On Oct 2, 2:49*pm, "GB" wrote:
PeterC wrote:

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.

I'm confused by that, sorry. I can see that pressing down on the handlebars
reduces the radius (but significantly distorts the tyre from a circle).
However, the tyre must still lay down 212.5cms of rubber onto the road per
revolution. So, where does the extra 1cm go when you only get 211.5 with the
weight on? I'm assuming no slippage between the rubber and the floor.

Mildly interesting fact, road races (running, marathons etc) are
always measured to be longer than they need to account for the
inherent 0.1% error when using a calibrated bicycle measurer. So in
the worst case, a marathon would be 42 metres longer (could be 15-20
seconds for a world class marathon runner).

http://www.coursemeasurement.org.uk/director-1.htm

Matt