Smart trainers come with a wide array of specifications and capabilities, but one key feature used to compare trainers resistance (also known as brake power). This is the number of watts the trainer can use to push against your legs, which is most noticeable when climbing and sprinting.
This power output is directly linked to the maximum gradient the trainer can simulate because as the hill gets steeper, the trainer needs to add more resistance to simulate the grade. The Kickr is advertised with a maximum simulated grade of 20%, while the Kickr Snap only goes up to 12%.
But here’s where things get interesting: given a constant gradient, resistance will change based on rider weight and speed (which is a function of power). This begs the question: where does that “maximum simulated grade” figure come from?
Obviously trainer companies are using different figures to compute their maximum simulated grade, since Tacx advertises the Neo 2 (which has the same maximum wattage as the Wahoo Kickr) as having a 25% maximum simulated grade. Remember, the Kickr is advertised as simulating only 20%!
In the end, we find that in the rarefied air of high-end smart trainers, the maximum resistance offered is always more than we’ll need. In fact, it’s safe to say many Zwifters are riding smart trainers which are quite overpowered for their needs.
There are four key factors which determine how much resistance you actually need in a smart trainer. Here they are…
#1: Your Weight
The more you weigh, the more resistance your smart trainer will apply on inclines. (Controllable trainers using Bluetooth or ANT+ FE-C on Zwift will apply more resistance the heavier you are–Zwift actually added this feature to ANT+ FE-C trainer control around one year ago, and it already existed in Bluetooth before that.)
And this makes sense. Imagine yourself climbing a mountain, then throwing on a 20kg backpack and continuing to ride. It’s harder now, right? That’s the increased resistance a trainer must apply to simulate rider weight.
At low speeds, the amount of resistance needed to simulate a grade is basically proportionate to body weight. So if you double your weight, you’ll double the resistance wattage. This makes sense if you think about it, because at low speeds the main force you’re overcoming is gravity. As your speed increases, air resistance increases, and that force becomes a larger factor.
#2: Your Sustained Speed (Power)
The harder you push, the faster you’ll go. And the harder you push, the more your trainer has to push back on you to accurately simulate a gradient. This is really a fitness factor, of course, since speed in Zwift is based on your power output. How high a wattage can you sustain for longer periods?
Like weight, at low speeds, the amount of resistance needed to simulate a grade is basically proportionate to speed. So if you double your speed, you’ll double the resistance. As your speed increases, air resistance increases, and that force becomes a larger factor.
#3: Gradient (and Trainer Difficulty)
Of course, the steepness of a virtual incline affects the level of trainer resistance. But we also need to remember that by default, Zwift sets Trainer Difficulty to 50%, meaning a 8% incline will only “feel” like 4%.
If you want to run trainer difficulty at 100%, you’ll need a trainer that offers more resistance than if you were running trainer difficulty at the 50% default.
About Brake Force and Brake Power
The maximum power output is also known as the “brake power” of a trainer. Here’s some insightful info from Tacx which gives you a look into how smart trainers work, and how these maximum gradient/resistance numbers are calculated:
The brake power of the trainer is the result of the brake force and mass inertia of the trainer and dependent on your speed. For example, a NEO Smart is capable of generating 2200W at 40km/h, while a Vortex can power up to 950W at this speed. A trainer with high brake power can generate high resistance at low speeds, so it can realistically simulate steep inclines.
The higher the brake force, the steeper the incline can be simulated at a certain weight. For example, the NEO has a brake force of 250N and can therefore simulate a slope realistically up to 25% at a weight of 75kg. The trainer’s brake force, expressed in Newtons (N), is defined by the structural architecture that causes the resistance, like the kind and amount of magnets. Which brake force is required for an accurate simulation is defined by the weight, slope, air resistance and rolling resistance. All these factors are taken into account in the simulation.
The mass inertia represents the needed energy to set the flywheel in motion or accelerate it. In contrast to an actual flywheel, a virtual flywheel can change the mass inertia precisely to the conditions; like speed, slope and weight. Therefore, a virtual flywheel is adjusted to accurately simulate the rider’s weight. This results in the most realistic simulation.
Here’s a summary table from calculations found on The Computational Cyclist’s page. While these may not perfectly match what you’ll find in Zwift, basic cycling physics are well known and these numbers compare quite closely to our Zwift tests.
|Gradient %||Body Weight (kg)||Speed (km/hr)||Resistance Needed (watts)|
#4: Your Sprint Power
What kind of wattage can you put out in a short (10 second) sprint?
In the end, this is what really matters. As you can see from the table above, the resistance needed on sustained climbs is actually quite low–so low in fact that every smart trainer on the market can accommodate any rider using the default 50% trainer difficulty, even on Zwift’s steepest climbs.
The peak resistance your trainer will need to offer is whatever you can put into a sprint. Strong sprinters will want trainers that offer 1500 watts or more, while weaker sprinters will probably never max out a trainer which tops out at 1000 watts.
Questions or Comments?
It’s a confusing topic, for sure! Post your questions or comments below.