The pickup for bike computers are mechanical reed switches. The
attraction from the magnet is all that is able to "open" or "close"
the reed switch, so it has to be a fairly flimsy mechanism.
This flimsiness tends to allow the switch to get sloppy when there is
a lot of vibration (i.e. riding off-road) or when the magnet is flying
by at a speed that causes the switch to not fully engage, but to
"bounce". This sends multiple "pulses" to the computer sometimes,
upping the apparent speed.
Some computers are better at filtering this out (they're smart enough
to ignore a series of pulses that tell the computer that the bike
accellerated from 20 to 80 and back to 20 in what appears to be three
or four revolutions).
Home of the $695 ti frame
>Much as I'd like to believe that I hit a top speed of 60.2 mph on my
>mountain bike, and that all of the trail markers understate the mileage
>covered, I think it more likely that the speed sensor is malfunctioning.
>Anyone else had similar problems?
>- Speed seems to jump around more than it should. I see wide fluctuations
> that mostly settle out after awhile.
>- Trip mileage is high. Almost looks like it is displaying KPH instead of MPH,
> but the MPH symbol is definitely there.
>- I am using a non-standard magnet on my wheel that is considerably larger
> than the poorly designed (IMHO) one that Polar provides.
>- I sent the unit back to Polar. They replaced the battery in the
> wrist receiver, and checked the cyclometer functions. They claim it
> checked out.