zero offset

Discussion in 'Power Training' started by newbie40, Jul 1, 2013.

  1. newbie40

    newbie40 New Member

    Jul 1, 2013
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    the zero offset on my power tap is 521 is this with in allowable range, will I be getting true power readings, or is the power tap faulty at this value


  2. daveryanwyoming

    daveryanwyoming Well-Known Member

    Oct 3, 2006
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    Your PT hub is almost certainly good at that zero offset self test value. I say almost certainly as it passes the PT self test (~ 512 +/- 12) but that is really just a comparison to known good units and only represents the unloaded strain gauge bias point. But still unless you have other reasons to suspect problems your unit is very, very likely to be good.

    I've seen a few units that passed the self test but didn't report torque accurately under load. They're rare and in all cases the rider had strong suspicions that the power numbers were way off but the self test does not actually test under loaded conditions so it is not an absolute test. But still the self test tells you a few good things, it tells you torque is actually being reported and it tells you the unloaded torque bias point is very close to known good units off the factory floor which makes it likely (but not certain) that the unit is good.

    If you're concerned about absolute accuracy then go get a big weight plate like a 20 kg Olympic weight plate with the big center hole. Have the weight validated at a local shipping center on certified postal scales (the UPS shipping centers here are happy to do this). Then carefully perform a static torque test using the known weights to measure the torque reported by the PT under load to the known torque you apply by hanging the known weight off a known crank length. Compare them as a percentage to determine percentage torque measurement accuracy under load. Since wheel angular velocity is simply a reed switch and is easy to measure accurately power accuracy = torque accuracy (power = torque*angular velocity). It takes a known weight and about ten to fifteen minutes (five once you get confident with the test) to do the torque testing.

    Google 'PowerTap Stomp Testing' for more info but use a known weight that has been weighed on accurate scales not just body weight and your bathroom scale as described in some Stomp Testing references. Sure you can do that but it often introduces additional error as it's hard to completely balance all weight on one pedal and also assure that pedal is horizontal and it's hard to know how accurate your bathroom scale is so much better to use a known and verified weight.