How Does an Indoor Bike Trainer Work? A Beginner’s Guide

What is an indoor bike trainer?

An indoor bike trainer is any device that lets you ride your own bike in a stationary position. Most indoor trainers keep the bike upright and add resistance, but this isn’t essential, as we’ll see when we discuss bike rollers in the next section.

Using your own bike is a key distinction between indoor bike trainers and the machines you find in a gym or spin studio. Cyclists put a lot of effort into adapting their bikes, changing components and getting a professional bike fitting in order to optimise comfort and power output, so using your own bike for indoor training is preferable to using a stationary bike.

It’s worth mentioning that while most people will use their indoor bike trainer in their home or garage, they can also be used outside, for example, in a garden, as long as you are happy to move the equipment and bike around, and most trainers require access to a plug socket.

The main types of indoor trainers: wheel-on, direct-drive, rollers and smart bikes

Indoor trainers can be broken into four broad categories: wheel-on, direct-drive, rollers and smart bikes (like the Zwift Ride).

Wheel-on trainers

Between direct-drive and wheel-on trainers, the latter is simpler and cheaper. These devices allow you to ride without removing the bike’s rear wheel. The device clamps the bike at the rear axle to keep it upright, and the rear tyre rests against a roller that provides resistance.

In general, the front wheel sits on a small block to raise it to the same level as or slightly higher than the rear, and to keep it from moving. With your bike’s rear tyre moving freely against the roller, there is some tyre wear, as well as more noise than direct drives. Power measurement is also less accurate, with typical wheel-on trainers listing accuracy at within 3-5%.

Direct-drive trainers

Direct-drive trainers attach to your bike in place of the rear wheel. By removing your rear wheel, you also remove the cassette (the set of cogs in the rear), so your direct-drive trainer must have a compatible cassette installed. Alternatively, you can purchase Zwift Cog & Click shifter. These act as a virtual cassette and shifter and are compatible with most 8-13-speed bikes.

With this type of device, your pedals drive it directly, eliminating tyre wear, reducing noise and allowing for more smart features. They also deliver a more realistic ride feel and the most accurate power tracking with a 1-2% margin of error, significantly better than wheel-on trainers. That said, these devices are heavier and the most expensive.

Rollers

Rollers are the simplest indoor trainer. Most of these trainers don’t help your bike stay upright; that’s up to you. One exception is the Wahoo KICKR ROLLR with its Safety Tyre Gripper that stabilises the front wheel.

On traditional rollers, the front wheel rests over one roller, and the rear wheel sits between two rollers. Like riding outdoors, you must start pedalling to catch and maintain your balance. This design helps refine your technique, as it requires smooth pedalling and stability to maintain your balance. Some modern rollers offer adjustable resistance, but most don’t, so they’re better for steady, low-to-moderate-effort rides. This design is less versatile, but it’s cheap, lightweight and doesn’t require an electrical connection.

Smart bikes and smart frames

A newer option skips the wheel-vs-frame question altogether. Smart bikes are all-in-one units with the trainer built in, while smart frames pair a bike-shaped frame with a direct-drive trainer. The Zwift Ride is a popular example of the latter: you clip it onto a direct-drive trainer (it’s also sold bundled with the Wahoo KICKR CORE) and ride without your own bike at all.

The appeal is convenience and integration. There’s no bike to mount or remove, the fit is adjustable to suit different riders and the handlebars have built-in shifting and steering buttons that work with virtual controls. That makes setups like this a tidy fit for a dedicated pain cave or a shared household trainer. The trade-off is that you lose the feel of your own bike and the fit you’ve dialled in over the years, and you’ll still want a compatible direct-drive trainer to bring the resistance and power data.

How resistance is generated: magnetic, fluid, air and electromagnetic

One important function of an indoor trainer is to provide resistance. Without it, you’d just spin away without any effort, and therefore without any training benefit.

Trainers rely on resistance generated in one of four ways: magnetic, fluid, electromagnetic and air, and some are more effective than others. Air resistance trainers are uncommon.

Magnetic resistance, using magnets and a metal flywheel, allows you to adjust resistance manually, as you would with a spin bike in the gym. This type of resistance is called linear resistance.

Fluid and air aren’t manually adjustable, but they provide progressive resistance that increases the harder you pedal, which more closely matches the feel of outdoor riding than the linear resistance created by magnets.

Electromagnetic trainers use electronically controlled magnets, allowing instant, precise control of resistance for a more realistic ride feel and a quieter ride.

Basic rollers rely on gear changes and the rollers themselves to provide some resistance. However, smart roller systems are available, and these offer adjustable resistance.

The role of the flywheel

Flywheels are used in all magnetic and some electromagnetic trainers. A flywheel is essentially a large disc that turns as you pedal and stores momentum to keep spinning when you stop pedalling, just like the momentum that keeps your wheels spinning and the bike moving forward when you briefly stop pedalling on a flat road.

Heavier flywheels perform this job better, as they store more energy when they get up to speed, meaning they will continue spinning for longer when you stop pedalling. This gives your indoor trainer a more natural ride feel compared to one with a light flywheel.

“Dumb” trainers vs smart trainers – what’s the difference?

A smart indoor trainer is one that can be controlled through a cycling app such as ROUVY. The standout feature of these devices is that they automatically adjust the resistance for you, allowing them to simulate the changes in gradient on climbs and descents, for example, if you’re riding Alpe d’Huez on the ROUVY app. This technology provides a more immersive and engaging experience, which has helped drive the growth of indoor training.

Smart trainers can also simulate drafting in virtual group rides or, in ERG mode, automatically adjust resistance to keep you at a desired power output (wattage) regardless of the gear you’re in or how fast you’re pedalling. This makes it much easier to complete structured workouts on your indoor trainer than outdoors.

“Dumb” trainers are those with manually adjustable (magnetic) or fixed progressive resistance (fluid and air). You can still use cycling apps with these indoor trainers, but you must adjust the resistance yourself using a knob or by pedalling harder.

How a smart trainer simulates climbs and descents

The routes found on indoor training apps, like ROUVY, have gradient data for the whole section of road. So, when you ride a route on your smart trainer, the trainer will automatically adjust the resistance to match the gradient you’re seeing on the route video.

The capability of your smart trainer can be a limiting factor here. For example, more expensive trainers can simulate higher gradients.

How smart trainers connect to apps like ROUVY

Connecting your smart trainer to an indoor cycling app is usually pretty straightforward. Much like a bicycle computer or heart rate strap, smart trainers use Bluetooth or ANT+ to communicate with your computer, tablet or smartphone.

Bluetooth is commonly found on all modern tablets, phones and laptops, but usually ANT+ requires a special dongle. You also need Wi-Fi to use most indoor training apps.

With both your device and trainer turned on, and Bluetooth activated, you open the app on your device and select the trainer to pair it.

What you need to start riding indoors

Getting started with indoor training is pretty straightforward; all you need is your bike, an indoor trainer, a subscription to an indoor training app, a device (phone, tablet or computer) with the corresponding app on it and ideally a powerful fan.

Road or gravel bike – Indoor trainers usually have tyre width limits, ruling out mountain bikes. Smart bikes and frames like the Zwift Ride are the exception, since they replace your bike entirely.

Indoor trainer – Any indoor trainer will work, but you need a smart trainer to use features like climb simulation and drafting. For direct-drive trainers, you may need to buy a compatible cassette or Zwift Cog and Zwift Click.

Training app subscription – There are multiple options with varied pricing and features.

Device – These apps can be downloaded on computers, laptops and Android or iOS devices. Connect the device to your TV to enjoy a more immersive experience of the real-world routes on ROUVY.

Fan – A powerful fan pointed toward your head will help keep you cool and prevent a pool of sweat from collecting below you.

Mat – Even with a fan, a lot of sweat will accumulate beneath the bike, so a washable mat is a nice extra. Some indoor trainer brands sell mats specifically for this.

Choosing the right trainer for you

The most important considerations when choosing a turbo trainer are budget, frequency of use, space, whether or not it needs to be stored between uses and how seriously you will take indoor training.

For example, if you want an indoor trainer for unstructured riding to maintain some aerobic fitness when it’s not possible to ride outdoors, a set of rollers is probably sufficient. If you want your trainer to provide accurate power readings for multiple structured training sessions per week during the winter months, you’ll probably want a mid-to-high-end smart trainer.

Entry-level

If you’re not willing to commit to indoor training, have a small budget, or plan to use it infrequently, a “dumb” wheel-on trainer or set of rollers would be a good starting point. For around €150-300, you can find many different rollers, a few wheel-on trainers from recognised brands and even cheaper direct-drive trainers, such as Decathlon’s Van Rysel D100. Many of these offer the majority of the functionality of the more expensive devices.

Mid-level

In the mid-level price range of around €300-600, almost all options are smart trainers, and you can find a range of more advanced wheel-on trainers, plenty of direct-drive options and Wahoo’s KICKR ROLLR smart rollers. These trainers are suitable for the majority of riders. All you will get by spending more is ‘nice-to-have’ extras. Devices in this price range should include power measurement and real-world ride simulation features.

Premium

In the premium range above €600, you get the top direct-drive smart trainers. These products can cost up to €1,700 (Tacx NEO 3M), and they offer impressive (and unnecessary) functionality such as mimicking road surface, simulating extremely steep gradients and movement while riding. Power measurement will also be the most accurate. These machines are also typically quieter.

Read in more detail about how to choose the perfect indoor trainer.

Once you’ve chosen the appropriate trainer for your needs, you’ll need to pair it with an indoor training app to help you stay motivated and reach your cycling goals.