On Tue, 15 Jun 2004 10:01:00 GMT, FranklynMint
<
[email protected]> wrote:
>Greetings fellow cyclists I have a mid-price cycle
>computer, which is set up according to instructions (double-
>checked and triple checked). On a long ride, recently
>(recommended by a friend, who said it was about 40+ miles,
>he hadn't measured it) i was quietly impressed by the
>distace i rode, which was longer than he'd said. But
>looking at a map later, I thought something was amiss. I
>reckon that the computer is about 1 mile out every 15. Is
>that par for the course with these gadgets? I'm not
That comes out at nearly 7% error, which I would think is
quite large. There are two main sources of error for these
things that I see:
1) The accuracy of the wheel circumference measurement
2) Sensitivity of the magnet + sensor (ie does one
revolution of the wheel register as one revolution in
the sensor)
The wheel circumference (C) is something that you should be
able to get quite accurately with a roll-test (where you sit
normally on the bike, keeping the front wheel straight, mark
the start and end of one (or more) wheel revolutions while a
friend pushes you and measure).
This should get it to within 0.5% at the worst (5mm error on
a 2000mm C wheel is 0.25% error). As C varies with tyre
pressure, ride weight etc this is probably more like 1-2% in
practice. Generally this bit is as accurate as you can make
it (within setting resolution of mostly 1mm on the computer,
ie about 0.001%).
What could be causing you trouble, assuming your C is set as
accurately as you can get it is the magnet sensor set up.
You want it as near to the hub as possible so that the
magnet sweeps past the sensor as slowly as possible. This
gives the sensor longer to react. If you've got it nearer
the rim and you're going at a decent pace the sensor may be
saturated and thus record a different number of revolutions
than there have actually been.
If your sensor works properly your distance should simply be
revolutions x C
so your only significant error lies in C (measurement error
and how it varies as you ride).
>interested in measuring to the millimetre, nor do I have
>more than a passing interest in the average speed, etc, but
>it's nice to have an idea of how far you've done.
>Arrivederci FM
Speed and distance are related, so getting one accurate
means the other should be as well. If you get your C to
within a few mm and get your sensor properly set up you
should easily achieve 1%, which is really quite accurate
when you think about it (1 mile in a hundred). Don't
forget that something that's 10miles in a straight line
might not be 10miles on a bike that weaves a bit (as a
front wheel tends to do a bit) so your distance reading on
the bike may well be accurate even if it doesn't quite
match what the map says.
As we're generally not looking for scientific precision it
doesn't matter overly much. My gf's computer was giving a
few % different to my computer so we tweaked hers to give
the same results. This isn't because mine was any more
accurate but just to make things fair (and hers was more
tweakable).
Frink
--
Doctor J. Frink : 'Rampant Ribald Ringtail' See his mind
here :
http://www.cmp.liv.ac.uk/frink/ Annoy his mind here :
pjf at cmp dot liv dot ack dot ook "No sir, I didn't like
it!" - Mr Horse