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Old 04-15-18, 12:00 PM   #26
wphamilton
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Magnetic reed switches can have maximum operating frequencies in hundreds of times per second, which on a bike wheel would be hundreds of miles per hour. So I think that having the magnet at the rim will make no difference whatsoever with respect to the magnet going by too fast for the sensor to read. If anything, I'd expect better performance at the rim because the faster attenuation of the magnetic field would mean less debouncing issues.

Reed switches have several advantages over hall effect switches, less power draw, cheaper, simpler, unaffected by electrostatic shock. The hall effect might last forever, but are more sensitive to current in wires, ESD and so on. Since a reed switch can last for billions of cycles, that's not really a problem. Convert a billion turns of the wheel into miles, and you'll see what I mean.

I think that the orientation of the magnet is a bigger concern than its position.
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Old 04-16-18, 07:47 PM   #27
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Wheel magnets are plenty accurate, no need to worry about where they are placed. I always had accurate mileage when I used them. I now use rotation sensors and they are just as accurate, especially when combined with a GPS head unit. It can calculate wheel size based on GPS miles/revolutions, and just take it from there. I switch my sensor and head unit from a 700c road bike to a 26" MTB and I get the same miles on either, and the same MPH, both based on the rotations, not the GPS, without doing anything. And these miles equal the miles my old magnet system would report. I had them running together for a while just to be sure and it never varied by more than .1 over a 50 mile ride.
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Old 04-16-18, 08:26 PM   #28
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Originally Posted by ItsJustMe View Post
High end ones might use a hall effect transistor, that would be the smart way to do it. Much more reliable than an inductive loop since a hall effect switch is extremely fast and will give you essentially an on/off switch no matter how slow the magnet is moving (or even if it's stopped altogether). Hall effect transistors are pretty cheap.

It's possible that newer comps have already switched to hall effects, I wouldn't know, my computer is a number of years old at this point.
Don't know of any current bike speedometers that use a Hall effect sensor, but Avocet made some that did back in the '80s. I still have one of theirs which uses a little ring mounted on the spokes at the hub instead of the single magnet used by the reed-switch models. One advantage of the Avocet approach was that it got multiple readings per wheel revolution and therefore a bit faster response at slow speeds.
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Old 04-17-18, 06:20 AM   #29
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Don't know of any current bike speedometers that use a Hall effect sensor, but Avocet made some that did back in the '80s. I still have one of theirs which uses a little ring mounted on the spokes at the hub instead of the single magnet used by the reed-switch models. One advantage of the Avocet approach was that it got multiple readings per wheel revolution and therefore a bit faster response at slow speeds.
The little Avocet pickups that worked with the ring magnets were just coils of wire. They were nice, simple, and somewhat robust (the connectors that slid onto the pickup's pins were not all that reliable). I'm still using these Avocets on three vintage bikes, and have a new Avocet 20 in the box. Very minimal, and a bit of a conversation piece.

These old Avocets do go through batteries quickly... mine last about a year. Maybe that's why there was an assumption that it used hall cells?


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Old 04-17-18, 06:44 AM   #30
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I never thought about the sensor tech in an avocet, but a hall would just require a single magnet just like a reed switch. I used my avocet for years, the ring eventually got to be too much of a pain and I stopped using it. I hope I threw it out, but it might be in a junk box somewhere.

GPS speed is good enough for me, I know it's unreliable, but close enough. Don't the new ones use an accelerometer?
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Old 04-17-18, 07:39 AM   #31
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Yes, FWIW the speedo sensors I have are reed switches. I haven't had them skip but it is certainly possible for them to miss a very brief passing of the magnet...
I use a Sigma bike computer on my inlines, wheel dia 74 mm.
I chicken out shy of 40 kmh/25mph, at which speed the computer is still doing fine.
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Old 04-17-18, 11:58 AM   #32
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Yes, of course RPM is constant. But like some one mentioned - it is important that you give the magnet enough time to "pull" the sensing switch in the pickup unit.

Believe it or not, when i used to get fierce while riding rollers, i could get pickup sending units to "fail" or skip when riding at speeds near 50 mph. This was mostly because the unit was simply nearing being worn out. But it is proof that there was actual "mechanical" activity in older cat-eye and avocet units.

............
Kinda, sorta, maybe!

It's a hypothesis, and a very reasonable one. Another might be that the wheel under such loads it experiences at that speed deflects and deforms more at the outer diameters. So maybe not so much the time of engagement between the magnetic flux and the reed switches, but maybe more that the magnet for that revolution was further away from the sensor.

But who knows. I'm not going to go out and test it. But likely there are other possible reasons too.
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Old 04-18-18, 11:06 AM   #33
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Jobst Brandt designed that Avocet computer. He claimed it was more accurate than the kind with the single magnet. He said the ring created a sine wave, and the computer counted the number of times voltage went to zero. I don't know enough to say he's right or wrong, but it sounds reasonable to me.
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Old 04-19-18, 04:42 AM   #34
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that's interesting that he designed it. The difference in accuracy is questionable in my mind. And the disadvantages of the design from a mechanical viewpoint outweigh them.
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