TR can answer a common cyclist argument...Outdoor FTP different than Indoor FTP?

Right? Now that there is some super cool FTP analysis algorithm. TrainerRoad can look at outdoor ride data, predict an FTP…TrainerRoad can take same athlete, temporally similar indoor data, predict an FTP.

Are they different?

I’ve always said they are not but there is a widely held colloquial belief that they are. TrainerRoad, please end this argument and release the data.

(unless I’m wrong. In that case never say anything. I’ll delete this post in a week or so.)


From listening to the times on the cast where they addressed this, my paraphrase would be this:

  • We have one actual, physiological FTP.

  • What differs is our ability to express that FTP based in / on / with different environments, bike setups, power measurement devices, training states, etc.


If you can’t put out the same power indoors as you do outdoors:

  1. Get a bigger fan
  2. Try again
  3. If result is the same go back to step 1

Like for like environments/set ups you can only physically deliver one maximum power effort so indoors and outdoors are theoretically the same. Add in out door cooling v hot stuffy environment etc and differences in what you can deliver outdoors and in develop.

@HLaB there is probably, over the entire population, some air flow cooling deficit indoors. However that is somewhat ameliorated by, over the population, a warmer ambient temperature on outdoor rides.

But that is neither here nor there! Let’s just look at the data & then we can quibble about confounds. For sure confounds exist…but, but, BUT there is no physiologic difference in an athlete if they are standing IN the house vs if they are standing OUTSIDE the house. Therefore, performance should be the same.


Here is the old TR Blog article on the topic:

Key Takeaways:

  • FTP is physiologically-based and does not change when riding in different settings or on different bikes
  • Ability to actually ride to your potential is affected by external factors
  • Experience, Motivation, Hydration, Nutrition, and Equipment can all influence how strong you feel.
  • Indoor and Outdoor abilities usually equalize if you practice both regularly.

Experience: The more often you ride the trainer, the more comfortable you will feel riding indoors.

Motivation: Purposeful training, on-bike entertainment, and Group Workouts can help motivate you to ride harder indoors.

Cooling: Cooling is the most important factor in indoor rides and you probably need a bigger fan than you think.

Hydration and Nutrition: Dehydration occurs more quickly indoors than outside, and eating and drinking are crucial on the trainer.

Equipment: Differences in power meters, riding position, and gear choice can dramatically change your experience when riding indoors.


I am 99% sure my cooling situation is solid. I keep my living room frigid (mid 60s) and I have a Lasko fan blowing on my bare chest from about 12 inches away. I’ve never needed Medium. I keep it on Low. I also keep ice water in my insulated bottles.

There are 3 options for why I hit higher power numbers outside for the same HR, RPE, and TTE.

  1. my kickr bike power meter might be significantly off from my quarq.
  2. erg mode pulls back on your pedals relentlessly and I think this makes the dead spot in the pedal stroke more intrusive. I think this tires you out faster than real world riding bc outside you get about 5-10 degrees of rest with each revolution of the pedals.
  3. maybe there’s a mental benefit of being rewarded with speed outside - could lower RPE potentially?

In a partial tangent, I wonder if others experience this too. Outside, when I recorded my best 12 min power (254), I did it by holding 270 and then my power dropped and by the time I finished the hill, I had averaged 254 for 12 minutes. Indoors, I can’t even hold 254 steady for more than a few minutes. And if you know your CP for a given time, it’s “supposed” to be a more efficient way to pace yourself but I do better riding over a range of zones than I do holding one steady power for the whole time. I’ve spent my entire SSB1 just working on doing the workouts as prescribed and hoping to develop my ability to hold a desired zone for longer and longer durations. One day, this might result in getting my outdoor and indoor numbers to match better.

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This seems very obviously like a power meter difference

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Yup, all those are covered above.

1 & 2 are essentially “Equipment” related and valid reasons for potential differences.

  • Different power data devices is a potential variable. That could theoretically be accounted for with a matched test using both devices together.
  • The trainer mode and flywheel inertia (related to bike gearing in use) are also relevant. Even in the best cases, it’s possible that the trainer still doesn’t match the functional feel of riding outside.

3 is about “Motivation” and also just as valid.

  • People use a range of options to work on getting what they can from themselves on the trainer. Even in the best case, it’s possible this falls short when compared to outside. I’d say it even goes to how a person views inside riding. Those that hate or dread it are unlikely to do well purely from that mental state, even with the best motivation in place.

All that to say that those influences are valid and play a part in the power data results.

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maybe. Both are supposed to be +/- 1 %

  • Sadly, we have seen far too many examples where these don’t live up to the claims when they are paired for testing.

Yeah, the kickr bike actually has a motor to simulate inertia better than a flywheel can do alone but it’s definitely not the same as outdoors.

Another thing I forgot to mention is that outside, even if you’re “steady, not rocking” your bike moves side to side a little underneath you. That might give you some leverage advantage over a rock steady locked in place indoor bike. Nobody can sprint as hard on a trainer as they can when they can make huge side to side movements right? There might be something going on on a smaller scale at non-sprint speeds.

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As the “Rocker Plate Guy” I also agree on the motion aspect. I left that out as the topic was focused on the TR perspective. But I also believe that rigid setups are not ideal and a valid consideration among those already listed.

It likely depends on the rider, power levels of interest, and motion device in use, but I believe adopting a setup that mimics the basic body / bike motion patterns we apply outside would make sense in bridging the In/Out power gaps.


That doesn’t mean they are both +/- 1% from the “true” value, just that repeat measurements of the same power vary by only 2%.

Re the op: I’d guess a lot of people actually use two power sources - their indoor trainer and their power meter outdoors, which could well throw the data off.

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so yesterday in the podcast, @Nate_Pearson talked about handling indoor ftp and outdoor ftp and applying secret sauce to handle those differences etc.

Are we saying now that indoor ftp issss the ftp and we just do a better job riding suprathreshold outdoors? Or would outdoor ftp be the ftp and we do a worse job riding suprathreshold indoors? Or should we ride indoors at percentages of our indoor ftp and outdoors at percentages of our outdoor ftp?

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Yeah I think that is a very real thing for a lot of people. On an individual basis (based on my experience with several athletes) small differences between pedal-based orthogonal strain gauge arrays & hub-based OTS are impossible to resolve.

However, in a population of athletes it all comes out in the wash. Some athletes will be a little bit more and some athletes will be a little bit less.

I think a more difficult confound would be how people zero their crank based power meters. Most people zero before they head out the door. There is a general bias to outside temp being higher than inside temp (otherwise, why are you riding outside?). This causes a power bias. If there is a higher outdoor FTP that is the most likely cause IMO.

Not according to Wahoo and all other claims I can remember seeing:

Quarq is similar with the use of “Accuracy” specifically.

  • image

You are talking about “Precision” and Wahoo & Quarq are stating “Accuracy”:

  • Precision = Repeatability of any given value

  • Accuracy = Trueness compared to the “real” value


Thanks for reposting that article, Chad. I clicked on it and noticed a typo I overlooked when writing it :wink:. Fixed!


I don’t want to speculate on how they are handling the delta’s we are discussing here and elsewhere (inside/outside, different bike setups, different power meters, etc.). I have NO idea what they are doing to address that.