Originally Posted by
slynkie
Some significant part of that difference is drivetrain loss. I think the rest can be accounted for simply by the wear and tear on my PT hub, either causing additional loss or just bringing it out of calibration. I don't have a ton of miles on it, maybe 5000, but I've abused the hell out of it, in all types of weather and roads, and the hub itself has never been serviced.
If there is significant "drivetrain loss", the Quarq should be calibrated to account for this or it is giving falsely high results which isn't right. You certainly wouldn't want a speedometer on your car that gave falsely high results. You wouldn't tell the cop that your speedometer must be off because of drivetrain loss
.
There is no reason that normal usage on a Powertap should cause it to give falsely low results. I have 20,000 miles on the my Powertap and the numbers certainly aren't going down because of wear on the unit.
Powertaps are also easy to calibrate but mine has never needed it in 20,000 miles so I don't think that that is a factor either.
Just based on your data for 1 ride and with 2 devices (maybe other Quarqs and Powertaps would be different), it appears that the two don't really correlate all that well. That is a little disappointing. Maybe these powermeters are only accurate to +/- 5% or so (at least in comparison to someone elses powermeter). Again, great post.