Big vs Small Chainring - Same Power (ERG Mode Gearing)


After install my Stages LR power meter, I was thinking “since its in the release notes I’ll bet its there but hiding” and so I tried the obvious and sure enough it worked.


Just checked the latest android version…

I’ve got an old iPhone kicking about somewhere so might fire that up and have a look.


I keep reading comments somewhat along the lines of:

  • big chainring = faster spinning flywheel with a lot of momentum
  • big chainring requires less effort versus little ring because of momentum
  • outside momentum on the road means you can hold speed without much effort

Will all that momentum on trainer, it stands to reason the power timeline for big ring would be smoother versus little ring. But it doesn’t for others and myself. Go figure.

The power timeline for my outside rides, both climbs and flat roads, looks similar to big chainring on the trainer (kickr direct drive).

Outside on the flats I apply power to pedals just like when climbing, and I do the same on trainer. I don’t just kick over the top.

I don’t have problems modulating power in big chainring when jumping from rest to work interval. If I did, that might be a reason to use little chainring as I agree that little ring makes it quicker and easier to handle resistance changes.

And I continue to find big chainring on trainer better replicates outside riding conditions.

Just for fun lets play a little matching game using ride data from 4 very different threshold efforts. And a ~5 min power timeline for each effort is below. I didn’t cherry pick, just grabbed ~5 min during a portion of the ride that I was applying constant power near threshold.

a. TR intervals in big chainring (53x15) in Erg mode on Kickr 2017 direct drive
b. Flat training ride into headwind at 4mph (something like 50x14)
c. Steady climb on 6% grade in little chainring (either 34x32 or 34x30)
d. Flat training ride into crosswind at 13mph with 20mph gusts (something like 50x17)

Can you correctly match each effort above to a timeline below?

Ride 1

Click for answer

b. Flat training ride into headwind at 4mph (something like 50x14)

Ride 2

Click for answer

c. Steady climb on 6% grade in little chainring (either 34x32 or 34x30)

Ride 3

Click for answer

d. Flat training ride into crosswind at 13mph with 20mph gusts (something like 50x17)

Ride 4

Click for answer

a. TR intervals in big chainring (53x15) in Erg mode on Kickr 2017 direct drive

They all pretty much look the same to me, and roughly felt the same. Good luck!

Based on these 4 power timelines, I guess that explains why big chainring feels more natural to me. Switching to little chainring would make that one power timeline above smoother, but then it starts feeling artificial to me. As always, your mileage may vary!


Isn’t the the smoothness of the power graphs a function of how the trainer applies resistance?

I am not sure exactly how the Kickr applies resistance, but if there is a minimum “step” or increment that the resistance can change, I think it makes sense that the power graph can be smoother in the little ring. When in the little ring, the flywheel speed is slow and the resistance is high. In the big ring, it is opposite. The trainer constantly adjusts the resistance, to keep the watts stable. The minimum increment mentioned above, will be a much smaller percentage of the total resistance, when the resistance is high (as in the little ring) than when it is low (as in the big ring), so the fluctuations should be smaller, resulting in a smoother power graph.

Does this make sense?


This might be nonsense, or missing the point or already covered in which case apologies.

Could the apparent riddle of why a faster spinning flywheel (pedalling in big ring) doesn’t necessarily result in a more even power profile than a slower spinning flywheel (pedalling in the small ring) come down to the simple fact that the spinning flywheel has the effect of smoothing angular velocity of the cranks, whereas the power profile is entirely down to when the rider chooses to apply force to the pedal? Stop pedalling and no matter how fast the flywheel is spinning the power reading will be zero.

In other words the spinning flywheel to some extent decouples the relationship between the angular velocity of the cranks and the power delivery from the rider. Might this mean that a smoothing of crank angular velocity doesn’t necessarily have to be accompanied by a smoothing of power delivery by the rider?


I think what you’re saying is essentially correct.

Look at the direto power profile for instance:

At 15kph speed you have a full span of 372W, where at 30kph the full span is 1007W. Like you said, there is a practical limit to what the resistance unit can adjust. Assuming the adjustment ability is around 0.5% of full span, that gives the smallest change at 15kph of ~2W and 5W at 30kph.

@Zahlmiac what’s odd is that when I’m in little ring it feels like less resistance. I’ve described it as pedaling down a false flat.

I’ve started to think of Kickr behavior in two areas:

  1. Large changes in resistance, going from rest interval to work interval.

I like to think of the Kickr as a black box, as it has its own gearing, braking, and control algorithm. And that requires turning your attention to bike gearing. Basic bike physics tells us the little chainring provides a higher torque multiplier. That higher torque multiplier is likely the key reason the little ring is quicker/faster to respond to rest-to-work interval resistance changes.

  1. Steady state during an interval.

During an interval, gearing doesn’t explain why little ring has smoother power graph/timeline. Your theory makes sense - it appears the Kickr “black box” makes larger power adjustments when the flywheel is spinning faster. And therefore the small ring shows less power variation (“smoother power”) because of Wahoo’s resistance control despite less flywheel momentum. What I find interesting is the little ring is so smooth, that it feels artificial and impossible to replicate outside except when I’m riding down a false flat.

This is all genuinely interesting, but getting back to the original question, I should state the following.

  1. Say you’re producing an indicated 250w at 95rpm in 50-15.

  2. You change to 34-15. Same 250w, same 95rpm. But now with less flywheel speed and the resulting reduction in the helping hand of inertia, your HR is higher and your RPE is noticeably higher. You may now be struggling when before you were not.

  3. I and others have tested this phenomenon. It results in a strange fact that if you ramp test in a high gear you get a higher level of maximum achievable indicated wattage than if you select a lower gear.

  4. Point 3 was verified by a forum member yesterday who unsurprisingly gets a lower ftp when he tests in the small ring vs big ring, by a factor of 20w.

  5. In other words if you have an interval of say 3 minutes at 300w that is not quite achievable at 95rpm in 34-15, with your HR and RPE “just off the scale”, you will find the same 300w at 95rpm doable by changing to 50-15.

  6. In summary, big ring will allow you to operate at a higher ftp, small ring a lower ftp, while all your actual efforts, pedal pressure, heart rate, RPE are identical in each of the two scenarios. Big ring = free watts, as far as the smart-trainer-and TR-app-measured output goes.

Separate but connected to the above is the different muscle groups that are being engaged and developed in the big ring vs small ring scenarios but maybe we should have a different thread for that!

Qualifier: the above is referenced from ERG mode, Kickr Snap.


I can’t tell the difference on my Kickr Core. I use powermatch. The power meter reads the power regardless of what gear I am in. When it’s hard, it’s hard.


@Darkgerbil thanks and great summary.

My premise: we train inside to improve outside riding, and FTP on trainer should match outdoor FTP.

My conclusion: TrainerRoad FTP using big ring (50x15), Erg mode, and power match has delivered consistent FTP across three different scenarios:

  • threshold riding on trainer
  • threshold riding on long climbs (20+ minute)
  • threshold riding on flat roads

Outside I ride by feeling/RPE, and not looking at power meter. After the ride I can see that what felt like threshold outside, was in fact threshold using TR FTP. For that reason I never thought of using anything other than big chainring. Bonus that it feels natural and more like riding outside.

And therefore I can’t comment on your summary about big ring resulting in higher ftp or “free watts.” All I know is that Erg/PowerMatch/BigRing results in an accurate and consistent ftp for both indoor training and outdoor riding.

p.s. my trainer is different - Kickr 2017 direct drive (wheel off). And for reference I’m using power match with Stages crank PM, so exact same power source for both indoor and outdoor.

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I think there’s clearly a spectrum of experiences with smart trainers, depending on what equipment is being used, regarding

  1. Indicated wattage vs real, actual wattage
  2. Changes to perceived power output, RPE and actual workload on body in small ring vs big ring
  3. Perception of “road feel” in different gears and cadences.

I had a feeling that if I had a power meter working with the trainer that it would be more a case of “watts are watts” and that I would not have the slightly bizarre situation where I can produce more indicated watts simply via higher gear selection.

I think, though, to get it all in perspective, all of these things are just peculiarities of the system (in my category of “no separate power meter”, at least) and whilst intensely interesting (to an anorak like me) none of it detracts from the fact that the smart trainer coupled with the brilliant Trainerroad system provides me with a fantastic training platform that (regardless of gear selection!) works me to the limit of my ability to suffer, via fantastically designed workouts, such that my FTP is on a nice upward trend, along with almost all of us!


As someone always using big chainring on trainer we can turn that around and it becomes… a bizarre situation where I can produce a lower ftp simply by switching to little chainring. Which means I would have a higher ftp on the road than inside. Or put another way, being able to produce more watts on climbs than on trainer.

I haven’t done an ftp on little chainring, just playing out the implications of your summary and comment above.

Pretty confident that my inside and outside ftp are consistent and aligned, using my trainer setup.

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I live somewhere very hilly, so all my outdoor rides consist of, effectively, steep hill repeats, and I’m relatively unaccustomed to riding hard on the flat - because there aren’t many. :wink:

I use a Tacx Neo trainer and choose to use a smallish gear for most of my harder intervals: the pedal stroke feel in a low gear more closely replicates the way I ride outdoors, and the way it feels ro ride up hills outdoors, than the pedal stroke feel from using a high gear on the trainer.

And I do ramp tests in that same lowish gear, so that I’m testing using a similar pedal stroke to how I do much of the training.

I actually find it “easier” to do hard (eg. threshold) trainer intervals in that same lowish gear. I do some work in a high gear, for the variety and differing training effect, but find it harder to “stay on top of” that high gear than I do the lowish gear.

As should be clear, this somewhat contradicts what you’re suggesting above. :smile:

The obvious explanation is that my endless riding of hills in low gears, and limited riding larger gears on the flat, means I’m better adapted (some combo of physically/mentally) to riding a low gear than a high gear, and this adaptation seems to translate to the trainer.


Just wanted to thank @DaveWh for his earlier long post, I thought it was great. But I wanted to pick up on @bbarrera’s point above. How does Erg mode work?

I think the quote has it the wrong way round. I think Erg mode has a control loop that applies braking force until power exceeds target. Once your power exceeds target, the control loop must DECREASE resistance, so that you maintain a constant power. If the braking force increased, you’d just increase power to compensate, and your power output would go up.

So think of the ERG has a brake until you reach target, and then it ‘slips the clutch’ as you exceed it so you cannot produce more power that target. Is that correct?

I noticed the same thing. I want to improve my climbing and discovered that having a slower turning flywheel and less inertia simulates more like a steeper climb or stiff headwind.

I actually wish there was a feature in TR that would allow us to adjust the flywheel inertial on smart trainers. I would prefer having as little inertia as possible, since my main goal is training for steep climbs.

Agee, I wrote it backwards

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You can do this by changing gears. Easier gear = lower flywheel inertia.

Other than changing gears, the trainer would have to have a virtual flywheel where the simulated inertia could be changed programmatically (or have the ability to accelerate the flywheel with a powered motor). So the capability would need to be designed into the trainer, before TR or any other software could take advantage of it.

AFAIK only Rotor power meters can handle oval chain rings correctly, because oval chain rings is one of the things Rotor is known for. (I reckon they officially support only their own Q rings, though.)

One question: usually when I ride, I really, really prefer the “feel” of the big ring. So if I have the choice between two equivalent gears, one is on the small ring, the other on the big ring, I can “feel” the difference and prefer to stay in the big ring. That is despite the higher drive train losses due to the higher chain angle.

Does anyone know why? And if I were to replicate that on a trainer, does it make the same difference as on the road?

I’m on the lowest gear, but the inertia is still to high and does not feel like I’m on a climb that is 8%+ grade.

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