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Answer to the Power Tap accuracy(1 post)

Answer to the Power Tap accuracyupandcomer
Dec 5, 2002 8:56 AM
The following is an email response I recieved from Adam Weaver at Cycleops:

The PowerTap measures power output at the hub, obviously, and doesn't
attempt to guess the drivetrain ineffeciencies of the particular bike. Power
training offers an objective point of reference, and whether it's measured
at the crank, chain, or hub is somewhat inconsequential as long as it is
repeatable, accurate, and reliable. Having said that, we find the Powertap
consistently measures between 3-10 watts lower than an SRM, or a known load
applied via a dynamometer to the bottom bracket directly. 8-10 watts is
typical, but we find that a new chain and new cassette reduce this
discrepency by about 5 watts. In order to calculate the drivetrain
inefficiencies in your own bike you'd need to measure it simultaneously with
a Powertap and SRM, or dynamometer or somesuch. Since all of these devices
offer plus/minus 2 percent accuracies it's probably splitting hairs. People
have applied power meters w/ much greater success to aerodynamic position
studies, rolling resistence studies, and the optimal cadence question.
Thanks for your interest. Take care.

Adam, PowerTap