Differences in Power/Heart Indoor vs Outdoor

Hi everyone,

Contrary to the mantra I usually read on the internet (“your Outdoor Watts are usually higher than Indoor’s”) for me this doesn’t hold water and it is the very opposite. I acquired a PM in May to start tracking my outdoor power numbers and after a few iterations I am getting a few conclusions like:

  • At 80% Max HR I can churn 65W more than at the same HR outdoors

Needless to say: it sucks.

I tend to observe that my HR readings are usually higher outdoors as well and I think this is impacting my performance considerably. The thing is: this is a problem that I don’t know how to address, so I am asking if someone has/had similar observations and how dealt with it.

Honestly I wished it was the opposite, but I can feel the performance improvements outdoors as well. But matter of fact I can’t hold my indoor FTP outdoors, limiting factor HR rate apparently.

To the equipment in question:

  • Elite Direto XR
  • Stages III Powermeter
  • Wahoo Tickr
  • Wahoo Bolt

I am fortunate to have two bikes, one that stays with the PM and the backup one that stays in the trainer.

Could it be that my PM numbers are nerfed or that my Trainer numbers are buffed? Should I attach my PM to the bike on the trainer and see if the readings match? Any suggestions are welcome!

Have a great day everyone and thanks for the attention.

65w is a huge difference. Surely one or both of the power sources is very inaccurate?

Suggest you put your PM to your trainer and have a test ride for comparison


That big a difference can only be explained by an incorrect power source. I suggest you put your outdoor bike on the trainer to do a comparison. Which one feels more correct to you?
And is the recording device the same in both conditions? Some settings in indoor apps can mess up things. I’m thinking about “double single sided power” for instance.

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If you’re using the same power meter, your FTP should not change whether your indoors or out. If watts are higher outdoors, you’re probably not cooling well enough indoors (or some other related factor). If you’re using a different power meter though, you will definitely need to calibrate the meter in some sense. Whether that’s performing an actual calibration or finding out how many watts or percentage of watts lower the indoor power meter always is and basing indoor efforts on this lower amount. But since you’re using different power sources indoors and out, it’s definitely a source inaccuracy. Your watts shouldn’t change whether you’re indoors or out if using the same equipment

Thanks, @david @app4g @MedTechCD @jeremy_l.

Since my PM is crankarm-based it was easy to get it on the trainer bike. I ran both datasets on Zwiftpower and here is the comparison:

I calibrated the Stages right before starting the workout to make sure the numbers were accurate as possible.

The question now is: Who holds the truth? Should I get another power meter borrowed to make this comparison? Should I calibrate the trainer and do it again?

Reading all the posts, I don’t see a comment about your L/R power balance. I am assuming your stages PM is left only?

Have you ever been able to test your L/R balance?
Eg. on a Watt bike at a gym. The actual power output is not important, but your balance is key. If I concentrate, I can get it to 50/50, but generally see a 48-52 when I ramp up my efforts.

Assuming there is a +/- 2% inaccuracy reading between the trainer and the Stages PM, that could be as much as 4% if the inaccuracy is +2% vs -2-%.

If your L/R balance is 45-55, then 45%*2 =90% is your power from the left side doubled. So 200W for 5-min doubled from the left side is 180W.

Hi @Gerald_M thanks for the reply.

I’ve done this workout in Rouvy (and a couple more) and Rouvy is able to get the pedal balance from Direto XR somehow.

Since the numbers seemed pretty balanced for quite a few workouts at different intensities I then decided to get a single-sided PM. I was also convinced by GPLama’s video in which he compares Assioma, Stages, and TACX altogether (Stages Power Meter (Single/Gen III) - Install, Ride, Data Review - YouTube).

But your explanation makes sense. Sometimes I get even a 51% L power even thought I am right handed.

I’ve got in touch with Stages support in Europe and they were super responsive. I am sending the PM for testing at their end and hopefully should get an answer next week.

I made another workout yesterday but this time I told Zwift to get the power data from Stages and connected the Direto XR to the head unit. Numbers were skewed again on the aggregate and the workout was much harder because of the inconsistent readings.

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Intersting that the direto xr actually has left/right pedal balance. I didn’t read about this (likely skimmed thru)

The graph/plot that you made, may I know what software was used to do the comparison?

Interesting. I wasn’t aware that a direct-drive trainer could measure dual-sided power. I learnt something today. I only have a left-sided PM on a no -smart wheel-on trainer.

The % difference on your short efforts can easily be explained by the +/- 2% and a 49/51 split in balance, but the 12% difference is a little harder to workout.

The relationship between the two lines from 30s onwards is very consistent, compared to your shorter efforts. Just looking at the graph I would say the hardware could be a large part of the problem.

As a side note:
If you are right handed, or rather kick with your right foot, you might have a stronger left leg as it’s the one holding the weight when kicking. My coach shared that with me in my days as a junior golfer. The left leg (right handed swing) holds majority of the weight through impact to the completed follow-thru. In the gym, I can lift more weight in a single rep on one leg vs the other.

Hi @app4g and @Gerald_M,

You can find more information here. I haven’t used the MyETraining software because it is awful but maybe Rouvy is able to pull the data by some internal interface and display it.

As to the graph I used it zwiftpower.com, like below:

AFAIK it should only work with Zwift workouts. I had the Zwift reading from Stages and my Wahoo Bolt from Direto so I generated two .fit files at the end. I have loaded the head unit one in zwiftpower.com and was able to crunch the numbers.

The relationship between the two lines from 30s onwards is very consistent, compared to your shorter efforts. Just looking at the graph I would say the hardware could be a large part of the problem.

I also agree with that. I tend to believe that the Direto has the most accurate one because it can pack bigger and larger sensors on its hardware than the slim crank arm. Let’s see what they say after receiving the unit. I would accept a 5%-ish deviation, but anything above 10% is ridiculous.

Makes perfect sense about the leg power, it bears a lot of load in spite of not being the dominant one. :grinning:


So, after some back and forth here is the deal:

I generated another comparison chart and the numbers stack up much better.

Actions taken:

  • Sent Stages for verification, came back ok.
  • Calibrated Stages prior to the workout
  • Update Direto XR’s firmware to the latest (78)
  • Calibrated Direto XR’s last week
  • Updated the wheel diameter based on my road bike wheel size, using the same value on both (Zwift and the Wahoo head unit)

Stages read slightly higher this time, but very well within the tolerance zone so it doesn’t bother me.

Now I believe that the power zones will line up much nicer and it will be easier to determine indoor vs outdoor efforts.

Thanks all for the contributions on the post!