# What’s the deal with flywheel inertia?

Does flywheel inertia really matter? I’ve read conflicting information on this forum and others that it does/doesn’t make a difference. GP Lama made a video saying the muscles will engage at different points in the pedal stroke depending on the relative speed of the flywheel. I, like all of us using TR want to maximize fitness gains… I’m so scared I’m missing out on potoential benefits I will alternate between the big ring and the small ring for my insterval sets so I can evenly distribute my time spent at “high” and “low” flywheel speed. Am I just wasting my time and wearing out two chainrings in the process? Or are there real benefits to be made?

First, you have to understand the initial reason flywheels were added to trainers. They are meant to improve “road feel”. While we are pedaling and rolling as we ride outside, the mass of our body and bike rolling along create a certain feel at the pedals. This is due to Newton’s 1st Law, objects in motion tend to stay in motion.

So, a flywheel is meant to mimic that feel of the inertia and the resulting feedback we get while pedaling. The ultimate goal being to make the pedaling feel like it does when we are pedaling on a flat road while outside.

The size, shape, mass and speed of a flywheel can change how it feels to pedal around the circle while riding a bike on a trainer.

In the old days, with simple wind, magnetic, and fluid trainers, the flywheel (and resulting road feel) was one of the major differences between brands and models. There are a couple of ways to do it, but generally, larger and faster spinning flywheels lead to a more road like feel. Of course, this varies as we have riders with different mass and traveling at different speeds in different conditions. And each one of those variables impacts what feels “right” to each person.

The trainer makers do some fancy math based on their target rider and conditions, and make a flywheel that aims to get close to that feel.

With the advent of ERG controlled trainers, we gain the ability to manipulate our flywheel speed and inertia with various gearing and “rear wheel” speeds. As such we can have an impact on our feel in training.

Without getting too much into the weeds, I think there is a difference in the uses of these different flywheel speeds/inertia. But I don’t think it will generally lead to massive differences (success or failure) if you are using the “wrong” setting.

We are probably talking about the marginal gains type of payback here. So I wouldn’t lose sleep over it. Try using high, medium, and low FW inertia. Use what feels more “right” for you and be happy with that. If in doubt, I’d pic something in the “middle” like small ring and middle to smaller cassette, or large ring and middle to larger cassette. Try them and see what you think.

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Ive done a bit of high inertia training, and low inertia training on my kickr. What I noticed immediatly was
when spending a lot of time doing lowish inertia training (50 / 28), the large fast group rides I really struggled putting out big power for a long time, and the constant surging nature of those group rides really drained me. On the climbs I was completely fine, and quite comfortable with that feeling of low inertia.

Now I swapped over to tri’s. Im training using higher inertia (53 / 14) as it replicates the conditions closer to a TT effort on a flat course.

I try to replicate the inertia for the course requirements, and dont be afraid to mix it up sometimes. Especially if you only exclusivley train indoors.

Thanks fellas!!! I really do appreciate the advice! I think I’ll keep it in the big ring for the majority of the time and bounce around the cassette to switch it up every now and again.

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Just understand that riding big ring little sprocket will be less responsive to wattage changes compared to little ring large sprocket due to intertia of the faster flywheel.

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Honestly it is possible you might get more results on the trainer by working on your aero position with a helmet.

Here is one indisputable fact - for the same change in resistance, the little chainring has a higher torque multiplier which allows faster acceleration for the same pedal force. Put another way, with little chainring you don’t need as much pedal force for the same acceleration.

That is all we know, full stop. A smart trainer in Erg mode is a complex system. On my Kickr, the flywheel is connected thru gearing to a resistance unit, and controlled by a small embedded computer. I think its better to think of it as a black box.

Haven’t seen any science on Erg mode chainring selection, everything I read is speculation. The little chainring feels ‘wrong’ to me, so I use the big chainring as it feels like all of the riding I do (both climbing and riding fast on flats).

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For what it’s worth I have found when using Power Match that using the smaller ring gives a better result. When using the big ring I found I was constantly chasing the power with massive increases in cadence. The TR support team suggested I use the smaller ring as the inertia of the Fly Wheel was too much. It helped a lot.

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