Spin Bikes: What is Better Light or Heavy Flywheels?

We’ve had a few questions lately asking whether it is better to have heaver or lighter flywheels on exercise bikes. It all started when Groom+Style reviewed the top 5 spin bikes, and gave top spot to the Keiser M3 plus (M3i and M3iX range of bikes).

The Keiser is a bike that Groom+Style absolutely love, and there are a lot of reasons why we gave it that top spot.  The Keiser M3 range of bikes do however have one of the lightest flywheels of any spin bike at 8lbs.

The Keiser M3 Plus Indoor Cycle with its rear positioned 8lbs flywheel.  Does it generate enough inertia to provide a real bike feel?

This has caused a few readers to question how Groom+Style can rate this bike so highly when there is quite a bit of older information “out there” mentioning that heavier flywheels are better.  

The story goes that light flywheels do not create enough inertia which can cause some riders to develop an “up-down” or choppy pedal stroke – which in turn can lead to stress on joints and ultimately injury.

In this article Groom+Style will explain why this light flywheel myth is incorrect, at least when it is applied to a high end, well engineered spin bike like the Keiser.

What Is the Flywheel on a Spin Bike and What Does It Do?

First off, let’s get this out there in case you are a complete beginner, looking to get into spinning. The Flywheel is the big round disc that spins around underneath the bike. It can be found at the back or the front and is the most prominent feature on most spin bikes.

The flywheels job is to help to simulate the feel of an outdoor bike – it does this by storing energy which is then used to smooth out the cycling motion to promote a consistent cadence (the speed at which your legs spin as they pedal).  

Specifically, on a fixed gear indoor spin bike, the amount of force you can exert changes during the pedaling motion.  At the point when your pedals are horizontal you are exerting maximum force, but when the pedals are vertical (straight up and down) you are applying a lot less force and the wheels can start to slow.  By having some momentum built up in the flywheel, you have less slowing of the pedal stroke and a more consistent pedaling action.

In addition, if you think of the initial feeling of resistance you experience when you start to ride a bike outside – that is the feeling the flywheel on an indoor bike is trying to replicate.  

If there was no flywheel, or an ineffective flywheel on your spin bike, it would be very uncomfortable to ride, and could eventually lead to the rider developing an injury.

Flywheel Designs – The Different Options Available and What is the Best Design

For the flywheel to perform its function correctly it needs to store a certain amount of kinetic (moving) energy to accurately simulate the smooth pedal motion of a road bike.

A “good” flywheel will typically take some force to set it spinning (think of the force required to get an outdoor bike going); and will typically take some force to make it stop (think of how your outdoor bike will keep going for a while once you stop pedalling, unless you apply the breaks of course).

Kinetic energy is a function of mass and speed, flywheels can, therefore, come in all different shapes and sizes.  Interestingly, the laws of physics tell us that large diameter and heavy flywheels store more energy than smaller diameter and lighter flywheels. However, flywheels that spin faster store much more energy than ones that spin slower.  This is the key point that you need to remember when thinking about the Keiser flywheel.

Bike Designers Have Two Main Options When Designing Their Flywheels

Therefore, spin bike and flywheel designers can either use:

– heavier flywheels which spin at relatively slower speeds.  For example, this is the option chosen by Schwinn etc.

The 37lbs flywheel on the Schwinn is positioned at the front of the bike.

– a lighter flywheel which spins faster.  This is the option chose by Keiser.  The Keiser flywheel is the fastest rotating flywheel of any indoor cycle bike in the world, it rotates 11 times for every single rotation of the actual pedals.

The Keiser flywheel is therefore designed to have more than enough inertia in the flywheel to ensure a smooth ride, while not being too aggressive.

In fact, from our experience, the Keiser spin bikes are the most comfortable and forgiving bike on the market for the ‘non-cyclist’ whilst giving excellent muscle recruitment feedback due to the constant resistance throughout the stroke itself.  There is no stamping motion promoted because there is no ‘grabbing’ of the flywheel at any point during the pedaling rotation.

The Impact of Different Flywheel Weight Designs on the Pedal Stroke and Muscles Recruitment

Heavier flywheel weighted bikes can have a tendency to pull the legs through the full 360 degrees of the pedaling action putting unnecessary pressure on the knees and actually removing the need to recruit muscles i.e. you can get lazy during your spin class which defeats the purpose.

In contrast, the Keiser’s lighter flywheel design can help to promote hamstring and glute recruitment on the upstroke of the pedal stroke helping you to work harder.

Bad Pedalling Technique and Bike Setup

At the end of the day, it is possible to have “bad” pedaling technique with any spin bike, and whether you choose to buy a bike with a lighter or heavier flywheel it is probably more important for long-term health and injury prevention that you:

– ensure that you have setup the spin bike to fit you correctly.  There are plenty of tutorials online about how to do this.

– that you stay focused throughout your spinning workout and focus on a smooth fluid pedal stroke.  If you find your pedaling action getting out of control, back of a bit and refocus on your technique.

Flywheel Weight Design and Implications for the Rest of the Spin Bike

Interestingly, the weight at which you design the flywheel can also have design implications for the rest of the bike.

Heavy vs. Lighter Flywheel Weight Conclusions

Based on this information you can see that not only is the Keiser’s lighter, faster spinning, rear positioned flywheel optimally designed, but it will have more than enough inertia to ensure a smooth and comfortable spin bike experience.

Although they are important, flywheels, however, are not the only factor you need to consider when buying a spin bike – you still need to focus on correct bike fit and setup, and your pedaling technique.

As a high-end spin bike is a big investment, if you are unsure about whether a bike with a heavy or lighter flywheel is best for you Groom+Style recommend that you contact your local cycling representative (of the brand bike you are interested in) and organize for test ride.