Flywheel Inertia / Momentum / kinetic energy

Having read the various very helpful threads on Flywheel “inertia” or more accurately momentum / kinetic energy

Some good thoughts by @mcneese.chad

And having watched Shane’s (@GPLama) video

Given that up until now I have always used the little chainring, in the middle of the cassette with my KICKR Snap, I have a couple of queries

  1. As someone who predominantly rides MTB or CX, but struggles to ride at higher speeds or hold the wheel during flat sections of occasional road rides (hills and rolling terrain fine) would training more in the big chainring on the trainer improve ability on flat road rides?

  2. Would the big chainring be better for longer VO2 intervals, where I find my cadence dropping as the interval progresses. I know these are hard no matter, but I wondered if it would be easier to stay “on top of the gear” as @GPLama suggests

Complicated topic, but interested in people’s thoughts


Nothing of substance to add. Just wanted to write that the video was really informative for me even though I ride a dumb mag trainer. It actually answers some questions I’ve been wondering about for a little while. :+1:

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I’m in the same mtb boat as you.
I ride in harder gears for exactly the purpose of building better leg speed and flat power as you suggested.

I can to this decision based on the same articles.


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The thread you’ve linked covers this topic in quite a bit of detail. To simplify it all down, your answer to 2 is yes, having more inertia will make the high wattage outputs (minimally) easier. However the trade-off is a trainer that is less responsive to changes in wattage, which isn’t such a big deal for sweet spot work, but is more of a problem for VO2Max and anaerobic workouts, which have larger wattage changes and shorter intervals.

The answer to 1. is … maybe. What will help you ride have sustained power on flat road rides is a higher FTP. I think consistent commitment to quality training will have a much (much!) more impact on your road riding improvement. Flywheel inertia is a very minute marginal gain (and I’d actually have to be convinced that it made any difference).


A workaround I use on the Kickr Snap is to shift to a harder gear a few seconds before a big wattage jump (sprints) and that solves the problem. I also do that on the back end of the sprint so I’m not spinning wildly and producing zero watts. Then as my cadence and watts stabilize I shift back to my normal gear.


Thanks Guys

Looks like a little experimentation is in order, mix things up a little, and see what happens


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he also does a video that covers the minimum power achievable in erg mode. Essentially, in a harder gear the minimum is higher for the same cadence. This would mean you might be too high in the rests between intervals, which negatively effects the next interval.

i concur with the need to change gear even in erg mode. Personally I have oval “Q” chainrings set in different positions so I can change the muscle emphasis between interval and rest if needed (although this is no different than people who stand for intervals achieve. I just am trying to never stand)


I use a Neo, which because of the virtual flywheel (vs. fixed mass flywheel) I believe has much more realistic kinetic energy modeling* than many (most/all) other smart trainers, but that kinetic energy appears to be related to speed not power, and hence for any given wattage so-called “inertia” is lower in easier gears and higher in harder gears. You sublty feel this in the pedal stroke.

That’s all fine and as I would expect.

In Erg mode it allows you to select easier gears in an interval if you’re simulating (training for) hill climbing and harder gears if simulating (training for) flat efforts. I mix up both within a workout.

*I believe the fixed mass flywheels in common use are unable to achieve high enough levels of kinetic energy to come close to representing the kinetic energy of an adult cyclist moving at speed on the flat. See the series of historic posts by poster TarmacExpert on the forums who investigated this in depth. NB even bigger flywheels aren’t an answer, because they’ll then be unrealistic at low speeds. It’s why the Neo’s resistance unit is really in a different league to every other smart trainer with their legacy-technology eddy current brakes vs. the Neo’s much more sophisticated and flexible motor brake…


Hehe …well I tried the big ring for a ramp test

What did I learn

Well …nothing

  1. It was still hard (as expected)
  2. improvements were probably due to better fitness not the big ring

I guess I’ll try it out on longer SS or threshold intervals to see if “keeping on top of the gear” helps

Confusing stuff :slight_smile:

i think I read somewhere that the Neo has a variable virtual flywheel. I have a bushido that has a fixed virtual flywheel. I think in theory it reduces the size of the flywheel if it is told to simulate climbing.

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Yep - ideally trainer resistance would match the resistance profile of an actual rider + bike under all conditions.

The inertia of the flywheel by itself just emulates the inertia of the rider and the bike. And is “fixed” - presumably to match some average rider/bike weight.

That’s where you want the trainer to step in and emulate the other forces at play - aerodynamic drag, gravity (if riding uphill - or downhill) and rolling resistance.

And from what I’ve read, the Neo is the best trainer on the market with these capabilities.

However, where this “road feel” is most relevant is in free ride mode - riding around or racing in Zwift for example. It’s not really needed for interval training in TR. for example, when ramping up from 40% to 120% FTP for a VO2max interval, do you really care about the road feel of the trainer during that several second step/ramp?


Perhaps there is a little more “feeling difference” between wheel-on Snap and big brother direct-drive Kickr. But as a big guy with a lot of a) low inertia climbing (I’m slow steady climber), and b) fast flat riding experiences, I’m going to say just pick a gear that feels natural and don’t overthink it. I’m used to 3-5mph grinding up steep climbs and despite that the little chainring on trainer feels unnatural. A year ago I did SSB-1 high volume in big chainring and promptly went out and set a personal best on HC climb (18 miles at 5%).

The only real difference I feel between chainrings on direct-drive Kickr is that little chainring has a higher torque multiplier and therefore less pedal force required to ramp from rest interval to work interval. I’m used to putting down big force changes on pedals, and prefer the feeling of big chainring on Kickr. Again, it’s all feeling. My climbs are better as a result of doing longer sweet-spot and threshold intervals, not because of chainring selection on trainer.

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It is entirely possible that i am overthinking this

esp on a snap

Perhaps I’ll just ride and suck it up :slight_smile:

FTP is increasing, so lets not worry too much

Analytically it’s fun, but probably irrelevant

thanks guys for the input