On Fri, 30 Sep 2011 17:10:01 +0100, Ron Lowe wrote:

On 30/09/2011 14:50, robgraham wrote:
The distance that my bike computer is measuring is not in agreement
with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is
traced on GE, and on the setting of the wheel size into the computer.
So taking those out of the equation, given a careful plotting of a
calibration road distance of 4.0 miles on GE, what is the accuracy of
their 'Path' measuring system?

The alternative question is how does one get a reliable distance to
obtain a calibration.

I've just had an argument with myself as to whether I increase the
wheel calibration figure or reduce it to compensate for the 5% high
error I'm getting on the 4.0 miles, and have won the argument by
realising that the entered circumference is the multiplier for each
pulse from the sensor, so counter intuitively (initially to me anyway)
to reduce an over-mileage, you reduce the computer wheel calibration
figure.

Rob

THe way I do it is this:

In the cycle computer setup, there will be pre-sets for common wheel /
tyre sizes, like 700x23. Those are usually close.

But there's usually a 'other' option where you can manually enter a
number corresponding to the distance covered per revolution ( the wheel
circumference ). To get an accurate measure of this, make a mark on
the ground, set the wheel with the valve exactly down at the mark, and
with your normal weight on the bike, make exactly 10 revolutions and
make another mark. Measure the distance, and divide by 10.

THe reason I say to do it with your normal weight on is that you will
squidge the tyre to some extent, so best to do the calb under normal
load conditions.

For my 700Cx28 tyres the 'book' says 214cm, carefully measuring 1 rev. gives
212.5cm, applying about 30kg on the bars gives 211.5cm so I use that.
The tyres are about 80psi; the other bike has 25mm tyres at 100psi and the
extra loading makes no discernable difference.

If you can find a measured mile[1] that far enough to eliminate most errors.
Bear in mind that 1 pass of the magnet adds 2m, even if it moves only 1cm
past the sensor.

[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.
--
Peter.
The gods will stay away
whilst religions hold sway