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Old 02-10-07, 12:20 PM   #1
gsk3
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Will a magnet boost affect my computer's speed reading?

I just bought an SI-90 wireless computer for my MTB, only to find that (duh) there was no way I was going get the magnet anywhere near the wireless reader on the fork. After kicking myself for a minute, I remembered I had some rare-earth magnets on my fridge (long story...). Sticking two of those on the provided spoke magnet brought them close enough (perhaps 6-8mm away rather than the recommended 2-4mm) that the computer started reading speed. These are crazy-strong magnets, so I'm worried about two things: will this affect my readings (perhaps by having a much longer period in which the magnet is "seen" by the reader), and is this going to damage my computer and/or spoke? My understanding is that it's basically counting the time between hits of the spoke, so it should be ok, I think, but I don't know.

Thanks,
Ari
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Old 02-10-07, 12:49 PM   #2
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I am not an engineer. But I have done coursework in EE and Microcontroller programming.

Shouldn't damage the computer, that much I can be certain of. The sensor is probably a Hall effect or simple magnetic switch. The worst that could happen is you can't get the sensor to ever think that the magnet has passed the wheel till you move it away. Well unless you somehow magically break the sensor with the magnet, but I've never seen that.

As for how the computer will read it, that will depend on a number of factors but primarily how the guy who designed the thing programmed it. I've programmed microcontrollers with speed readings and there are a few ways to do it. One is to measure the off time between hits, like you say. The other is to measure the hits per second and display that. The ease of doing each would depend on the hardware you're working with and is puerly academic for the discussion we're having. I'm willing to bet however, that some computers work based on the former method, and some on the latter (Low end sigma sports, in particular...)

If it's measuring the time between hits, it again is an issue of programming semantics. You could reset the counter as soon as the 'switch' is hit on, or wait for it to be switched 'off' before resetting the counter.

I won't comment on the dynamics of the magnet sensor itself with a big magnet, because I can't pretend to know about those things.

www.dansdata.com <-- This dude loves magnets though and I bet you could ask him. Might take a little while to get an answer however.


Long story short, give it a try, see if the reading's accurate.... I'd test at all 3 speed ranges, 0-10, 10-20, 20-30.
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Old 02-10-07, 04:05 PM   #3
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The key thing here is if the sensor electronics are able to discern whether the magnet is either near the sensor or is not. For the case of a very strong magnet, the worst case problem would be that the sensor would not be able to detect that the magnet has moved away, and would compute a low or zero speed.

The best way to check if things are OK is to try it out. You should not see any erratic speed readings when you are riding at a constant speed.
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Old 02-10-07, 04:13 PM   #4
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Forget about putting on a stronger magnet. If you want better readings, put a SECOND magnet on your wheel.
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Old 02-10-07, 04:17 PM   #5
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Why can't you get the sensor near the magnet?
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Old 02-10-07, 04:17 PM   #6
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Quote:
Originally Posted by caloso
Forget about putting on a stronger magnet. If you want better readings, put a SECOND magnet on your wheel.
I think the OP wanted better as in accurate, not better as in faster.
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Old 02-10-07, 04:26 PM   #7
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Should be fine. I think it's just a magnetic switch that measures the time in between trips. If you want to be sure, do the math: ride on the road for like 100 feet at the same cadence, take that speed, the rollout on your tires, and your gear ratio, and figure out how fast you should have been going. If they match up, you're golden.

Alternatively, ride it on some road you know the exact length of, then see if your computer agrees.
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Old 02-10-07, 06:42 PM   #8
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I may test it against Google Maps tomorrow, but it seems to be working ok (at least judging by the no skips criteria dbc suggested). It seems to be strong enough to trip it at a distance but within an inch or so turns off (tested by wiggling back until it registered some speed gains). The magnet, by the by, is similar to this:
http://www.allelectronics.com/cgi-bi...2%22#34;_.html

Phantoj: Can't get the magnet closer because my spokes stay a good 1.5-2 inches from my fork. It's an old Marzocchi M3.

Caloso: I'll wait until the girlfriend gets home to try the second magnet trick. 40mph here I come!

Thanks guys,
Ari
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Old 02-10-07, 08:28 PM   #9
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Quote:
Originally Posted by gsk3
Phantoj: Can't get the magnet closer because my spokes stay a good 1.5-2 inches from my fork. It's an old Marzocchi M3.
How is the M3 special? I had no problem putting a computer on a Z3... You have to rotate the computer about the axis of the fork leg until the sensor is close to the spokes. Also, closer to the hub might work better...
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Old 02-11-07, 07:09 AM   #10
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Quote:
Originally Posted by Phantoj
How is the M3 special? I had no problem putting a computer on a Z3... You have to rotate the computer about the axis of the fork leg until the sensor is close to the spokes. Also, closer to the hub might work better...
Oh, duh. I tried a little bit of rotation, but assumed it needed to be oriented the way it was in the diagram. I'll try again later. Thanks.

Ari
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