Downhill Power

[mfez]

I have been noticing that when pedaling downhill my PRE is far less than the watts readout. The watts read maybe 5-600, but I feel like it's more like 200-250. Is power overstated on downhills, which is then corrected in the software, or is it just inaccurate?

[coachboyd]

It sounds to me as if something might be a little bit off with your profile. If you can send me a file from a ride that you did by yourself to boyd (at) totalcyclist dot com I will take a look at it for you and see if it needs a slight tweaking.

[rruff]

I have been noticing that when pedaling downhill my PRE is far less than the watts readout. The watts read maybe 5-600, but I feel like it's more like 200-250. Is power overstated on downhills, which is then corrected in the software, or is it just inaccurate?

Small errors in primarly wind scaling and CdA can become greatly magnified on fast downhills. If you are tucking, and therefore reducing CdA compared to your CDs, then you will get a higher power reading than reality. If your position is the same as it was for your CDs, then this would indicate that your CdA x Wind Scaling (Aero) is likely too high.

[MultiRider]

I noticed the same thing and checked it out by pedaling backwards while descending. As pointed out by rruff, it is important to be in the same riding position as you were for your Profile if you want it to be 100% accurate. If you did your Profile in the hoods and then go into a tuck on a downhill, you can expect to see 20-50w while pedaling backwards. Which is kind of the basis of the iAero -- it determines what watts you should be putting out at a given speed, compares that to a DFPM, and the difference is the aero advantage from whatever you changed (wheels, helmet, riding position, etc). But that is a different topic.

Anyway, I had problems of 25-100 ghost watts while pedaling backwards on downhills at speeds from approx 25-40mph. I sent an out-and-back ride file to Coach Boyd, he returned a slightly tweaked Profile, and my iBike has registered 0 watts when coasting yet been sensitive to light pedaling forces ever since. I'm very happy with the result.

[cwwees]

Any time I'm pedaling over ~30 mph, the power reading is balderdash. My unit regularly reads 600W or more when I know I'm only putting out 100. Maybe there's a little from my position, but I've got months of data showing this consistently every time I descend.

[coachboyd]

Can you send me a file where this is happening to boyd (at) totalcyclist dot com.

I'll check out your profile settings and see if I can make an adjustment for you.

[rruff]

Any time I'm pedaling over ~30 mph, the power reading is balderdash.

What does it read when you are going down hill and not pedaling?

[Jimbobob]

WOW! That is exactly the problem that I am having with my Ibike. 600, 800, 1200 hundred watt readings while soft pedalling downhill. WAY off. What did I do wrong? How do I fix this?

[coachboyd]

Send me a ride file at boyd (at) totalcyclist dot com and I will take a look at it for you.