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Speedometer missing spoke part

Old 06-15-19, 10:50 AM
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rseeker
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Speedometer missing spoke part

I have an old speedometer with the sensor on the fork but the part (magnet? piece or steel?) that goes on the spoke is missing.

I also have a part for the spoke from a different bike.

What are the chances that I'll get correct speedometer readings if I install the spoke part?

I'm calling it "spoke part" because I don't know whatcha call it .. maybe sensor target.

And I don't know if it needs to be a certain size/strength of target, or conversely if the sensor simply detects a binary "the sensor target is passing" signal, and counts those to get RPM. Probably that...


Last edited by rseeker; 06-15-19 at 11:16 AM.
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Old 06-15-19, 11:01 AM
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Berg417448
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Any spoke magnet will probably work if you can get it close enough to the sensor. Its just counting RPMs.
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Old 06-15-19, 11:29 AM
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caloso
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https://www.biketiresdirect.com/prod...l-wheel-magnet
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Old 06-15-19, 11:45 AM
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It's just a magnet that induces a current in the coil of wire that is the sesnor. The pulse of Alternating Current is what the computer is counting. So any magnet of sufficient strength to induce a detectable current would work.

What the computer is doing is a frequency=speed calculation. It has some version frequency counter, probably in conjunction with the clock crystal.
((pulse/time)*wheel circumference=rate) There's nothing fancy going on in there to screw up.

2 wheel magnets makes you twice a fast!

Last edited by base2; 06-15-19 at 12:00 PM.
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Old 06-15-19, 11:46 AM
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philbob57
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That's just a nagnet; any magnet will do. if you can't get it close enough to the sensor, get a stronger magnet. Search for 'neodymium magnet.'
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Old 06-15-19, 11:59 AM
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rseeker
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Originally Posted by philbob57 View Post
if you can't get it close enough to the sensor, get a stronger magnet.
I'm thinking of a certain Breaking Bad episode ...


Last edited by rseeker; 06-15-19 at 12:04 PM.
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Old 06-15-19, 02:09 PM
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If you pm me your mailing address, I'll mail you one. For some unknown reason I have several in my box of sh..., er stuff.
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Old 06-15-19, 03:00 PM
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rseeker
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Originally Posted by Camilo View Post
If you pm me your mailing address, I'll mail you one. For some unknown reason I have several in my box of sh..., er stuff.
Wow, that's extremely nice, and I'll take you up on it. Thank you very much. Sending PM.
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Old 06-23-19, 05:19 PM
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rseeker
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Originally Posted by Camilo View Post
If you pm me your mailing address, I'll mail you one. For some unknown reason I have several in my box of sh..., er stuff.
It arrived. Again, thank you so much. And nice stationery (if you remember).
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Old 06-24-19, 09:41 AM
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Originally Posted by base2 View Post
It's just a magnet that induces a current in the coil of wire that is the sesnor. The pulse of Alternating Current is what the computer is counting. So any magnet of sufficient strength to induce a detectable current would work.
I think it's actually a reed switch not a coil, pretty much like a door/window burgler alarm. I seem to recall playing with a loose one and being able to hear it actuate if I held it close to my ear along with a magnet. Also I know my speedometers can be triggered by shorting their contacts, have had to do that several times while debugging. Just pulled out a meter and found a 1v bias across the contacts (probably with very high source resistance, only 1.5 uA flows if I short it, though that may be a time average of pulses, didn't get out the scope). That said, there might be coil based ones too.

But pretty much anything that securely attaches a small magnet to a spoke should work, and N/S magnetic polarity doesn't matter. I'll often test mine with a fridge magnet.

Alas mine worked right up to the mile 49 rest stop at the county line on a century, then wouldn't register anything more. Multiply removing and reinstalling it in the base didn't work, replacing the battery didn't work... Probably messed up the wiring leaning against a trail sign for a picture. I've bought and broken enough of these that if I ever get my act together and fix them I should just bring an entire spare sensor/wire/mount along.
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