by **MitusRider** on Fri Apr 09, 2010 4:47 pm

No problem, I look a good discussion and I'd be the first to admit I'm no expert.

The math models are correct, given perfect input. E.g a rider of known weight and surface area, who could hold a power constantly, on a consistent road surface (with known rolling resistance of drivetrain/tyres) with perfectly measured slope gradients and known wind resistance (and no drafting) the maths will work.

I've owned an iBike and Powertaps. The iBike when calibrated correctly (e.g. rider weight, frontal area and rolling resistance are set-up correctly through calibration) is extremely close to Powertap under good conditions riding solo. This device is fundamentally based on a maths formula constantly being computed, but also measures the slope accurately, wind resistance using a wind port etc. Now imagine taking away that knowledge of slope, wind and rolling resistance, frontal area knowledge and replacing it with estimates (or the best information Google maps or some weather station can deliver). It might be close, but then it might not be...and it will vary depending on the complexity of the ride and the environmental conditions. You just can't know.

The Powertap of course does away with all the guesswork/environmental factors by actually measuring the power output at the wheel.

Like I said, I'm no expert just a bike rider. There are whole discussion groups dedicated to this topic!

Cheers