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Old 01-17-06, 01:11 AM   #1
willtsmith_nwi
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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).
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Old 01-17-06, 01:14 AM   #2
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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.
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Old 01-17-06, 01:17 AM   #3
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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.

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Old 01-17-06, 02:41 AM   #4
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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?
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Old 01-17-06, 04:39 AM   #5
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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 04:44 AM.
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Old 01-17-06, 05:15 AM   #6
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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.
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Old 01-17-06, 06:22 AM   #7
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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 !!
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Old 01-17-06, 07:23 AM   #8
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How accurate do you NEED it to be???

I'd say this would be an excellent practical joke to play on other riders.
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Old 01-17-06, 07:25 AM   #9
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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 !
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Old 01-17-06, 09:20 AM   #10
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aww, i wish people ride thier bikes here...id do that to someone. dang

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Old 01-17-06, 09:42 AM   #11
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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...
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Old 01-17-06, 11:11 AM   #12
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But if you attach TWO gps units to your friend's front wheel ....

...now you've got a practical joke!
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Old 01-17-06, 11:16 AM   #13
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Then I'm going for three !!
I thought about that but then I'd get ahead of them, wouldn't I?
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Old 01-17-06, 11:21 AM   #14
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Just think how fast you could go by putting magnets on all 32 spokes! Of course, I might have to remove the playing cards...
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Old 01-17-06, 01:15 PM   #15
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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
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Old 01-17-06, 01:43 PM   #16
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They don't even need to be 180 off. The computer would recieve two signals per revolution regardless of position.
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Old 01-17-06, 01:48 PM   #17
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Nah, the magnets add weight and wind resistance - just change the computer calibration to a really high number!
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Old 01-17-06, 01:53 PM   #18
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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.
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Old 01-17-06, 02:30 PM   #19
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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.
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Old 01-17-06, 02:37 PM   #20
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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.
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Old 01-17-06, 02:41 PM   #21
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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.
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Old 01-17-06, 02:59 PM   #22
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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.
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Old 01-17-06, 04:31 PM   #23
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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!
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Old 01-17-06, 04:44 PM   #24
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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.
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Old 01-17-06, 04:49 PM   #25
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Quote:
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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.
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