Inconsistent PowerTap readings

Triathlon Forum
1

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…

2

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.

3

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.

4

True, good point.

5

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.

6

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.

7

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/gforum.cgi?post=3713515;search_string=;#3713515

8

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.

9

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

10

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”.

11

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.

12

Thanks for the help.

Now, to track down a LYC to borrow…

13

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.

14

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.)

15

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.

16

“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.

17

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

18

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

19

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!

20

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.