Originally Posted by
jgwilliams
Yes, I probably will in due course. They're a bit expensive and I can't afford to just go out and buy them on a whim. I got the Topeak one at a bargain price and it's been quite good up to now. I have a spare cadence sensor because it was ultra cheap and turned out to be not a very good one. I managed to pick up a genuine Garmin one to replace it on Ebay quite cheaply. But the two Garmin magnet-less sensors I have are for the mountain bike, as I find the combined ones can get knocked out of place quite easily during some vigorous downhill activity. I once did a really good time (at least, it felt like it) on a local trail only to find that the wheel magnet had been knocked askew and, once again, the Garmin had registered 0 mph for most of it.
When you get the bill for the diagnosing effort, you are going to wish you just bought a sensor!
Nothing is going to be perfect.
The 520+ doesn't (as far as can tell) have a general problem with this. The GSC-10 doesn't (it's been a round for a very long time). Topeak is a competent company (so their sensors should be fine).
The issue with magnets is one reason I mentioned the other type. It's possible that your issue is the sensor(s) moving around a bit. Maybe, the magnet is weak.
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This thread would be more appropriate for the Electronics subforum.