Can Powermeter be off or affected by trainer?

seancbegley

 Hello all,

  It's been a while since I've posted. Today I did my bike test (couldn't do earlier in the week) and was very surprised by my wattage numbers during the entire ride.  They seemed very low compared to usual (I was in the 150 wattage zone for most of the 2 X 20' intervals) but the resistance on the trainer (Magnetic Blackburn) was up there and you could absolutely feel the resistance.) I may have broken 200 a few times.  I routinely ride well over 200 watts for majority of my trainer rides and was really pushing today. I am a little tired and fatigued but no way I was that low during this test.

Could something be off w/ the powertap computer, hub, or something that I had set w/ my trainer have affected it.   I have noticed that it seems my numbers are not consistent somewhat frequently on this trainer and wondering if that could be the cause. I am considering moving up to a kurt kinetic fluid although not the best time for me to drop the $$$.  I'm still getting used to training w/ the powermeter and interpreting everything in training peaks, etc..

Any input on this is helpful.  I'd really like to think that with all the training I've been doing my wattage is a lot better than what shown.

 

Thanks! 

-Sean

dhalligan

Hey Sean,

I'm not familiar with the blackburn trainer, but I would ask this.  Prior to this test, have you done a 2x20' test on that trainer with your powertap?

If so, when was your last test and what was your FTP?

Dave

 

mgraffeo

Sean,

the powertap computer has an autocalibrate feature, which works very well on the road.  Every time you coast for more than a couple of seconds, it re-zero's the torque, to account for big swings in temperature, etc.

On the trainer, I seldom (if ever) coast.  Therefore, I have found several times where the powertap's torque offset is not at zero when I've checked.  The way to do this on the PT computer is to

1. stop pedaling, and hit mode until the arrow is at the Watts line
2. Hold down Select until the arrow starts blinking, and the word "watts" disappears.  release the select button.  This number should go to zero if torque calibration is working properly (again, IME, doesn't always do it on the trainer)
3. hold down select for 2-3 seconds to manually re-zero the torque.
4. press select (don't hold down) to return to the "watts" screen, and ride as normal

I did have one situation last year where it was reading something like -3 instead of zero, which had a significant effect on the watts on the screen.  Now, for every ride, I ride easy for a minute or two, and perform the above procedure, just to make sure.  Even if it reads zero, I still re-zero it, since small differences in torque show up as bigger differences in watts.

Good luck!

Mike

Kevin Walter

I agree with Mike. I find recalibrating the PowerTap to be key at the start of every ride. Just a small difference in calibration can really affect the wattage output.

kathleennelson

Sean,

I bought a pt this past Nov and have had some of the same trouble you are experiencing.  I have sent my pt back and forth to Saris 3 times.  They replaced the torque tube and also gave me a new cpu.   I am still have trouble with it dropping transmission.  I use a One up USA trainer, which uses a centrifugal pressure resistance system (CPR).  Rich at Wheelbuilder thought maybe my trainer was interfering with the transmission to the cpu, so I called One UP and they said they have never had anyone call about problems with transmission from a pt.  The guy I talked to also said that most of the guys in the shop use ONE UP and pt, and no problems ever.  So, we decided that it wasn't the trainer.  But, initially Rich and I thought that the One Up trainer was a magnetic trainer and he said that could possibly cause problems with transmission.  He suggested to me when we were brainstorming about the problem to try and use a fluid filled trainer to see if it made any difference..  So, you could try that if you have access to a fluid trainer.

The weird thing about my problem is it is random;one day it works and the next day it is dropping transmission.  So, I usually have to stop my ride, get off and zero the torque.  I'm kind of tired of sending it back and forth, so for now I am living with the problem.  I am waiting to get the bike outside and see if riding it on the road makes any difference.  If not, then the cool thing is it is under warranty and Saris has been great about fixing stuff.  That is if they can figure out the problem.

TH3 FRB

Could be a calibration/offset issue with the PT as mentioned above.

Have you routinely used the trainer with power? Many people, myself included, find it difficult to match their outside wattage on the trainer. So if you have been doing all your training and testing out on the roads and then jump on the trainer for a session it could feel significantly more difficult to hit the same targets. There are several theories/explanations for why but it absolutely happens.

seancbegley

Thank you all for the reply. To answer some of the questions above:
1- Yes, I did my initial test (I think November) when the outseason plan started. My norm power was much higher (around 200 or 205) for my bike test.
2- I have not re-calibrated the powertap, and didn't even think of that, so that could be an issue. My friend also mentioned could have been how I had the trainer roller set against the wheel and if it was consistent with my previous test, which likely, it wasn't.
3- I've done all my riding/training since my last race in October on the trainer or on a spin bike at work. I try to get at least one ride a week on my actual bike so I can use my powertap but unfortunately, as of late, a lot of my bike rides have been at work on a spin bike, so I have to focus more on my HR, cadence, and feel. Still been pretty good workouts considering though.

All of your post so far have been very helpful, and I think I need to remember to recalibrate my powertap and also ensure i have the resistance w/ the trainer more consistent.

I also agree that Saris has been great w/ service. I had to send the CPU back last year after it started having some issues and conked out (right before IM Kentucky.... I had to go back to using an old school Cateye computer for the race after having trained for months w/ the Powertap). But they sent me out a new CPU right away, no cost or anything. I'm happy with them.

Thanks again for all your help.

cwhyte

Rule #1: Be very careful about trying to draw conclusions about your power or your PM based on a single ride.

I've owned multiple PTs for about 7 years now and I have to say that my experience is much different than what I'm reading above.

First off, ever since they came out with firmware version 2.21 on the PT Pro computer, which is probably at least 4 years old now, the auto-zero torque feature is quite flawless. In addition, when a PT's torque goes off it's quite noticeable. And in the very few times where my torque has gone off, my PM never once read low power, it always reads high power. There has never been a doubt when my torque has needed to be zeroed. Also, assuming it is off, you only need to coast once. Even though I rarely coast on a trainer, I always coast at least once early in the ride.

I'd say in the last 4 years I've owned a PT (and I currently own 3 PTs) I've had to zero my torque maybe 3 times. Keep in mind, I'm someone who looks at my power data after every single ride.

I wouldn't read too much into that one ride. If you really want to figure out if there was a problem with your PT then go look at your data. If you don't see "0" as the min power and torque then that would indicate a problem. However, make sure you did some coasting early in the ride too.

Thanks, Chris

seancbegley

FYI- I rode today doing Saturdays workout on my tribike (other test I was on roadbike) and recalibrated the powertap. I got numbers more consistent with what I am usually pushing. I don't know if there was anything to do w/ the difference between my road bike and my tri bike. (I have my road bike in my apt. and tri bike at my weekend getaway place where I am now). I've had better numbers on the road bike but do realize that the different set up, subtle body position change, etc... can impact the ride. Anyways, what Chris said makes sense about not drawing conclusions from one ride.

I appreciate everybody's help and can honestly say that these bike workouts have made a difference. I can notice the difference in my strength, recovery, and power, and the wattage (when the CPU is working correctly is showing improvements.

Thanks again for everybody's help and post.