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Inconsistent PowerTap readings...

 

   


The_Mickstar

Jan 19, 12 11:39

Post #1 of 21 (3198 views)
Inconsistent PowerTap readings... Quote | Reply

I have a PowerTap Elite+ hub built into a wheel. I use it with a Garmin 310XT. (I do not own a LYC.) The batteries in the hub are good, and, after waking up the hub, I perform the Garmin calibration with the bike in a stand before every ride.

I was doing power-based intervals on the rollers Tuesday night, and had the following issue. NOTE: I've experienced this a handful of times before, as well. But, it doesn't happen very often, and definitely not every time I do intervals.

I was doing 10x3min with a specific power target. So, during the work/rest intervals, I was simply switching between a harder (work) and easier (recovery) gear.

For the first 3 or 4 intervals, there was no issue. Switching between the two gears, the power levels (and speeds) were consistent, as you would expect. However, the next interval, when I returned to that same harder gear and cadence, the reported power was noticeably lower (on the order of 15-20W). I actually had to shift to one gear harder (verified by the accompanying change in speed) to get to the power level that I was targeting. I finished that interval, shifted back to the recovery gear, and it, too, was reading noticeably lower than it had previously, despite the speed being the same as it had been on prior intervals. (All that to say that it wasn't just an issue of me mis-counting the shifts between the work/recovery gears.)

At that point, I got off the bike and started messing with the Garmin calibration and things got confusing. But, when I've experienced this same issue in the past, I would just keep riding and, sometimes, the reported power level would return to the (higher) initial values on subsequent intervals.

Has anyone experienced anything like this?

Like I said, it's not very often that it happens. But, when it does, it's damned annoying!

I usually just forget about it after a couple of days of it working fine, but I did remember to send their customer service a message this time. We'll see what they say...

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


Nate Pearson

Jan 19, 12 11:49

Post #2 of 21 (3186 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

The_Mickstar wrote:
I have a PowerTap Elite+ hub built into a wheel. I use it with a Garmin 310XT. (I do not own a LYC.) The batteries in the hub are good, and, after waking up the hub, I perform the Garmin calibration with the bike in a stand before every ride.


I was doing power-based intervals on the rollers Tuesday night, and had the following issue. NOTE: I've experienced this a handful of times before, as well. But, it doesn't happen very often, and definitely not every time I do intervals.

I was doing 10x3min with a specific power target. So, during the work/rest intervals, I was simply switching between a harder (work) and easier (recovery) gear.

For the first 3 or 4 intervals, there was no issue. Switching between the two gears, the power levels (and speeds) were consistent, as you would expect. However, the next interval, when I returned to that same harder gear and cadence, the reported power was noticeably lower (on the order of 15-20W). I actually had to shift to one gear harder (verified by the accompanying change in speed) to get to the power level that I was targeting. I finished that interval, shifted back to the recovery gear, and it, too, was reading noticeably lower than it had previously, despite the speed being the same as it had been on prior intervals. (All that to say that it wasn't just an issue of me mis-counting the shifts between the work/recovery gears.)

At that point, I got off the bike and started messing with the Garmin calibration and things got confusing. But, when I've experienced this same issue in the past, I would just keep riding and, sometimes, the reported power level would return to the (higher) initial values on subsequent intervals.

Has anyone experienced anything like this?

Like I said, it's not very often that it happens. But, when it does, it's damned annoying!

I usually just forget about it after a couple of days of it working fine, but I did remember to send their customer service a message this time. We'll see what they say...


That could be your rollers heating up and reducing the rolling resistance or the resistance on the belt of the rollers.


If you had access to a second power meter, like a SRM or Quarq then you could tell if it's the PT's fault or the roller's fault.
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The_Mickstar

Jan 19, 12 11:51

Post #3 of 21 (3184 views)
Re: Inconsistent PowerTap readings... [Nate Pearson] [In reply to] Quote | Reply

Nate Pearson wrote:
That could be your rollers heating up and reducing the rolling resistance or the resistance on the belt of the rollers.


If you had access to a second power meter, like a SRM or Quarq then you could tell if it's the PT's fault or the roller's fault.


Except that I would expect a more gradual change if that were the culprit. And, I would not expect the power level to return on subsequent intervals.

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


Nate Pearson

Jan 19, 12 11:51

Post #4 of 21 (3183 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

True, good point.
---------------------------------------------------------
http://www.trainerroad.com - Indoor Cycling Training System


nickwhite

Jan 19, 12 12:31

Post #5 of 21 (3164 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

If you were using the LYC, I would ask if you have the auto-calibrate (or something like that) feature enabled. Not sure if there is such a thing for the Garmin.


Why Do You Race? // FCA Endurance // Competitor's Creed // Twitter: @_NickWhite


The_Mickstar

Jan 19, 12 12:42

Post #6 of 21 (3148 views)
Re: Inconsistent PowerTap readings... [nickwhite] [In reply to] Quote | Reply

nickwhite wrote:
If you were using the LYC, I would ask if you have the auto-calibrate (or something like that) feature enabled. Not sure if there is such a thing for the Garmin.


That's a good question, and, honestly, I'm not sure. I can tell you that my normal routine is to calibrate prior to starting the workout, and that's it.

The thing that bothers me is that I seem to do the same handful of different types of workout over & over, and only very rarely see this issue. If it were a setup issue, I would think I would experience this more frequently, or at least all the time during a specific type of workout.

It probably wouldn't be a bad idea to try to track down a LYC to borrow. Of course, never knowing when/if it's going to happen...

I'll have to look into the auto-calibrate possibility.

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


coloradoveto

Jan 19, 12 12:58

Post #7 of 21 (3131 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

I'm not sure if your problem is similar to mine, but mine had to be sent in to cycleops to be worked on.

http://forum.slowtwitch.com/...rch_string=;#3713515


The_Mickstar

Jan 19, 12 13:02

Post #8 of 21 (3126 views)
Re: Inconsistent PowerTap readings... [coloradoveto] [In reply to] Quote | Reply

While not exactly the same, it certainly sounds similar. I have a message in to their customer support. We'll see what they say. (Although I expect that they'll immediately blame the Garmin.)

Thanks.

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


JollyRogers

Jan 19, 12 14:57

Post #9 of 21 (3095 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

What was your zero offset? Have you checked your zero offset when you're experiencing unexpected power readings?


The_Mickstar

Jan 19, 12 16:46

Post #10 of 21 (3077 views)
Re: Inconsistent PowerTap readings... [JollyRogers] [In reply to] Quote | Reply

JollyRogers wrote:
What was your zero offset? Have you checked your zero offset when you're experiencing unexpected power readings?


With the version of firmware that I have on my 310XT (3.70), the only options when you select Calibrate on the Ant+ Power menu are...

Current: This always reports -2371 for my unit after cal. It's some sort of unit ID.
Auto Zero: This is a checkbox, which I have checked.
Torque: This reports 0 after the cal.

When the cal is complete, a message pops up which says Calibration Successful.

So, I'm not sure what you mean by checking the "Zero Offset".

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


JollyRogers

Jan 19, 12 18:29

Post #11 of 21 (3052 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

zero offset is the raw value that your PT hub reports unloaded. unfortunately Garmin elected to call this "calibrate" when it's not calibrating, but setting a zero offset point.

"in spec" for a PT is 500-524. an out of spec zero offset can lead to quirky power readings.

i've never used a 310, but to get zero offset with the PT LYC you have to go into the test menu. the joule returns the value directly when manually zeroing. the edge 500 returns the zero offset value when using the "calibrate" function

if the 310 won't return a raw torque value, see if you can borrow one of the devices i mention above to get the raw torque value (zero offset) of your hub.


The_Mickstar

Jan 20, 12 4:49

Post #12 of 21 (3025 views)
Re: Inconsistent PowerTap readings... [JollyRogers] [In reply to] Quote | Reply

Thanks for the help.

Now, to track down a LYC to borrow...

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


JollyRogers

Jan 20, 12 13:48

Post #13 of 21 (2997 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

Joule or Edge 500 are the easiest. If you get an LYC, you'll need to take a look at the full users manual at Cycleops's web site unless the LYC owner is very knowledgeable. My guess is that very few folks know how to tab through to that function.


The_Mickstar

Jan 20, 12 15:30

Post #14 of 21 (2990 views)
Re: Inconsistent PowerTap readings... [JollyRogers] [In reply to] Quote | Reply

I actually do have access to an Edge 800. Do you happen to know if it will do it? (I assume so, if the 500 will.)

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


The_Mickstar

Jan 22, 12 7:04

Post #15 of 21 (2931 views)
Re: Inconsistent PowerTap readings... [JollyRogers] [In reply to] Quote | Reply

JollyRogers wrote:
zero offset is the raw value that your PT hub reports unloaded. unfortunately Garmin elected to call this "calibrate" when it's not calibrating, but setting a zero offset point.

"in spec" for a PT is 500-524. an out of spec zero offset can lead to quirky power readings.

i've never used a 310, but to get zero offset with the PT LYC you have to go into the test menu. the joule returns the value directly when manually zeroing. the edge 500 returns the zero offset value when using the "calibrate" function

if the 310 won't return a raw torque value, see if you can borrow one of the devices i mention above to get the raw torque value (zero offset) of your hub.


Update: I calibrated my hub with an Edge 500 yesterday. It's calibrate screen gave the same info as on my 310XT. The only difference, is that the "Torque" field reported "0.00" instead of just the "0" that my 310XT reports. Is this the zero offset you were talking about? I was expecting to see something in the 500s.

I'm going to borrow a LYC today to do the check than Cyclops requested. Not sure exactly what it is, but here's the directions they gave me...

If you have a Cervo (CPU), follow these steps for calibration testing:
M=Mode
S=Select
Press and hold M and S until “Set Edt 12345” appears. Press S until “t” flashes. Press M 4 times. Turn the cranks to move the wheel and a number will appear. What is the number after the “3 M”_ _ _? This is your hub calibration. Please let me know what this 3 digit number is.

Thanks for the help so far.

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


The_Mickstar

Jan 22, 12 11:18

Post #16 of 21 (2907 views)
Re: Inconsistent PowerTap readings... [JollyRogers] [In reply to] Quote | Reply

JollyRogers wrote:
"in spec" for a PT is 500-524. an out of spec zero offset can lead to quirky power readings.


I borrowed a buddies LYC. After playing with it, I see that the zero offset value is also what Cyclops customer service was asking for. My hub reports 517. So, it appears to be "in spec".

My buddy is going to let me borrow his LYC for a little while. I'll check it again if it starts acting "wonky" while I have it.

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


Bonesbrigade

Feb 2, 12 20:00

Post #17 of 21 (2827 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

Did you get this resolved? I seem to have had a similar issue on my ride tonight.

I'm using an edge 500/PT SL
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The_Mickstar

Feb 2, 12 20:45

Post #18 of 21 (2816 views)
Re: Inconsistent PowerTap readings... [bonesbrigade] [In reply to] Quote | Reply

Nope. I have a LYC to check the zero offset now. But, I haven't experienced the issue since I've had the LYC.

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


Bonesbrigade

Feb 3, 12 8:13

Post #19 of 21 (2771 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

Thanks.

What does the LYC do for you that the garmin 500 doesn't? When I do the calibration, the 500 gives me the same number every time - 25099 - I assume this is sort sort of unit ID which is not relevant to the calibration? Even though my torque value was 0, it was still reading like 20-25 watts low.

Hopefully this is just an isolated event as you seemed to have experienced - very frustrating when you are in your second set of a 2x20 though!
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The_Mickstar

Feb 4, 12 11:34

Post #20 of 21 (2741 views)
Re: Inconsistent PowerTap readings... [bonesbrigade] [In reply to] Quote | Reply

bonesbrigade wrote:
Thanks.

What does the LYC do for you that the garmin 500 doesn't? When I do the calibration, the 500 gives me the same number every time - 25099 - I assume this is sort sort of unit ID which is not relevant to the calibration? Even though my torque value was 0, it was still reading like 20-25 watts low.

Hopefully this is just an isolated event as you seemed to have experienced - very frustrating when you are in your second set of a 2x20 though!



Correct. My understanding is that the number your Garmin displays is some sort of unit ID. But, if you get a LYC to use, you can get your actual hub ID, which is not the same number. (Figure that one out!)

The LYC will let you get the actual torque offset value for your hub. If your hub is within tolerance, it will be 512 +or- 12.

I feel your pain. My goes whacky so infrequently, I doubt I'll catch it while I have the LYC. (I'm just borrowing one.) Even if I do, getting the hub rebuilt is $350 so I'll probably just live with it and hope it doesn't happen in a race.

Steve

"If you ain't first, you're last." Ricky Bobby Talladega Nights


MAS

Feb 20, 13 21:56

Post #21 of 21 (1959 views)
Re: Inconsistent PowerTap readings... [The_Mickstar] [In reply to] Quote | Reply

I believe I'm having the same problem as Mickstar, except that my combo is Edge500 + PT G3 + Supermagneto trainer instead of rollers. I also calibrate (manually from Garmin menu) PT everytime before I start the exercise (after I have clamped in & tightened my back wheel to the trainer), trainer is indoors in a room with steady temperature. I have also auto-zero for PT set on in Garmin menu.

I wasn't having any problems for about 3 months that I have had the PT hub. Practically almost always on the trainer once I had started pushing the pedals I would continue until the end, with no coasting or rest in between. Last week for some reason I coasted a bit (5-10s) in the middle of a 45min tempo-level effort. I believe this is the process that does auto-zero PT. Right after the coasting my power output dropped some 10w-15w, even if all the other indicators were the same: how it felt, speed sensor (separate Garmin spd+cadence) and heart beat.

Yesterday I was doing a 2x20min set and coasted 10secs after the first set to auto-zero (rest 3min between sets) and again strange things happened: I was getting about 10w lower reading for the 2nd interval right from the start even if feeling, speed sensor and heart beat again all confirmed that I was putting in a clearly harder effort on the 2nd interval. After the 2nd interval and super-quick cool down I got off from the bike and did manual calibration from the menu: reading was 522, which was exactly the same I got when I manually calibrated PT before I started the session.

In my case this problem seems to be clearly linked to auto-zero set "on" and coasting in the middle of a training session to activate auto-zero.

   
 
 
 



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