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Re: Garmin Vectors Going Prime-Time [djmercer]
djmercer wrote:
Y-Tri wrote:
Ummm...........
What "issues" with Q-rings and Power2Max?
I have both and love them both!
Working great!
-YT


+1


[Update: Uh-oh. See MTM's post below.]

Ray's post verifies that P2M (like many other crank-based PMs) assumes constant rotation. If you can assume constant rotation speed then it simplifies power calculation, which is why manufacturers do it that way. When rotation is constant then all you need to do is get the average torque and multiply it by the (constant) angular velocity to get power. However, *by design* Q-rings don't have constant angular velocity so in order to get the correct power you ought to use the *weighted* rather than *unweighted* average torque where the weights are given by the changing angular velocity. If you don't do that the unweighted average torque for Q-rings will be biased, so the power reported by the PM will also be biased. The amount of bias will depend on how far the true angular velocities are from constant which, in turn, will depend on how eccentric the rings are -- but for "typical" non-round rings the bias will be in the ballpark of 2 or 3% (that is, if you're truly producing 250 watts then a Q-ring equipped power meter that assumes constant angular velocity will read something like 5 or 8 watts high, so unless you're aware of this you might think that a simple change to Q-rings gave you a 5 or 8 watt boost in power). P2M says that they only measure cadence by a single event trigger once per crank cycle so they can't observe changes in angular velocity.

[Edited to add clarification:] This is not a problem that applies only to P2M. It applies to all crank-based power meters that assume constant angular velocity. Because the Polar chain-based power meter measured chain speed and chain tension, it probably would have been immune to this particular problem. Because the Power Tap measures at the rear hub (and wheel rotation speed is typically faster than crank rotation speed) and also because its power measurement is time-based rather than event-based, it also should be less affected by this type of measurement bias.
Last edited by: RChung: Aug 16, 13 13:46

Edit Log:

  • Post edited by RChung (Dawson Saddle) on Aug 16, 13 10:04
  • Post edited by RChung (Dawson Saddle) on Aug 16, 13 10:20
  • Post edited by RChung (Dawson Saddle) on Aug 16, 13 13:46