Powertap G3 correct out of the box?

[DutchMike]

Hi all! This is my first message on this forum. I received my first PowerTap (G3) yesterday (yeah!) and rode this morning for the first time, but I'm getting higher values than expected. I'm hoping that someone can clarify this. After Googling and reading on several forum, including this one, I realized that I should have calibrated the Powertap and I didn't. But it was set (defaultly) to auto-zero and it was indeed reading 0 watt when I didn't do anything. The torque was jumping between 0.00 or 0.09 when I checked during calibration. Is there something like a calibration slope (next to the bias) and can that be off? How would the calibration set the slope? How much error could I expect on a new Powertap without calibration (with power and torque outputting 0 when doing nothing)? Ehm.. sorry, lot's of questions This was the ride: http://app.strava.com/rides/21049234 I had an average of 247W over the ride, while I was expecting about 200W-220W with the speed I was going (33,5km/21mph; I had a +-3mph tailwind in a 45 degree angle or so and rode on the hoods) and also given my current fitness. When I use power calculators online I get values of 200-220W. I have been training for 3 months or so. Due to an injury and subsequent surgeries I didn't train at all for almost 6 years before that. My heart rate was approx. 160bpm and I think my AT is at approx. 168bpm. I'm male, 33, 181cm/6ft, 75kg/165lbs. Or should I just trust the values? That would be just awesome.. Thanks! Michiel

 

[bgoetz]

You cannot actually calibrate the powertap, it can only be stomp tested and then sent back for calibration if it is off. The calibration on the Garmin head unit (assuming that is what you have) is for the unit to communicate with the powertap. That said my guess is the calibration is fine, it is the settings in your Garmin that are off. Set your Garmin to record zero averages for power, when it is set to nonzero averages it does not average in zero power, which is what is causing your average to be high. Also be sure it is set to record every second, it should default to this, but I have heard stories about it not. I also do not use auto pause, but that is really a personal choice.

 

[DutchMike]

Thanks for the reply!

The Garmin (Edge 500) was set to include zeros when averaging power (I have it set to non-zero for cadence) and it records every second. It was originally set to non-zero for all and 'smart recording'.

So I should do the stomp test? With the weights hanging on the pedal? hmm, but that seems like a hassle. But maybe I should. The numbers are just too high.. 3.3W/kg seems just too high for this commute ride and me being way out of shape. Or maybe I'm just going to be happy about it

Thanks again!
Michiel

 

[HillyGoat]

Hanging weight on the pedal is quite easy and will tell you if the PM is accurate or not. I have 2 G3's and they are right on.

 

[bgoetz]

Yeah stomp testing is easy. I had a good website to enter the #s , but I can't seem to find it. 247 is realistic it was only a 24km ride. I would say most reasonabally fit individuals in your weight range could TT that at 247, just based on fitness. So if you were at a steady pace, just under threshold you fall to the more fit side of the spectrum.

 

[alienator]

So I should do the stomp test? With the weights hanging on the pedal? hmm, but that seems like a hassle.

It's about as much of a hassle as inflating your tires and lubing your chain. Do the test.