Indoor/outdoor FTP differences and Kickr '16 power discrepancies


#1

I’ve been training indoor/outdoor and racing for a while now and I don’t remember when was the last time I did an indoor FTP test that resulted in an increment on my figures. In fact I think that all my PBs have been done while racing, after uploading your activity you receive an email from TP telling you that an FTP increase was registered, always makes me smile. Of course, a new FTP while racing makes you wonder what would the real FTP be If you did not have all those miles of racing before you set your new best 20min power. Anyways, that’s how I’ve been keeping track of my FTP.

I have a quarq elsa on my road bike, a quarq Dzero on the gravel bike and an SRM on my MTB. The three of them give very similar power readings. On the other hand, my wahoo kickr '16 always seems to be way off… around 30w less. It was until recently that I found out about the Power Match option in Trainerroad (I don’t know if Zwift offers something like this) but before that I found myself training indoors “in the dark” regarding which FTP to use since it definitely felt much harder to move the 310w FTP that my training peaks suggested. I had two factors against me, 1: the fact that I always feel stronger outdoors and 2: the different (lower) reading that my kickr showed. This was a bummer since the whole point of buying all this equipment was precisely to train as accurately as possible.

So, to round up my questions:

I know it’s not just me who feels stronger outside, is there any reason for this other than the psychological aspect of being outside? (I’ve thought about the fact that the bike is fixed, this might alter the result). Is there some sort of handicap you can consider in order to pick the right FTP to train inside, lets say a 10% decrease in the FTP when training inside?

Is my kickr '16 defective or are everyone’s kickr readings about 30w lower than their other powermeters? Is there any way to override this within the kickr hardware so I don’t have to rely on the Power Match option which I only know it exists in Trainerroad?

Thanks in advance,
ig: poncho_arroyo


#2

Cooling, and the ability to move around a bit more when riding outdoor, as well as all the psychological benefits of being outdoors/in a real world riding situation are pretty major factors. Especially cooling.


#3

I have a Kicker 2 and it also reads ~30w lower than my PM (Stages). I just use TR and Kickr (with an old bike attached) for proper training and a combo of higher FTP and RPE for outdoor riding/hill training. The ramp test is a godsend for my TR training, prior to that I used to estimate my indoor FTP because I never did the 20min test. Sorry, not much help other than to confirm that you’re not the only one with a substantial Kickr/PM differential.


#4

If you have accurate and calibrated power meters that are located at the same position on the bikes, or if you use the same bike both indoors and outdoors, then the outdoor power reading should be the same as the indoor power reading. If you are using a crank or pedal based power meter outdoors vs. a KICKR indoors, then the power will read differently due to drivetrain losses and the difference in power meter accuracy/precision. Additionally, I have a friend with a TACX NEO who says that 300 watts on a TACX is much harder than 300 watts outdoors (same bike). He thinks this is becuase his trainer uses magnetic resistance, with no flywheel simulation to ease through the dead spots.

So, long story short, I use the same FTP settings indoors as outdoors, but I always use trainerroad powermatch when doing indoor intervals. And, I alway calibrate my crank-based power meter before each workout in TR and do a Trainer-road KICKR spin-down once a month.

My KICKR v1, using the strain gauge, reads 20-30 watts lower than my SRM power meter, depending on the power output; 200 watts is more aligned than 300+ watts. The KICKR in non-strain gauge mode is even more out of sync with the SRM. I switched back to strain gauge mode because it was so bad. I am not aware of any way to over-ride the KICKR calibration, but you could try checking your belt tension. A tighter belt will have more accurate readings. I did that, but it did not help in my case, so I contacted Wahoo about purchasing a calibration kit last year. They said that they no longer offer it, so I am stuck with auto powermatch, since it is not clear that manual powermatch even works.


#5

BTW, I found that the 20 min FTP test over-estimated my FTP power, so I recently did a 60 min indoor FTP test and was happy with that result, although the test was freakin’ hard.


#6

I don’t have the same issue between PM and Kickr readings since I have a dumb trainer. But I use a stages PM on my road bike for indoor and outdoor training. For training I always use the FTP from Trainer Road tests because it is in the environment my indoor training will take place at. Indoors you have different motivation that plays a factor, a difference in cooling and regulated intervals with no breaks in power targets.

Outdoors you have the motivation of chasing someone, being in a pack or simply the joy of seeing a finish line or the top of a hill. You also have much greater cooling and greater frequency of rest, coasting and changes in FTP. All of this combined (cooling alone is a big factor) can cause a higher FTP outdoors. I have consistency raced with NP above my FTP from TR but I keep my FTP at TR test results for training purposes and because it reflects the training environment for me.

Lastly, I believe there is TR podcast on this topic and with the new features you can adjust your FTP for indoor workouts and outdoor rides. In your case, it might make sense to utilize TR Test FTP for indoor trainer workouts with Power match on and use your TP FTP estimate for outdoor rides. Hope this helps.


#7

Worthwhile reading:


#8

Well, while that is not a good thing to hear I’m glad I’m not the only one and true, the ramp test is a much better option!


#9

Although I have read of the drivetrain losses, it escaped my mind to consider it. I’m not sure that alone means 30w loss of power but it sure adds up.
I’ve never used the NEO but while that reasoning seems logic, I am experiencing the same issues and the kickr has a heavy flywheel.
I did not know that you could change the way kickr calculates power, when it is not using strain gauges how does it calculate power?

And lastly, regarding manual powermatch I also have the same feeling. It seems like the powermatch curve is not a constant in which it is always +/- a certain amount of watts, rather it seems like the difference is exponential as you input more power, the difference increases.


#10

The power calculation uses standard electrical math looking at voltage, current as well as flywheel speed. They can back solve for the power based on measuring those values closely, after also confirming with an external power measuring device. The calibration process for most smart trainers is done to compensate for the various differences in each unit and each particular environment (temp and humidity) where the trainer gets used.

First, understand that power is graphed on a cartesian coordinate system. An overly simple example is a line sloping from low on the left, upwards to the right. Picture two of these, but at slightly different angles. They both slop the same basic direction, and even cross over at one point.

  • Imagine the two different colored lines are the power curves of each device.
    image

  • That would be an exact match between the two meters.

  • The difficulty with manual adjustment, is that you are picking one location to apply a fixed delta offset between the 2 meters. That’s fine, as long as you are at or very close to that point.

  • As you move away from that crossover, the deltas will change.

  • That’s why the power match is a good idea, because it actively looks at the delta and adjusts in effort to keep everything aligned.


#11

Sounds exactly like you’re describing drive train loss on top of the effect of your Q-rings. Drive train loss is ~7%, plus the ~2% skew from the oval rings and with 310W FTP that brings you to around 30w less from where spider-measured power reading is from and the freewheel of the KICKR (the PM also is +/- 1-2% accuracy as well).

Most people find indoors to be a lot harder for the same watts than outdoors too though, especially when you’re not factoring in stuff like the adrenaline rush you get on race day or in a pack. Richie Porte this year said he just can’t do things like Zwift or any real training indoors because it’s feels “too hard” for him and that he “gets dropped” on the box hill course on Zwift. This is someone who probably has top 5 watts-per-kilo in the world.


#12

Thank you! I’m glad I posted my question, that graph was spot on.


#13

Thanks, where can I read more on the 2% power skew you mention? It interests me.


#14

The more oval it is (like QxL or Osymetric), the more skewed it is – note that this doesn’t affect your training numbers or anything as long as you’re using the same equipment (the constant offset is effectively normalized out).

Stages claims 4-5%
https://support.stagescycling.com/en/support/solutions/articles/1000158537-can-i-use-the-stages-power-meter-with-oval-or-osymetric-chain-rings-

Others suggest 1-4%


#15

So, not a direct response to your question (though I’ve had my share of issues with accuracy between my Stages and Kickr Snap), but sort of a tangential recommendation.

You can disable your Kickr’s internal power meter and tell it to use your power meter as its source. From the Wahoo Fitness iOS app (this feature isn’t available on Android), tap Senors, choose your trainer, then scroll to the bottom. Activate the “Control w/ ANT+ Power Meter” option, then key in your power meter’s ANT+ ID in the section just below.

From then on, your Kickr will actually use your power meter as its primary power source for all apps—meaning it will basically work like a more accurate version of Power Match for TrainerRoad, Zwift, etc. While it doesn’t necessarily “fix” the inaccuracy issues, it will give you the same baseline to work with indoors and out.

Hope this helps.


#16

I honestly don’t worry too much about these ‘discrepancies’ between different systems.

When I’m on my Kickr I calibrate with the ramp test and then do everything Chad tells me to, and I get better. When I get on my road bike, my improvements carry over: I do a given hill faster, my heart rate is lower for my commute etc. As long as things get better, I really don’t care that my Stages PM gives me different numbers than my Kickr. It’s not like my heart has my ‘real’ FTP engraved on it: there is no single correct answer to make the Kickr more accurate than the Stages or vice versa.

What’s important is that improvements on one translate to the other.

  • Pasta

#17

Could you please share a link that discusses using different FTP for indoor and outdoor rides? Would love to know how to do this…


#18

I already linked it a few posts above:

  • (see “The Differences Between Testing Outdoors V. Indoors” section in the link)

#19

That’s more around testing indoor vs outdoor. I thought lonestarcyclist was referring to a software feature that allows users to maintain an indoor FTP and an outdoor FTP.

I use Assioma pedals. They are known to be very accurate. I consistently get a ~30w difference in FTP, with outdoors being higher.


#20

Sorry, I misunderstood and missed the “using” aspect of your post.

In general, my first attempt would be to get the inside work environment as cooled and flowing as possible for air.

After that, I think it is possibly to use a decreased workout Intensity setting to compensate for any remaining delta between the In/Out FTP.