PT Problem (another one)

Not 100% sure what it could be but few questions:

1. This is done outdoors on the trainer
2. How long have you had the PT? (could be battery related issue)
3. The PT is tight in the cradle?
4. The wires look ok to you? no nicks or anything?
5. Is your recording interval set at 1.26 seconds? (I usually see my power jump over the place a bit with 1.26 seconds recording intervals but 15-20 - I'm not sure if that's the primary reason).

Some other PT owners may have better suggestions/thoughts.

Robin

gggossel said:

I get to where I know I'm in the zone, and the power drops down 15-20W, then back up, then down, etc. Approximately every two seconds (I have it set on 1 sec rec/ refresh) It's doing this in all zones, and I know my pedaling isn't that erratic.

Sounds like the "precession" that is unique to PowerTap. What was your cadence at the time? The effect will disappear when you are pedaling at ~95rpm of cadence.

It's explained in the TaRwPM book, but the chapter comparing four major bicycle power measuring devices is available online here:
http://www.peakscoachinggroup.com/freeinfo/Power_Tools.html

I strongly recommend that you get a copy of the book if you haven't yet.

Hope this helps,

Ken

if you mean that it drops by 15W, that's the nature of power output.

if you mean it drops to 15W, there's something wrong.

nrhorwitz said:

Not 100% sure what it could be but few questions:

1. This is done outdoors on the trainer
On the trainer.
2. How long have you had the PT? (could be battery related issue)
~One month.
3. The PT is tight in the cradle?
Yes.
4. The wires look ok to you? no nicks or anything?
Zip-tied and neat, no cuts or nicks.
5. Is your recording interval set at 1.26 seconds? (I usually see my power jump over the place a bit with 1.26 seconds recording intervals but 15-20 - I'm not sure if that's the primary reason).
I have the rec set to 1.26, and changed the refresh to 2sec. I would hold my cadence steady at 93, approx. 240W and it would dip to 224, back up to 240, down to 218, back up, etc. My lousy technique is smooth enough not to cause that large of a fluxuation and has not up until tonight.

Some other PT owners may have better suggestions/thoughts.

Robin

Sorry, I should have been more specific.
Going to sleep on it because I was too po'ed to even finish tonight. I'll try it on the road tomorrow to see if it still does it.
Thanks for everyone's suggestions.

gggossel said:

Originally Posted by nrhorwitz
Not 100% sure what it could be but few questions:

1. This is done outdoors on the trainer
On the trainer.
2. How long have you had the PT? (could be battery related issue)
~One month.
3. The PT is tight in the cradle?
Yes.
4. The wires look ok to you? no nicks or anything?
Zip-tied and neat, no cuts or nicks.
5. Is your recording interval set at 1.26 seconds? (I usually see my power jump over the place a bit with 1.26 seconds recording intervals but 15-20 - I'm not sure if that's the primary reason).
I have the rec set to 1.26, and changed the refresh to 2sec. I would hold my cadence steady at 93, approx. 240W and it would dip to 224, back up to 240, down to 218, back up, etc. My lousy technique is smooth enough not to cause that large of a fluxuation and has not up until tonight.

Some other PT owners may have better suggestions/thoughts.

Robin

Sorry, I should have been more specific.
Going to sleep on it because I was too po'ed to even finish tonight. I'll try it on the road tomorrow to see if it still does it.
Thanks for everyone's suggestions.

My first reaction would be this: It's normal seeing that kind of fluctation. There are some "dead spots" around your pedal stroke which allows for some changes in the watts. If you try this, 40 rpm on a friendly grade, you should see some kind of smoothness with your watts output. If that does work, I'd be inclined to believe your powertap is in perfect working condition.

nrhorwitz said:

My first reaction would be this: It's normal seeing that kind of fluctation. There are some "dead spots" around your pedal stroke which allows for some changes in the watts. If you try this, 40 rpm on a friendly grade, you should see some kind of smoothness with your watts output.

To be a bit more precise, when using a PowerTap your power will appear smoothest when pedaling 60/1.26 = 47.6 rpm, or any multiple thereof (e.g., 95.2 rpm).

(Nine times out of 10, I can tell whether someone was using a PowerTap or some other brand of powermeter just by looking at the flucuations in power when plotted in, e.g., WKO+.)

gggossel said:

I have the rec set to 1.26, and changed the refresh to 2sec.

Isn't the default refresh frequency every 3 s (i.e., the display shows a rolling average of the last three samples)? If so, changing it to every 2 s would cause the displayed values to be "jumpier".

acoggan said:

To be a bit more precise, when using a PowerTap your power will appear smoothest when pedaling 60/1.26 = 47.6 rpm, or any multiple thereof (e.g., 95.2 rpm).

(Nine times out of 10, I can tell whether someone was using a PowerTap or some other brand of powermeter just by looking at the flucuations in power when plotted in, e.g., WKO+.)

Thanks, Dr. Coggan.

acoggan said:

Isn't the default refresh frequency every 3 s (i.e., the display shows a rolling average of the last three samples)? If so, changing it to every 2 s would cause the displayed values to be "jumpier".

Not necessarily jumpier from how I'm thinking. Think abou it, you might have wattage going from 185-200 in 3 sec refresh but with two or one sec refresh, you'd see 185-191-200 and so forth.

tomdavis80 said:

Not necessarily jumpier from how I'm thinking. Think abou it, you might have wattage going from 185-200 in 3 sec refresh but with two or one sec refresh, you'd see 185-191-200 and so forth.

It's a rolling average, not just a different display rate. IOW take 4 instantaneous 1 second readings of 185, 188, 199, 200... You'd see 191 after the third second and 196 after second 4 and so on. You wouldn't get instantaneous jumps to whatever the power was at that particular moment. Longer averaging smooths the data.