Why flywheels suck – a quick lesson in the science of indoor cycling

It’s a widely-held belief that the size, weight, and shape of the flywheel on your smart bike or smart trainer has the most impact when it comes to creating a realistic road feel experience on smart bikes and trainers. But a quick high school science refresher will explain why achieving a realistic road feel with a flywheel is practically impossible. And, more importantly, why you should care.

Newton’s First Law

Even if you didn’t pay full attention in Physics class, you’ll likely remember this: an object in motion tends to stay in motion, unless a force acts on it.

You, your bike, and your wheels rolling along the road – while friction and air resistance try to slow you down – all create a specific pedaling experience. This is why it feels very different to suddenly accelerate up to 30 km/h from a static start compared to getting back up to 30 km/h after freewheeling for a few moments while zooming along a smooth road.

This quick and simple refresher in Newtonian Physics explains precisely why flywheels were first added to spin bikes and, later, trainers and smart bikes. They simulate that same inertia and, therefore, provide a more realistic “road feel”.

Bigger is better but not enough

As any rider who’s been unfortunate enough to take a fall from their bike will have felt all too painfully, we and our bikes actually store significant kinetic energy while we roll along on our Sunday coffee rides.

It makes sense then that we’d need to mimic the same amount of energy to create the most accurate road feel indoors too. How can that be achieved? Well, if you double the weight of a flywheel, assuming the speed remains consistent, you’ll double the amount of energy it stores.

That’s why smart bike and smart trainer manufacturers have added heavier and heavier flywheels in the hunt for a more fluid, more comfortable and more realistic indoor cycling experience. But it’s not quite that simple.

The problem is that you’d actually need a flywheel that is 10 to 20 times heavier than current standards to mimic true road feel accurately.

As a 500kg smart trainer isn’t exactly practical, manufacturers use lighter flywheels of around 6 to 20kg which speed up quickly and, therefore, riders can only put a little force into the flywheel with each pedal stroke.

Does the science matter?

Clearly, it’s impossible to accurately simulate the same outside riding forces on a flywheel-based indoor bike or trainer. But, as long as your smart bike or smart trainer is easy to use and connects to your favorite training app, should you really care?

The answer is yes. And for three reasons:

  1. The lighter flywheel means your muscles engage at different points in the pedal stroke compared to riding outside. A flywheel-based system doesn’t train the cycling muscles you’ll use when out riding with friends, so it’s less efficient.
  2. The constant force required throughout the entire pedal stroke to keep the flywheel moving also makes it harder to hit the same power numbers when training indoors. Again, this makes your indoor training less efficient.
  3. Finally, the less comfortable and realistic your indoor cycling experience, the harder and less enjoyable you’ll find it to log those winter kilometers in the pain cave.

Is there an alternative to flywheels?

Fortunately, there is an alternative to the standard flywheel system. Robotics overcome all of the problems of flywheels to provide the most realistic and effective indoor cycling experience. Learn more about the TrueBike and TrueTrainer,