Replacing Old Power Meter Question

Equipment
1

Hi There,

I’m buying a new Stages Gen 3 Left Crank Arm PM to replace my Stages Gen 2 Left Crank Arm PM. Is there any process i should follow to see if the new PM power data matches the old PM?

Thanks!

2

Impossible to do without another power meter that could be used with each Stages, to see how close they may or may not be. Without that, there is no practical way to get reasonable data for comparison.

  1. Redo your FTP test (via your preferred method) and move forward with data from the new PM. This is the primary recommendation with any new power measuring device.

  2. It is a good idea to start a new season (and possibly make an old season) related to the times you use each power meter.

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3

Makes sense. So there’s no good way to keep the old pm on a trainer bike and keep the new one on the bike I use outside?

David

4

Well, sure… you can have them installed o two different bikes. Your OP apoke of “replacement” and asked about “matching” them. Unless you have a very accurate smart trainer or another power meter, it’s not practical to compare one to the other. If you have one of those you can run some level of a comparison. It might help you know a rough delta between them if you plan to keep and use both meters.

5

Not impossible to do without another PM to compare with. I used to compare power meters using my lemond trainer. Record specific cadence and gears and see what watts you get. Compare to new power meter using same protocol and then figure out the % difference if any. I’d usually test 3 different gears at a cadence that would usually equal common intensities I’d ride at. Example. 53x18 100rpm, 53x16 100rpm, 53x15 100rpm. This would usually give me mid zone 2, mid zone 4 and somewhere in zone 5. I think it’s important to test across a few zones so when comparing you can more accurately judge % difference between powermeters. I’d slowly and smoothly get up to my desired cadence and once I was steady at required rpm hit the lap button and ride for anywhere from 1-4mins, I did not find any different results with longer laps. FYI when I first decided to try this slightly unconventional method I ran through the protocol 10 times on the same power meter and results were all within 3 watts which for me is less than 1% of FTP and within the acceptable range of error for most powermeters. Strange process but this got me through 4 power meter changes quite successfully.

6

Agreed, “impossible” is a bit overboard, but “impractical” is quite realistic. It depends on a VERY consistent trainer and not one that is subject to tire pressure, roller pressure and such. There are likely too many variations between a typical bike and trainer setup to trust the data more than the tolerance ranges.

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7

What I’m reading is that i could do your testing protocol, @matthiask, while using a Kickr connected to my iphone to adjust the resistance and having the power meters connected to my Wahoo Bolt to record the power meter data? That seems like it would be a consistent testing environment. Considering the Kickr is fairly consistent in ERG mode.

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8

This would also work and give you within +/- 1-2% accuracy.

9

@matthiask Sounds good. +/- 1-2% seems pretty good to me.

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10

If you have a Kickr I’d do a slightly different protocol. Get the Kickr wattage to display on your phone and get the stages wattages to display on your bike computer.

Now, get the kicker to 100 watts, pedal smoothly for at least 10-20 seconds, record wattages for both, 150 watts, record, 200 watts, etc.

Repeat the process with the new Stages v3.

After that you’ll have a nice little chart showing you how they compare.

11

@AJS914 That makes sense as well. I’ll probably take a day to track the differences between the PMs

12

BTW, I just did this with my Stages. It’s my first power meter and I wanted to know how close it correlated with my Tacx trainer. They were both within 5 watts at every power level.

13

I’d say just use the new PM and you’ll see what it spits out. After 42 days, your Performance Manager Chart will have adjusted itself.

I have three powertaps on three bikes, and they all read a little differently. I ballpark FTP with the middle figure.

The truth is in the sensations and the cells, not the numbers.

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14

@RobertK Totally. The PM is there to quantify perceived exertion. I’m trying to have the transition be as simple as possible.

15

@mcneese.chad As a followup. I ended up testing my old and new power meter by using the Kickr as a constant during Carson -5 connected to my iPad, while the power meters was connected only to my head unit. I did Carson -5 twice, once with he old pm and new pm. What I found is that the new power meter reads 4-6 watts higher on average compared to my old power meter. I plan on upping my FTP by 5w to account for the higher reading and if I use the old PM, I’ll just adjust the workout down by ~2%.

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16

Depending on the power(s) tested, that difference may be within the power meter and trainer tolerances. Sounds like you have a good comparison and can adjust well now. Good work. :smiley:

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17

I agree 100%. It was three intervals at 255,266, and 262. The power meters all ran higher than the kickr, but power match should adjust the Kickr resistance to the power meter reading if my understanding is correct?

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18

Yup, Power Match will set trainer based on the power meter and you will have good power data matching your outside efforts.

19

Wonderful, thanks for all your insight Chad!

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