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Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP
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I’ve read some of the discussions about indoor vs outdoor FTP differences, and it sounds like cooling is the biggest factor.

I generally use TrainerRoad during the week (3-4 workouts) plus 1-2 outdoor group rides on the weekends. I use a Computrainer indoors in TrainerRoad and have the FTP set to 254, which is where I tested doing a 20 minute indoor FTP test 10 weeks ago. The workouts did seem to start getting “easier” (still tough, but nothing insane). I planned on retesting my FTP sometime soon but have been avoiding it a bit.

Recently I bought PowerTap P1 pedals that I’ve been using outdoors. They’re calibrated and working great. I went for a long ride, about 60 miles with 4,000’ of climbing which ended with a really long climb, so I decided to do a 20-minute FTP test just to see what I’d get. My legs weren’t fresh but I decided to try it anyway. In that test I averaged 295 watts for 20 minutes. Based on that I should have a 281 FTP, correct? That makes me really happy, especially since I went into it with fatigue in my legs.

I feel like there are a lot of factors though:

1) Cooling outdoors is better than indoors
2) Computrainer indoors VS. Powertap P1’s outdoors
3) I haven’t tested FTP in 10 weeks and have been riding consistently, so maybe I just made gains

So, what’s my FTP???
Should I restest on the Computrainer to verify? Argh those 20 minute tests are tough. My end goal is to make sure my indoor FTP is set correctly so that I can get those most out of my indoor sessions.
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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I have a computrainer and a C1 powermeter. I found a difference in power readings of about 10%, (which seems rather large to put down to drive chain losses to me). Nevertheless the computrainer measures at the end of the drive chain and your P1s are even further up the chain than my C1s. The power measured by your P1s should be higher, indoors or outdoors, than that measured by the computrainer. Have you tested your P1s on the computrainer? If not I think you need to find out so that you can compare FTPs accurately.
I use the FTP number from my C1 to set my zones and do my analysis as that is what I'll use in races. I put a lower number into the program that drives my computrainer. I use Golden Cheetah.
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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cbr shadow wrote:
I’ve read some of the discussions about indoor vs outdoor FTP differences, and it sounds like cooling is the biggest factor.

I generally use TrainerRoad during the week (3-4 workouts) plus 1-2 outdoor group rides on the weekends. I use a Computrainer indoors in TrainerRoad and have the FTP set to 254, which is where I tested doing a 20 minute indoor FTP test 10 weeks ago. The workouts did seem to start getting “easier” (still tough, but nothing insane). I planned on retesting my FTP sometime soon but have been avoiding it a bit.

Recently I bought PowerTap P1 pedals that I’ve been using outdoors. They’re calibrated and working great. I went for a long ride, about 60 miles with 4,000’ of climbing which ended with a really long climb, so I decided to do a 20-minute FTP test just to see what I’d get. My legs weren’t fresh but I decided to try it anyway. In that test I averaged 295 watts for 20 minutes. Based on that I should have a 281 FTP, correct? That makes me really happy, especially since I went into it with fatigue in my legs.

I feel like there are a lot of factors though:

1) Cooling outdoors is better than indoors
2) Computrainer indoors VS. Powertap P1’s outdoors
3) I haven’t tested FTP in 10 weeks and have been riding consistently, so maybe I just made gains

So, what’s my FTP???
Should I restest on the Computrainer to verify? Argh those 20 minute tests are tough. My end goal is to make sure my indoor FTP is set correctly so that I can get those most out of my indoor sessions.


[/url]Start with 280W, also use HR to check your progress. There is no such a thing like “correct FTP”… it is ballpark at any given time. You will not be able to repeat that every day.
There are a lot of variables that influence performance. FTP number allows to have set of zones created to allow you to have repeatable training load in given environment where test was performed.
Having 20min power from outside climb ride for training in the basement on the trainer is good for starters (for your first indoor ftp test) but useless if you are looking for precision.

You will get the most out of our indoor sessions not by setting your FTP to the closest 2-5W (impossible anyway) but by being consistent in training, eating well, sleeping well, and properly recovering as well listening to your body, if you are tired rest if you are sick rest etc.
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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Yeah, not unusual with outdoor hill courses vs indoor training to have variations in FTP, even if yours does seem a bit on the high side.

I've actually come to err on the safe side and use the lower of my two on race day - I find that after the swim, it's hard enough for me to hit that lower %FTP anyway.
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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Experts can chime in, but is a climb an accurate way to get an FTP guesstimate? I can generate a shit ton more watts on a climb than a 20 minute flat section. Or maybe a watt is just a watt, but the CT tests that I did had a flat course with maybe a 1% grade (not that this reflected an accurate way f doing it, just what I did)
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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cbr shadow wrote:
I’ve read some of the discussions about indoor vs outdoor FTP differences, and it sounds like cooling is the biggest factor.

I generally use TrainerRoad during the week (3-4 workouts) plus 1-2 outdoor group rides on the weekends. I use a Computrainer indoors in TrainerRoad and have the FTP set to 254, which is where I tested doing a 20 minute indoor FTP test 10 weeks ago. The workouts did seem to start getting “easier” (still tough, but nothing insane). I planned on retesting my FTP sometime soon but have been avoiding it a bit.

Recently I bought PowerTap P1 pedals that I’ve been using outdoors. They’re calibrated and working great. I went for a long ride, about 60 miles with 4,000’ of climbing which ended with a really long climb, so I decided to do a 20-minute FTP test just to see what I’d get. My legs weren’t fresh but I decided to try it anyway. In that test I averaged 295 watts for 20 minutes. Based on that I should have a 281 FTP, correct? That makes me really happy, especially since I went into it with fatigue in my legs.

I feel like there are a lot of factors though:

1) Cooling outdoors is better than indoors
2) Computrainer indoors VS. Powertap P1’s outdoors
3) I haven’t tested FTP in 10 weeks and have been riding consistently, so maybe I just made gains

So, what’s my FTP???
Should I restest on the Computrainer to verify? Argh those 20 minute tests are tough. My end goal is to make sure my indoor FTP is set correctly so that I can get those most out of my indoor sessions.

Hey there!

You are spot on with you analysis of possible discrpencies between indoor computrainer power and outdoor P1 power. Since power measurments are not 100% accurate, a large part of the discrepency can come from the power meters themselves. Is it possible to put your P1 pedals on the bike you use with the CompuTrainer? In TrainerRoad, we can use the power from the pedals to control the CT via PowerMatch, allowing you to eliminate one of the variables.

Other than that though, it can be hard to estimate your FTP without doing a test. Your actual FTP should be the same regardless of whether you are indoors or outdoors, however as you mentioned, the temperature and humidity indoors may effect your body's ability to express high percentages of FTP- therefore you may not be able to simply use your outdoor FTP of 281. If you are looking for a "quick and dirty" way to assess your fitness you could choose to just do one of the 8 Min test intervals and multiply the result by 0.9. While not perfectly comparable to your 20 min test from before, it can certainly give you an idea your progression without the pain of a 20 min test.

You could also choose to continue with your indoor training and increase your FTP by 3% and see how that feels. If you are still able to easily complete all your workouts, maybe add another 1-2%. If that's too much and you can't finish your workouts, back it down a little bit. This method allows you to go by feel and listen to your body. While not as scientific as periodic testing, it will ultimately lead to similar results in applicaition.

I hope this helps! Let me know if you have any more questions :)

Get Faster with TrainerRoad
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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Have you compared your P1 readings (on your bike computer or watch) compared to the watts being read from your CT? I've had the C1 and P1 meters and both were very close to the same reading the CT was giving.

And I'd suggest using whatever FTP you are getting indoors, if you plan to use TR... if you change your FTP to 280 from 254 you aren't going to be able to do the workouts!!!
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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I listen to a lot of experts on podcasts and they all say that your indoor trainer power ave will be up to 20% less that what you will achieve outdoors.
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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  • Your P1's may be measuring higher than your Computrainer - do a side-by-side test

  • Common to have an indoor FTP and outdoor FTP - Motivation, distractions, terrain, cooling all play a role

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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [holograham] [ In reply to ]
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holograham wrote:
  • Your P1's may be measuring higher than your Computrainer - do a side-by-side test

  • Common to have an indoor FTP and outdoor FTP - Motivation, distractions, terrain, cooling all play a role

Agreed regarding point 2. My training partners and I all have some variance between indoor and outdoor power. I have a Tacx Neo, SRM, PowerTap and Vector 3 pedals. I know and trust all of my power sources. My indoor power is approximately 20 watts lower than outdoor and it is pretty darn consistent. I adjust my training accordingly for it and don't worry about much at all. Its been this way for a decade and as long as I put in the right work I get the right results.
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [Greatwhite7] [ In reply to ]
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Greatwhite7 wrote:
I listen to a lot of experts on podcasts and they all say that your indoor trainer power ave will be up to 20% less that what you will achieve outdoors.

That's ridiculous. If you lose 20% of your power indoors, that is the only thing you should be trying to change. 10% is still not acceptable. 3-5% is the most I would tolerate.

Which experts are these?

Watts are the stimulus to adapt, not your perception of effort.


3 Months of Paradigm Shifting Swim Instruction for Cheap // Your Professional & Private “Critique my Fit”

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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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IMO, I would not worry about your FTP, just keep inching up your critical workouts on trainer road by percentage FTP or absolute wattage (what I do). If you are completing these workouts at higher and higher wattages (assuming these are targeted FTP workouts) then you can safely assume you are getting fitter.

I have not measure FTP in about 7 years and set race power according to outdoor simulation rides/runs at the particular distance near the race.

FWIW, when I did measure FTP my outdoor measurements were always 5-10% higher (never 20%). And, as some others have posed, I also found that my outdoor test done on a 8 mile ~3% climb was highest on road bike on hoods and lowest when I stayed in aero on tri bike (a good 20-25 watts watts). Likely obvious I know, but speaks to difference, even outdoors, from position. If I ever tried to race off a percentage from road test (which I did early on) I would be trashed for the run every time. Same for training off road measures, couldn't finish workouts.

If you have been going 3-4/week for 10 weeks, you should have already be progressing your workouts significantly. My unscientific recipe is successful completion of a critical workout 2 times results in a step up of intensity.

Train off trainer numbers, race off road numbers with specificity to the race.
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Re: Indoor TrainerRoad & Computrainer FTP vs. Outdoor Powertap P1 FTP [cbr shadow] [ In reply to ]
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Rohan Dennis's coaching team wrote something up for us on this topic.

"If you’re training with power year-round, both inside and outside, you might have noticed that you hit your best power numbers while outside, or that the same power might feel easier outside compared to inside. This isn’t your mind playing tricks on you, but a well-documented phenomenon. This difference is more pronounced with shorter, higher-intensity efforts, especially when you’re out of the saddle (think sprints). The differences are smaller for longer, lower-intensity efforts, and can be caused by more factors. Before you read on, here are some key points to remember:
  1. Understanding why you can crank out more watts outside can help you identify specific weaknesses—like poor flexibility or core strength—that may be holding you back.
  2. There is no set formula for how much indoor power differs from outdoor power. Every athlete is different.
  3. If you have no idea what your individual differences are, using Relative Perceived Exertion (RPE) during training both indoors and outdoors will allow you to dial in your specific differences faster.
  4. If you use a different power meter for indoor riding and outdoor riding, be aware that differences in accuracy and calibration between the two power meters may result in different readings for the same output."

You can find the full article here: https://thesufferfest.com/...utdoor-power-numbers
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