General Discussion Triathlon Talk » Powertap P1 pedal power reading low? Rss Feed  
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2016-08-23 4:16 PM

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Subject: Powertap P1 pedal power reading low?
Either my legs are shot, or the power from my Powertap P1 pedals are reading low. The power I am generating in TrainerRoad for the RPE level I am riding at is much lower than expected. I bought and started using the pedals 2 months ago, and I have put around 30 hours on them after installing the batteries that came with them. I have not seen a low battery warning, and the LEDs are green.

Has anyone seen this issue? I am getting ready to troubleshoot by replacing the batteries and using the pedals on my friends bike that has a crank PM, but I'm wondering if anyone else has seen this problem.


2016-08-23 5:21 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?

Do you mean the power is lower than it was before?   Or it's always been lower than what you expected it should be?

If the latter, (said sarcastically) welcome to the world of power LOL.

I don't have P1 pedals but have been looking at them, haven't heard many complaints of this nature. 

 

2016-08-23 6:19 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?

Where do your power expectations come from?

2016-08-23 7:08 PM
in reply to: #5196381

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Subject: RE: Powertap P1 pedal power reading low?
Good questions... I was using Virtual Power from TrainerRoad with a KK trainer and my FTP with virtual power was 260 (20 minute test). After the Powertap pedals and a period of riding less than normal, the 20 minute test FTP was 206. Lower, but I wasn't surprised.

The first month or so of workouts using the Powertap P1 power gave me power readings that felt like they were in line with the new FTP. A subsequent 20 minute test a month later showed my FTP increased to 215.

In the last few weeks, I am struggling to get my power over 175 watts. Today I got in a big gear and stood up and could only get to about 225 watts.

At first I thought increased running volume could be taking a toll, but lately my legs feel pretty good. When I stood today and saw the numbers, it convinced me that something is probably wrong.
2016-08-23 8:18 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?

Has your bike volume decreased while your run volume has decreased?  "Legs feeling good" is not always a good way to judge how your bike fitness is.  I know when I used to marathon train I would have days where my legs would feel really good after a recovery wee.  But because I was down to riding only once a week, my bike fitness was still way below previous levels where I was biking more regularly and not running nearly as much.

2016-08-24 6:53 AM
in reply to: #5196432

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Subject: RE: Powertap P1 pedal power reading low?
I've kept bike volume the same.

I was open to the possibility that it was because of bike fitness or because my legs were not recovering from the runs, but I've been doing tris for a while and have never seen this kind of bike performance drop off. That and the fact that the big gear while standing on the pedals only produced power that is 30 watts over my FTP continues to lead me toward the conclusion that the power may be wrong.

I'm going to move on to changing the batteries and testing with a bike that has a crank power meter to see for sure.

Thanks for the responses.


2016-08-24 7:11 AM
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Subject: RE: Powertap P1 pedal power reading low?
Originally posted by roserc

I've kept bike volume the same.

I was open to the possibility that it was because of bike fitness or because my legs were not recovering from the runs, but I've been doing tris for a while and have never seen this kind of bike performance drop off. That and the fact that the big gear while standing on the pedals only produced power that is 30 watts over my FTP continues to lead me toward the conclusion that the power may be wrong.

I'm going to move on to changing the batteries and testing with a bike that has a crank power meter to see for sure.

Thanks for the responses.


I "think" you can do this with the P1 pedals but check with Saris. I have done it with Quarq, Powertap hubs and Garmin vectors.

You hang a known weight on them, it gives you a torque value and you can check if they are correct or not. While this sounds like a PITA at first, once you have done it you can repeat it quickly and easily if ever you doubt your PM

If you have a dumbbell and can find a way to hang it off your pedal that would be a start.







Edited by marcag 2016-08-24 7:11 AM
2016-08-24 7:46 AM
in reply to: marcag

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Subject: RE: Powertap P1 pedal power reading low?
I had issues with mine (early production pedals with cap issues) and had to send them back. (Fantastic customer service btw.) I would start with batteries, cheap & easy. What do your files look like? I had numerous drop outs on trainer rides, files showed zeros all over the place. Then they started registering super high #s outside. Like 800 watts when I was pedaling easy, and then low #s on hills, like 30 watts. It was obvious mine had problems. Saris ended up replacing the right pedal, I assume left was ok. Never found out what problem was, but they have been good since. Hope you figure it out.
2016-08-24 12:55 PM
in reply to: #5196381

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Subject: RE: Powertap P1 pedal power reading low?
Thanks again for all the info. I like the idea of testing the torque. How did you get a torque reading?

That would be easier than testing them against another PM, and I like that I can do it anytime.

First, I'll change the batteries and do a TR workout and see if anything changes...
2016-08-24 4:42 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?
Originally posted by roserc

How did you get a torque reading?



When you do a calibrate with your Garmin does it show a Torque value ?
If not, what does it show ? some will say offset, some torque...
2016-08-30 3:25 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?
Problem solved... I'll go through the details in case someone else can benefit from what I learned.

I changed the batteries, and although I had read in the manual to use lithium batteries, I forgot and installed alkaline batteries. My first ride with those batteries was a mess. The power was all over the place, and I thought that the pedals were defective and had finally gone totally bad. After a call to the Powertap customer support, I switched to lithium batteries.

Lesson one - Use lithium batteries.

My first ride on the trainer with the new lithium batteries gave me power readings closer to what I would expect, so I thought that the problem was resolved by a simple battery change. I got an hour into a two hour TrainerRoad session, and for some reason I decided to turn on my Garmin Edge to see what power it was reading. As soon as I turned it on, the power reading in both TrainerRoad and the Garmin Edge dropped to the level it had been when it was reading low. I estimated that the power had dropped about 40%.

Just to confirm that there was still a problem, I went back and did an FTP test a couple of days later, and the new FTP was 40% lower than my previous FTP. As a result, I called customer support again, and they told me that it might be some sort of issue with Ant+ and two devices. They recommended that I try Bluetooth and get the bike out on the road to see what happens.

So today I took the bike out, and the problem was still present when I looked at the power reading with both the Powertap app and TrainerRoad app on the iPhone via Bluetooth. Still low. Then I finally did something I should have done earlier - I double checked the settings for the pedals, and found that the crank length had somehow been set to 110mm in the Garmin Edge.

So at that point, it all fit... When I changed the batteries, the pedals reset the crank length to the default (I'm pretty sure it is 172.5mm). My crank length is 175mm, so the default is close enough that the power output made sense for the first half of the first workout I did after changing the batteries. When I turned on the Garmin Edge halfway through, it fed the 110mm crank length to the pedals, and they started calculating a lower power.

The difference between the 175mm crank length and a setting of 110mm is around 40%, which was my estimate of how far the pedals were off. So it all fits.

Lesson two - If there is a problem, check all the settings that are being sent to the pedals, even if you have not physically changed them.

Hope this might help someone else... Thanks for all of your help.




2016-08-30 4:33 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?

Or, to sum up....  RTFM!     Said in all jest!  

Glad it all worked out for you.  Looking at those myself but have yet to pull the trigger

2016-08-30 6:29 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?

So when you set the crank arm length in your Garmin Edge, it actually sends that data back to the pedals in order for it to calculate power and transmit that power back to other devices such as your laptop or phone?  

I guess that sort of makes sense, but my initial thinking was that each device (Garmin, laptop, phone) was responsible for setting their own crank arm length properly and thus it would be possible to have different lengths, and different power readings if they were not set the same. 

2016-08-31 10:09 AM
in reply to: #5196571

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Subject: RE: Powertap P1 pedal power reading low?
You are right. Each device that you use as a head unit sends the information when you start a workout. The issue is that TrainerRoad looks at the pedals as a generic Ant+ power meter and does not have a setting for crank arm length.

When I use TrainerRoad on the laptop with an Ant+ USB stick, the pedals use the setting that was last set by a head unit since TrainerRoad does not have a crank arm length setting.

As I found out, if I turn on a head unit while using TrainerRoad on a laptop, that head unit will send it's crank length setting to the pedals, and the power will begin to be calculated using the information sent by the head unit.

Understanding that the setting is sent to the pedals was the key to figuring this out...
2016-08-31 4:03 PM
in reply to: roserc

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Subject: RE: Powertap P1 pedal power reading low?

That's a good find.  I would have never thought it worked that way and my guess is a lot of other people might run into this issue as well.

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