distance travelled would be measured correctly. But the current speed function times the difference between magnet passes. Hence the speed jumps up and down.

  20. #20
    Senior Member
    Join Date
    May 2004
    Posts
    1,398
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by TheRCF
    I'm not exactly sure if the OP had the same thing in mind as I have wondered about or not, but here is my question:

    I have noticed that the actual speed tends to jump certain numbers. For example, mine might show 16.9 and even if I try to just barely speed up, it will then go to 17.2 - same problem if I slow back down.

    I'm not sure it can't or never has shown the inbetween speeds, but it sure doesn't do it much if it does at all.

    I figure it has to do with how the computer software works in relating seconds to rpm (rounding things off basically).

    So I wondered if I put a second magnet across from the first and cut my calibration number in half, would that cause me to get more accurate numbers? Sure, it isn't a big deal, but certainly more accuracy is better than less.
    Yeah, this is kinda the point. With a large wheel the computer has to do a LOT of guessing (is he slowing down or speeding up?). This is more amplified at lower speeds. In the end the odometer will be the same. But two magnets might make the computer a bit more responsive.

    Three magnets would probably be optimal, but I don't think the computers take numbers that low for circumference.

  21. #21
    Senior Member
    Join Date
    May 2004
    Posts
    1,398
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by ghettocruiser
    If the magnets were not 180 off, the distance travelled would be measured correctly. But the current speed function times the difference between magnet passes. Hence the speed jumps up and down.
    It already does, that's the point.

  22. #22
    Senior Member BlazingPedals's Avatar
    Join Date
    Dec 2004
    Location
    Middle of da Mitten
    My Bikes
    Trek 7500, RANS V-Rex, Optima Baron, Velokraft NoCom, M-5 Carbon Highracer, homebuilt recumbent
    Posts
    7,022
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I think multiple magnets would make the readings more stable, but might result in a loss of accuracy. Here's how: if the circumference measurement is an odd number and you add a second magnet, you can't set the computer for half - it'll have to be half minus 0.5 or half plus 0.5. You could be off by 1 for each wheel revolution!

    OTOH, one integer difference is only going to introduce something like a 0.5% error or less. Make it up by not wobbling as much when you pedal or cutting your corners differently.

  23. #23
    Huachuca Rider webist's Avatar
    Join Date
    Jun 2002
    Location
    Charlotte, NC
    My Bikes
    Fuji CCR1, Specialized Roubaix
    Posts
    4,274
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    This thread raises an interesting point regarding computer accuracy which until now I hadn't really given much thought to. Saturday, I was certain that I had completed a 30-mile ride. My computer registered only 29.97 miles however. I think I'll get another magnet!
    Just Peddlin' Around

  24. #24
    jur
    jur is offline
    Senior Member
    Join Date
    May 2005
    Posts
    6,106
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    From a computer calculation perspective, having 2 magnets and entering 1/2 the wheel circumference could yield faster speed updates and smaller quantization steps, depending on the circumference resolution, for low speed.

  25. #25
    later free_pizza's Avatar
    Join Date
    Sep 2004
    Posts
    2,471
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by jur
    From a computer calculation perspective, having 2 magnets and entering 1/2 the wheel circumference could yield faster speed updates and smaller quantization steps, depending on the circumference resolution, for low speed.
    that would work perfectly provided that the magnets were placed at exactly the right points.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •