Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

An IF of 1.965?

For a 14.6mph average?

Yeah, ignore the IF, FTP is set at 200 watts in Garmin Connect.

Here's a file from my SRM for comparison- http://tpks.ws/O5aU

Which has a more believable figure for the average power- ~150 watts.

I was pushing on today, but I really don't believe I was pushing as hard as the Powertap thinks.

Calibration figure for the Powertap was 506, which sounds about right, so what have I done wrong?

Yes, just comparing the 5sec numbers you can see something is very wrong:

SRM: 649W
Powertap: 1,217W

Disengage the brakes next time?

Looks like it's reading double, ish.

Is it a second hand PT? Might need a new torque tube.

What's the torque offset?

Brand new, this is the first time it's been used. How do I check the torque offset, or is that the calibration figure it gave me before I set off this morning? If so, 506.

512-520 is normal apparently.

From a bit of Googling on this it is suggested that including zero's can skew the data- I checked and my Garmin was recording zero power data, if that makes sense.

I've turned that off, we shall see what the ride home brings.

Would have thought that would work the other way though? See how it goes. If still not right, try the static calibration thing (something to do with hanging weights on your pedals).

If that's off I have to send the wheel to Saris, I believe.

This is not the "welcome to Powertap!" experience that I was after.

Fingers crossed it's the zero thing, and that the Powertap is fine.

Would have thought that would work the other way though?

I'm not sure, but I'd have thought this. Worth a try though.

From the looks of the graph though I'd guess it was something else. Unless of course your commute to work always comprises 600-1000 watt sprits between lights.

Not on Fridays

I've uploaded the same file to Training Peaks, http://tpks.ws/NX1i

Average power now down to 203 watts, which is perfectly possible.

God alone knows what is going on here.

runs back to SRM

Not including zeros will skew data upwards, as AP goes up without 0 power readings.

It's absolutely necessary to include zeros. As they count.

Fucksticks, so it's possible that the Powertap is pre-borked?

Repped.

I find the Training Peaks very handy, it's worth it for the PMC/calendar I think.

I rode my new bike with a PowerTap this morning, which I calibrated before I set off.

http://connect.garmin.com/activity/491220383

I'm not sure that I agree with that average power however, any thoughts chaps?

^^^ If you look at the power graph, it looks like you are either doing nothing (presumably stopped at lights as the speed graph drops away also) or you are riding at c.400W and often more. The periods where you are actually riding only appear to be a couple of minutes at a time so >400W is certainly not impossible but are those the sort of numbers that your SRM gives you?

No, and I trust the SRM.

I've just re- calibrated the Powertap, and this time it showed "Calibration 508" and "Torque 0.00".

It didn't show torque previously- which suggests that a metric was missing during the calibration process.

I'll see what the ride home brings, and over-ride Training Peaks suggestion that I increase my FTP to 332 watts, based on this mornings commute.

I say it looks like it's reading double. At a rough look. As you said earlier.

Right, I took the long way home, the following is what I did, what I observed and my conclusions.

Before getting on the bike I ran the calibration, which was 507 and the torque was zero.

I set off and did some laps of Regents Park, power was what I expected to see.

Then just sort of randomly wandered South ish, looking for a combination of stop/start and longer uninterrupted roads.

After a few periods of track standing the power figure crept up- until I was putting out 400 watts just pootling along.

I stopped and re-calibrated- and as I did so noticed that with no force applied the torque was reading 9.98.

Off I set again, and again the power "looked right" at first, but then crept up again.

I rode for a longer period this time, then stopped and re-calibrated- torque at rest was 13.48 this time.

Once calibration was run power values were (once more) back to where I expected to see them.

I think that what is happening is the Garmin 910XT is performing an auto-zero when it thinks I am coasting - but of course with a fixed drivetrain you never truly coast.

I need to disable the auto-zero, and I cannot for the life of me find that option.

Scratch that- in the Ant+ Power screen you go to sensor details, then about, and you can disable auto-zero in there.

Hopefully, Great Success!

Go to the Power graph on this file and hit 5 second averaging- http://connect.garmin.com/activity/491527282

You can see how the zero-offset is climbing.

Go to the Power graph on this file and hit 5 second averaging- http://connect.garmin.com/activity/491527282

You can see how the zero-offset is climbing.

Why are you saying that? That's just your best 5s effort charted