Inertia – What is it?

You might already be familiar with the physics concept of Newton’s 1st Law: objects in motion will stay in motion unless acted upon by an outside force, and objects at rest will stay at rest unless acted upon by an outside force. Simply stated, unless an outside force comes into play, things will continue to do what they’re already doing. The property that dictates how much force is required to change their behavior is called inertia.

To expand upon this concept, objects can have low or high inertia, relatively speaking. When an object has high inertia, a larger external force is required to alter its behavior. Conversely, a low inertia object requires a smaller outside force for change to take place. High and low are generally relative terms, meaning it’s easiest to compare to objects to determine if one has higher or lower inertia than another. For instance, a 10lb kettlebell moving at the same speed as a 50lb kettlebell will have less inertia. 

From left to right represents increasing inertia at a given speed.

And yes, there is plenty of latitude to make this concept much more involved, but for the sake of understanding indoor bike trainers, let’s keep it simple.

What does this have to do with trainers?

In the case of stationary and indoor bike trainers (and in the fashion of keeping things simple), inertia plays a role in how much indoor cycling feels like outdoor cycling. More specifically, the inertia of the rotational component of the bike trainer is what creates this sensation. The term “flywheel” is used when it comes to indoor cycling trainers – regardless if talking about smart trainers, mag trainers, fluid trainers, spin bikes (think Peloton bikes) or even cycling rollers.

An example of an external flywheel meant to add inertia

Many companies will tout “high-inertia flywheels”. What this means is that they’ve attached a heavy (typically steel) mass to the outside of the trainer to increase the overall inertia of the trainer system. But why? 

When you’re riding outdoors and you stop pedaling, your bike doesn’t instantly stop because the inertia of your “system” (you + your bike) maintains your forward motion until external forces slow you down (wind resistance, gravity, rolling resistance and system friction). 

Thus, product designers add heavy flywheels to increase the overall inertia of their system, so their bike trainer sustains momentum as it would outside. 

Interestingly, in the world of smart trainers, this can be accomplished virtually (electronically). In a way, inertia can be programmed into the trainer app to have a feel very much like that of a heavy external flywheel.

Indoor bike trainers can feel more, or less, like riding outdoors when compared with one another. Much like kettlebells.

But one thing to consider is, do you want your trainer to be portable? Do you want to have a trainer that you can set up everyday with little effort? Do you need to travel with your bike trainer? Do you want to take a trainer to races for warm-up and cool-down efforts? Do you have a permanent training room (aka “pain cave”)? 

Can differences in inertia affect my outdoor cycling?

Well, yes and no. In my previous life as a biomechanist with a focus on cycling, I kept up with the science of topics just like this. And as a scientist, I can say that there’s simply not enough financial backing to promote extensive research. 

There are approximately a dozen unique papers (dating back to the 1970’s) that investigate the physiological impacts of low and high inertia bike trainers, or more commonly the scientific, standardized research stationary bikes. None of the papers culminate in a conclusive outcome due to the lack of consistency is research design. So I can’t speak to whether or not your cardiovascular or muscular systems will have a predictable response. Or said differently, there’s no concrete evidence that suggests one is more “efficient” – which in the case of a cyclist means more speed for a given effort.

Research on efficiency is measured via gas exchange – VO2max

However, although the physiological science (efficiency) is not conclusive, biomechanically speaking, there is a more consistent agreement on a topic called “pedaling effectiveness”. 

Simple, yet accurate depiction of force generation during a pedal stroke – courtesy Patria Hume

This has to do with how well a cyclist can apply a perpendicular force against the pedal at any given point in the pedal stroke. Generally speaking, when a rider applies a perpendicular force against the crank arm (EF), their work is applied to the crank and thus the drivetrain system – this is a good thing. As force applied moves away from perpendicular to the crank (RF), the less effective it is in generating meaningful drivetrain force.

High inertia flywheels allow people to maximize their strengths as a rider, which oftentimes has negative consequences on pedaling effectiveness. In short, the more inertia from a flywheel, the more a rider can cheat their way through the pedal stroke to generate power. 

Meanwhile, low inertia systems, which will on their own come to a rolling stop much faster, require the rider to be constantly generating effective force throughout the pedal stroke. Low inertia trainers amplify the weak spots in the pedal stroke. 

So which bike trainer style is best?

Here’s where opinions come in to play! As a former owner of a cycling and triathlon coaching business, I can say that coaches have a lot of influence on rider’s perspectives of this topic. Unfortunately, they’re generally related to the coaches preference, and occasionally lose focus on what makes a particular individual a better rider.

So to answer the question of which is best, you have to ask yourself, what would benefit you most as a rider – a high inertia bike trainer, a low inertia bike trainer, or maybe you’re fortunate enough to own both? Do you as a rider need to work on your pedaling effectiveness? Or is your pedal stroke already very effective?

Here at Feedback Sports, we ride the Omnium Portable Trainer for two reasons: to stay fit when the weather prevents us from riding outdoors, and to tackle very specific interval workouts. 

While not everybody will share our opinion, the low inertia of the Omnium exploits weaknesses in our pedaling effectiveness and provides immediate input that we can make improvements. This is a training tool, just like hypoxic training or interval training – it’s a marginal gains strategy. 

The fun part is that after a winter of indoor bike training on the Omnium, we feel very effective once we hit the pavement – the pedal stroke is just a little bit smoother. And while it may be psychological, we feel more efficient too – which means we feel like we’re getting more speed from a given amount of work. Personally, I’m OK with placebo when it comes to spring riding sensations! Anything that makes me feel more fit, I’ll take!

Whether or not you should purchase a high inertia or low inertia bike trainer is up to your specific needs as a rider. 

We want our readers to understand that they each have an upside, and that there’s biomechanical evidence that suggests low inertia bike trainers can make a positive impact on your outdoor cycling performance. Is it absolutely conclusive? No. But it is generally more consistent than most cycling-related research.  

Omnium Portable Trainer – a Low Inertia Bike Trainer Option

Obviously, the low inertia aspect of the Omnium Portable Trainer is a result that’s literally in the name – “portable”. We designed it to be portable. It just happens to have the added benefit of presenting riders the opportunity to improve their pedaling. 

That said, do you have a need to set up and take down your trainer daily? Will you ever want to travel with your trainer? Do you have space constraints for your indoor cycling? Do you want to ride outdoors, possibly in inclement weather – the Omnium is quite happy in rain and mud? Do you need your trainer for warm-up and cool-down at bike races? 

If you answer yes to any of those, the Omnium Portable Trainer might be ideal!

The Omnium Portable Trainer is good indoor or outdoor

Thanks for reading. 

Feedback Sports’ Marketing Director, Thomas McDaniel, has a M.S. in Clinical Biomechanics from the University of Georgia and a B.S. in Exercise Science from the University of Missouri. His experience as a researcher spanned 5 years and included physiological and biomechanical aspects of cycling. In a previous career, he taught bike fit courses at a leading educational program for retailers and physical therapy clinics around the world.