Why is the wattage on trainers so different then out on the road?

Hi Trainer road team! I am using Garmin Vector 3 pedals on my bike. When I ride outdoors on the road rides I can totally feel what wattage I am at when pedaling. (Within about 30 watts) when I put the same bike and power pedals on a trainer
( kinetic wheel on roller) not smart trainer the “feel” is way off. The same amount of power in my leg feel only produces 50 watts compared to feel at 200 watts. I guesstimate there is a 150 watt difference. At Highest gear on bike I cannot produce the wattage for training numbers or FTP tests that I can do on road with same bike/power meter pedals. Thoughts? Thank you in advance for your expertise.

Could just be flywheel inertia…I feel the same and this is what I’m left with. Engagement around the pedal arc is slightly different.

  1. Depending on a variety of factors when riding outside (rider weight, general riding history, road surface conditions, road pitch, typical riding speed, etc.) and the specific trainer in use, there can be real differences in “feel”.

    • Much of this boils down to momentum, inertia, etc. differences between outside and that provided by the trainer. The flywheel on a trainer is intended to mimic the feel we get outside. The size, weight, and typical rotation speed (RPM) of that flywheel will give a range of “feel”.
    • This can impact the muscular loading and feel around the pedal circle. It is somewhat like the difference in feel between equal wattage when on a flat grade vs a steeper climb.
    • There are ways to run some math and physics to estimate the trainer feel and compare to outside feel, but those are well beyond me. But without that, the common experience is that most trainers still lack enough inertia & momentum and fall short of outside riding.
    • The Kinetic Road Machine (that it sounds like you have) is regarded as pretty good, but still not perfect for many riders. So, this may be a notable factor based on your comments.
  2. You don’t mention this, but a common issue when moving inside is insufficient cooling. Getting 2 or 3 REALLY GOOD fans can make a massive difference in the RPE (Rate of Perceived Effort) and totally change what you feel. So, this could be worth a look too.


“Road feel” is the #1 benefit of a direct-drive smart trainer. On my Kickr 2017 direct-drive trainer, 180W zone2 rides feel the same indoors and outdoors. To achieve that I use resistance mode (Standard/Level mode).

Keep in mind that the feel/calibration of the KK Trainer has been modeled after specific conditions:
KK Power Curve

  1. We started by defining the “average rider”

We created an “average rider" assumed to be:

  • 165 lbs.
  • Riding a 23 lb. bike
  • With 170 mm crank arms
  • Up a 1% grade
  • At sea level
  • With no wind
  • On rough asphalt

I know it’s not your metric, but damn is that a low weight for an “average”"


haha yes. those are just their assumptions. Who knows, the may have reversed engineered the “average rider” to fit their fluid resistance :stuck_out_tongue:

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Great feedback, thank you for taking time to answer

I am obsessing about my 144 lbs. So thank you! :joy::blush:

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Having the same power curve as outside, means that the same power is needed for the same wheel speed. That does not guarantee the same feel.

The feel is related to inertia and we need to dig a bit deeper to understand what is going on.

It starts with the fact that no matter how smooth your pedal stroke is, with perfect kicking, pulling and pushing, you will always apply the highest force using your biggest muscles, which is when pressing down.

Now inertia is the reluctance of the crank to change its speed. Out on the road this happens because change in speed means big change in Kinetic energie, especially at high speeds like on flat roads. When climbing you have a lower speed, giving lower inertia and crank changes speed with less extra force.

Now what does this mean for the pedal stroke? With high inertia, you can spin at a high cadence and apply enough force during the time the pedal is in the downwards movement, to keep you going. All is good because your largest muscles are providing the bulk of the power.
With low inertia, if you try to do the same thing, the pedal will move away much faster as you press down on it. That gives your large muscles less time to apply their force and therefore less power. So in order to produce the same power as outside, now the other muscles need to provide more force than they are used to in the remainder of the pedal stroke.

This also explains why the cadence naturally goes down when climbing, it is to give your large muscles more time to apply their force and therefore produce more power. Of coarse the consequence of a lower cadence is that this increase the force needed and tires you out faster.

This is what happens indoors if the inertia is not the same, most likely lower than what you experience outside. Your large muscles are not able to apply all the power that they can, while the smaller muscles are being asked to produce more power than they are used to.

With a smart trainer it is possible to adjust the inertia of the trainer and thereby the ride feel, by choosing a higher or lower gear when in ERG mode. This will change the flywheel speed and therefore the inertia of the trainer.


I got the heavier flywheel when I used s KK Rock & Roll. It made a huge difference to the feel of the trainer with the higher inertia

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Many people say riding indoor is harder but i wonder is there anyone who says oppesite?

I have been doing my workouts indoor for 13 weeks and gain a lot watts (did ssbase high v 1/2) and wonder a lot when i go outside can i produce same or more watts? The weather in my country does not let me go and try it and you know covid situation. So i prefer saying at home but so curious about new fitness that i have.

I won’t say it’s impossible, but the number of those that feel they have more power inside vs outside is likely very low. Based on number complaints about low numbers inside, I think is much more common. Getting equal power is a challenge and requires specific steps and setup to achieve. More power inside seems rather unlikely to me.


Agreed but I think that’s most likely down to RPE / Microbreaks / Cooling etc. than actual power output?

Sure, just depends on “what you want to measure”. For any real evaluation, you have to pick at least one variable to aim for as “constant” and then judge the results by one or more other measures. Power is the most “stable” value when compared to options like RPE, heart rate or other options. So using Power as the constant and then rating the others seems most beneficial and reliable to me.

Yeah absolutely - I agree with you.

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My guess is other than the psychology of being inside versus outside, it comes down to:

  1. You’re constantly pedaling inside to meet a power goal. You have no actual breaks. No coasting going downhill etc.
  2. If you do “back pedal”/coast on the trainer, you lose momentum very fast, and you have to work hard to get back on power (at least with a fluid trainer).
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I suspect that the responses here are usefully trying to explain some perceived difference, but are assuming a degree of exaggeration in the original post.

If that’s the case then great, but if someone is seriously recording 50 W indoors when it feels like 200 W then there’s something significant wrong somewhere. Cooling and inertia and so on are absolutely important to feel, but unless you’re training in a very hot, very still room, then that’s too big a difference. Way too big for short intervals, but even too big for long ones, I’d say. If the heat were enough to make 50 W feel like 200 W for me then I’d be very aware that I was overheating massively.

Same vector pedals? Same bike? Zero offset before each ride? Are you able to do a consistent flat (e.g. 200W) effort for a decent length (say 10 minutes) outside to check that you’re comparing like with like?

For me, I’ve always found power a bit easier outside, but I’d say something more like 10% with modest fans.

This makes a lot of sense. I guess this explains why doing 180watts down hill feels harder than doing 300 watts uphill!

Despite the fact that riding indoor might feel harder studies have shown that it is NOT more effective. Effectiveness of training is pretty linear with watts produced, and since you can produce more power outdoors, several studies have shown that training outdoors is more effective. This is of course unless you cannot hit intervals properly outdoors due to traffic or road conditions.

However, my indoor power and outdoor power are very close. Around 50 to 80 Watts on max sprint and maybe 5-10 Watts FTP.