How smart trainers measure power (principles & accuracy)
Smart trainers are an important piece of equipment for anyone that wants to improve their cycling performance. They propose interesting ways of training with a wide range of metrics. Like power meters, smart trainers also measure power. But since it’s not their sole purpose, many people wonder if smart trainers are less accurate than dedicated power meters. So what’s the truth?
Do smart trainers measure watts?
Smart trainers do measure watts, a metric of power. They do it differently than dedicated power meters. Their accuracy is nonetheless very good on the latest models. Smart trainers will measure the power along with the cadence (RPM) and speed (MPH) which makes them very useful to train.
Having the three metrics is very interesting while training. Indeed, watts do not mean everything. Adding context like speed, cadence, and slope is a great way to understand where you struggle and train accordingly. Pair it with a heart monitor and a smart trainer will give you a wide range of metrics to improve your performance.
Do smart trainer have power meters?
Smart trainers do not have power meters. They do measure power but in a different way than dedicated power meters (not with strain gauges and a cadence sensor). The power measured by smart trainers is however as accurate as power meters. Thus there is no need to invest in an additional power meter.
How smart trainers measure power then?
Smart trainers measure the rotational speed of the rear wheel with a dedicated sensor. Smart trainers by nature can introduce resistance in the wheel which is essentially a resistive torque. With these two metrics, the smart trainer can deduce the power since power = torque x angular velocity.
Contrary to power meters, where both the torque and angular velocity are measured, only the angular velocity is measured on power trainers. The torque is not measured directly because it is already known. Indeed, the manufacturer knows which torque corresponds to which level of resistance. There is a law that is specific for each trainer but known by the manufacturer since they need to make the resistance match the slopes of your training app.
In the past, some smart trainers used to have strain gauges to measure torque as power meters do. But the location of these gauges made the trainer unreliable. Indeed, they had to put the strain gauges far from the front part of the drivetrain (like on the crank or pedals).
Considering that you can have a gear ratio of 5:1, it means you need to put strain gauges 5 times more sensitive. On the Wahoo Kickr it’s worse than that since you have an additional reduction due to the belt with the flywheel. The ratio between the crank and the flywheel is around 40:1, meaning you would need strain gauges 40 times more sensitive!
In the end, these strain gauges happened to be easily damaged during transportation while being hard to calibrate. That’s why they were ditched at some points. Below is a video where the wahoo CEO explains why they abandoned power meters on their trainers and how it improved accuracy contrary to beliefs.
Smart trainers are basically electric motors generating motion, it’s not much different than electrical bikes. The only difference is that instead of always helping by adding power, a smart trainer can also generate resistive power. In other words, it can subtract power from the one you generate.
Basically, the manufacturer did some testing and calibration to know what electrical instruction combined with sensors measurement corresponds to a power value. It won’t be surprising that they measured the power with a power meter on cranks or pedals and did their calibration thanks to that. After that, it is easy for them to say: “this value of electric current + this value of angular velocity + whatever sensor = this amount of power”.
How accurate are smart trainers’ power?
The power accuracy of smart trainers ranges between 1% and 5%. Trainers above $1000, such as the Wahoo Kickr or the Tacx Neo, have an accuracy of 1%. The power of these trainers is as accurate as power meters. However cheap ones, such as the Tacx Flow Smart at $370, have an accuracy of only 5%.
Do all smart trainers measure power?
All smart trainers measure power. Power is the fundamental operation of smart trainers. It needs to add a resistive or additive power according to the slopes it simulates.
Please be aware that some brands claim to be ‘smart’ despite needing to buy an external sensor. Don’t be fooled, these aren’t smart trainers and won’t measure power.
Should I use a smart trainer or power meter?
Smart trainers are preferably used for training but cannot be used outdoor. On the other hand, power meters can be used outside but are more limited when it comes to training possibilities.
Choosing between the two is not an easy answer and depends on what you are looking for. To help you out I have written a thorough article: Power meter or smart trainer, which one to buy first?
Do you need a power meter with Zwift?
A power meter is not required for using Zwift. Indeed, a speed sensor is enough for Zwift to make an approximation of the power. To use Zwift without a power meter, your trainer needs to be on their list of supported trainers. The calculated power is however much less accurate than a power meter.
To make this estimate Zwift did some in-house testing to correlate speed measurements with power. Therefore, for all the wheel-on trainers they have tested, they can estimate the power by the speed measured with a speed sensor. This is what Zwift calls “zPower”.
If you have a smart trainer, you can use Zwift directly without buying a power meter. Indeed, smart trainers have power meters embedded.