High power readings on descents

[ciclista]

What would cause the unit to display very high power readings while descending (pedaling)? The power numbers can go well over 1000 even if soft pedaling, and I know there is no way that can be correct.

[Morocco Mole]

Curious what sort of speed does this occur at, eg. is it fine at 20mph but you get spikes above 30mph ? If you go into setup and go to the coast screen while descending what sort of number are displayed? You want to see it hovering around 0, if not then you Aero calcs are off and you need to redo your coastdowns.

[coachboyd]

I'll be glad to help you with this. Can you send me a file where this is happening to boyd (at) totalcyclist dot com? I'll take a look and get you taken care of.

[ciclista]

Thank you for your quick response. I did a new calibration and so far it looks like the numbers are better. I'll be checking the coastdowns as described on the "checking calibration" posting this weekend.

Alex

[cwwees]

Hmm... I don't know if I posted this or hit the wrong button...

I always get absurd power readings on a descent. It reads 500-1000 W when my subjective assessment is 100-200 W. I don't think it kicks in magically at one speed, but above 30-35 mph, the readings are obviously bogus. It's been that way for about a year through Gen i, Gen ii, and Gen iii units, multiple coast-downs, and everything. I ride this hill twice a week year-round, so I can send loads of examples.

Charlie

[coachboyd]

If you can, send me a solo ride file to boyd (at) totalcyclist dot com. I will take a look at the file and see if anything looks out of the ordinary. You can also upload the new firmware and software and give that a try. The new firmware makes it much easier to get a reliable profile, especially with being able to set your own crr when doing coastdowns.

[ciclista]

Hmm... I don't know if I posted this or hit the wrong button...

I always get absurd power readings on a descent. It reads 500-1000 W when my subjective assessment is 100-200 W. I don't think it kicks in magically at one speed, but above 30-35 mph, the readings are obviously bogus. It's been that way for about a year through Gen i, Gen ii, and Gen iii units, multiple coast-downs, and everything. I ride this hill twice a week year-round, so I can send loads of examples.

Charlie

I had the same issue, even when soft pedaling I was getting high power numbers. I did a new calibration paying good attention to the tilt and wind offset prior to starting the cal and the problem was solved.

[cwwees]

I learned to reset tilt and wind just before calibration one or two cal cycles ago - when it was cold and the readings drifted pretty badly.

I have high power readings from every calibration over the past year. My opinion is that the drag formula doesn't account for varying laminar and turbulent flow at speeds above 40 mph. It's also possible that I tuck naturally in such high speeds, so my position doesn't match the calibration position - which is intended to match my "most common" stance rather than the position I take up during a descent.

[coachboyd]

Do you have the cadence sensor on your bike?
If you are descending at high speeds and get into an aero position this will lead to a spike in high watts. If you have a cadence sensor however, when you are coasting power will automatically go to zero.