How do they measure rider power output?

[Globalti]

First came across this in the Michael Hutchinson book and have now seen it in relation to the Tour. Is it done by a speed/pedal RPM comparison?

 

[chris42]

There are 3 main methods
SRM Cranks which have a strain meter (don't know the correct term) in the bottom bracket.

Power tap.
Similar to SRM but located in the rear hub

Polar
Which is the least accurate which works on the movement of the chain and the cadence I think.

 

[Squaggles]

Speaking of SRM has anybody got the live SRM data on the cyclingnews website to work ?

 

[Keith Oates]

I've had no luck with that, is it for real are are they taking the mickey!!!!!!!!!!!!!!!!!!!!!

 

[Keith Oates]

Talking of NO NO's, I can't get any joy with NOS either today. Anybody more lucky than me!!!!!!!!!!!!!!!!!!!!!!!!