Matching up power meters

power-meter

#1

Ok, so here’s my predicament.

On my Shiv I have a Rotor inPower single sided PM. On my Venge I have a Pioneer right side PM. I use Wahoo ELEMNT Bolt to collect the data.
Both PMs work well, never had a signal drop out.
I have an indoor FTP taken from a ramp test using Wahoo Kickr as the power measurement. I also have outdoor power based off a 20min test, last done on my Shiv. The values seem about right.

Here’s the thing though, I’m confident that the Rotor crank is spitting out accurate numbers as compared to my Wahoo Kickr it just feels about right. However I’m not convinced the Pioneer is as accurate and it just feels a bit lower, like 10-20%. Does anyone share my frustration?

Is there some sort of validation test I can do to see if it’s just my imagination that they are different?
If the Pioneer is indeed not helping my ego by reading low is it possible to override the settings to bump it up closer to the rotor? I tried reading through manual but I lack the PhD in Engineering required to understand it.
Thanks for your help


Team Zwatt Power Meter Users - your experience so far
#2

Simple test here.

Set trainer road to use your kickr as power source and to work in ERG. Disconnect all other power devices.

Now fire up the first bike on the kickr and use your head unit to record only the power meter on the bike. Do a ramp test but stop once you get say 4 steps above threshold. Now repeat this for the other bike. Make sure you don’t accept the new FTP that is calculated.

You now have 4 power files all of them based on the same workout with the same control so your only variable will be the power meters. You can compare the power reported for each step and see what the % difference is.

Remember each power source is ± 2% so your perceived 10% may not be as far from their specs as you think. Additionally it doesn’t really matter if a power source reads high or low as long as it reads consistent and you know what that consistency should look like (especially between varying sources in your case)

Edit to add:
You have two different positions between a TT and road bike which will also affect your power and RPE. I think @chad said on the last podcast that 5% was nothing to worry about which really puts your bang on target based on the variances inherent in your power sources. Though it’s odd that you are seeing the low power on the road bike.


#3

This is the best solution ^^

After that, just get ready to drive yourself insane thinking about the discrepancies. :slight_smile:


#4

That’s an awesome method of testing. I’ll do that.
Better get a thru axle adapter for my Kickr.
I’ll report back with how they went. I guess if one is out I can get zones for roadie and zones for tri bike.
The challenge will be if one is consistently lower is it possible to adjust the power meter to bring it in line with the other. My OCD tendencies kick in and I like seeing things matching up.
Thanks for the tip


#5

I am so glad you posted this. I don’t have a solution beyond the recommended test technique, but I can attest that I have the exact same issue. I have a kickr and pioneer l/r powermeter on my road bike. The pioneer is consistently 10-20% lower than the wahoo. Interestingly, the power I’ve generated on field power tests–8 minute or 20 minute–with my pioneer have been very close to those from my kickr, but well above those from my pioneer on the same indoor test.

After months of frustration and uncertainty, I accepted the delta, ignore the pioneer indoor and periodically do an all out 8 minute interval outdoor for my sanity’s sake.

I’m really looking forward to looking into the issue with more detail in the coming month once Pioneer’s advanced pedaling metrics become available through the Wahoo head unit (dcrainmaker overview) With the advanced metrics it will be interesting to see if there is anything particularly different with how the power meter is measuring things on the trainer versus outdoor.

Sorry I don’t have anything constructive to add at this point. I’ll follow up if anything interesting comes out of the advanced pedaling data. But wanted to let you know that you’re not alone. I feel much much less insane!

Cheers and best of luck troubleshooting.


#6

I asked a very similar question just yesterday!: Comparing power meters on multiple bikes

You have a huge benefit with the Kickr though. With just a dumb trainer and different power meters on different bikes I have no constant to check both against.


#7

I run a Pioneer on my CX bike, it aligns almost perfectly to my Stages, and InfoCrank, all three of which are about 8-10% lower than the Kickr. I’m certain the Kickr reads high, but to your comment, I would be concerned about 20% discrepancy.


#8

Mitch we need to start a support group. I’m glad I have found someone in the same boat as me.

Yes it will be interesting to see what comes with this marriage with Wahoo.

I’ll let you know if I come across a solution.

Cheers,


#9

Yeah I agree the Kickr is probably higher than what is realistic on the road (off the Rotor PM).


#10

I have done a quick comparison between my Stages, Assioma, and kickr, and they are all pretty close - well within tolerance from my first quick check. This consisted of running them all at the same time, and recording 3 different files, then comparing power for different intervals. Determining start/end is tricky, but I was able to manually do it well enough for a quick and dirty test.

I do plan to do some more testing and use DC Rainmaker’s tool to compare the files - I think this is the best way to see what is going on, and be able to compare at different power levels and cadences. Since they were all pretty close I haven’t been all that motivated.


#11

Ok, so here’s something I hadn’t thought of until now…

On the Shiv I’m running 165mm cranks (Rotor inPower), on the Venge I’m running 172.5mm cranks (Pioneer).
I wouldn’t have thought crank length would impact power output this much, but I guess if I’m lacking torque in turning the longer crank I’ll be producing less power…or am I reading into that too much?
Interesting the bike I have on the kickr is an old beat up tri bike with 172.5mm cranks. With the Kickr reading power from somewhere other than the crank it’s a different source, and therefore I shouldn’t expect them to be the same anyway.

Can someone just tell me to keep it as it is and to just go get stronger? Maybe Occam’s Razor applies here


#12

I agree with the ramp test method, you have 1 constant in the situation being your Kickr, you have something to compare against the other two. You do have to think about left only and how that compares against a full system. If you have a slight imbalance then that would effect things.
My situation is a bit different. I have my Kickr, a Pioneer dual on my road bike but my TT bike uses a Powertap rear hub… so I can’t compare that against anything.
If I knew someone with a Kickr Snap or some pedals I could compare but alas.


#13

Assuming you can use the same rear wheel on each bike then you can do something similar. Just go through your gears from lowest to highest spending about a minute on each and holding your cadence constant. Give the tyre and trainer time to cool between attempts.


#14

Good point @brenph… if my rear wheel fit both bikes and both have same chainring sizes, then yes I think I could do same ramps and use cadence (plus maybe a speed sensor?) as a constant. Alas, my rear wheels and gearing are anything but interchangeable though! 1x and thru axle on one bike and 2x and QR on the other.

In my situation, I’m resigned to 1. finding someone in my area (Fairfield county, CT anyone??? :pray::joy:) with a wheel-off smart trainer I could beg some time on, or 2. just trying a full ramp test on both (days apart), assuming my exhaustion point should be within a few watts from one day to next (after all we’re trusting it for FTP testing now).


#15

Can you at least fit the same (or same type of tyre) and pump to similar pressures. Ultimately comparing is all about minimising the variables.


#16

Same tire @brenph? Sure. But how does that help without some known power constant to measure both against? I’m on a Kurt Road Machine dumb trainer.

Wait, are you suggesting I use TR Virtual Power as the constant? Hmmm, I wonder if that could work actually. Same exact tire and tube (actually moved rim to rim), pressure and drum tension. Ride ramps to virtual power only in TR with a speed sensor and record power meters separately to head unit for comparison?


Comparing power meters on multiple bikes
#17

Ah! My bad. I missed that you were using a power tap wheel.

If you read some DC Rainmaker power stuff he says that the location on the bike matters a lot and the further from the point of application (the pedals) the lower the number you should expect from the PM.

I suppose you could use virtual power as your constant. To do that I guess you need to make sure you let the turbo cool down fully between runs.


#18

Sorry, I’m not on a Power Tap wheel. I’m on two different bikes, each with its own Stages crankarm. And a dumb, wheel-on trainer, the Kurt Road Machine.

I take your point about trainer cool down though. I guess the time spent switching tires and tubes will take care of that!


#19

Sorry I haven’t read the entire thread, but you’re comparing power from a TT bike and a road bike. That might be the key difference. I lose around 20 - 30W when on my TT bike due to the position. As long as you’re happy that you have an accurate picture of FTP on both bikes does it matter if they are not the same?

I was convinced that the Stages on my TT bike gave a different reading to the P1S on my road bike. I put both powermeters on the same bike, hooked them up to different headunits and did an indoor ride. To my surprise they gave very similar readings and tracked almost indentically.


#20

Using the DC Rainmaker tool for comparing different power meters makes it really easy to compare different ones