You wheel speed on the trainer doesn't correlate directly with your speed on the bike, so it isn't a useful independent measure of your progress, but it is an important part of your power calculation. Remember:
Speed x Force = Power.
- How do I get an accurate speed reading?
- My speed is different from my speed on the bike outdoors
- WHY is wheel speed different on the trainer vs outdoors?
- My speed is different from another speed and cadence sensor
- I don't see my speed in my training app
- Signs Fliiiight is not getting a good speed reading
- GPS based speed
How do I get an accurate speed reading?
To ensure you're getting an accurate speed reading,
- Install one (1) speed index, and slide it towards your wheel rim.
- Update your wheel rim circumference in the 4iiii app if your wheel is smaller than 700c (or 29" MTB).
That's it! Your wheel speed is Distance (# of wheel rotations x circumference of the brake rim) over Time. As long as your trainer has the correct rim circumference, and can detect the spoke index, you have the correct speed reading for the purpose of your power calculation.
Find out why this is different from your speed on the bike.
My speed is different from my speed on the bike outdoors
There are two things Speed can refer to when it comes to indoor training.
Simply, how fast your wheel is spinning.
This value matters as a part of the power calculation. There is not a direct relation between how fast your wheel spins on the trainer and how fast you'll go on outdoors at the same effort.
Keep in mind that if your Speed reading is wrong, your Power reading must also be wrong. If your Power readings seem reasonable, read the next section to understand why a "weird" speed reading on its own isn't necessarily a concern.
If you use a simulation such as Zwift, the app will calculate Virtual Speed and Virtual Distance, using data including your power output, your weight, the grade of the virtual terrain, and drafting simulations. It should approximately match what you'd achieve outdoors for a similar course and effort.
This is the value you'll use to measure your progress on the bike if you want to know "How far?" and "How fast?"
ERG mode doesn't allow for a virtual speed to be calculated because there's no virtual terrain. Instead, you'll focus on power.
Why is wheel speed different on the trainer vs outdoors?
Power is is king when it comes to interval workouts in ERG mode. In this mode, you could spin your wheel at 70km/h or 15km/h, but it won't change the difficulty of the workout. Whatever your wheel speed and gearing, the trainer will adjust the resistance make sure you hit your target Power in Watts.
Reminder: Spinning your wheel faster in ERG mode improves the road feel. Shift to your top gear!
In a simulation, you use your gears like you do outdoors, while the trainer creates the feeling of hills by increasing and decreasing your resistance.
This means your wheel speed more closely approximates your wheel speed outdoors, but there are still situations where they diverge significantly. For example, if you stopped pedalling on a downhill outdoors your wheel would continue to spin rapidly, but indoors, your wheel will coast down.
I'd like to try interval ERG workouts, but I'm accustomed to measuring my rides in speed and distance! How can I find out how "far" I've come?
Riders who aren't used to tracking power sometimes want a more familiar benchmark to put their ERG workouts in perspective. Zwift's "Training" mode is one solution. In this mode Fliiiight will create the resistance for the ERG workout you've selected, while in the simulation you move through the virtual course based on your power, just like when you use a simulation app with a powermeter. Be prepared for the complete disconnect between the resistance provided by the trainer and the course on your screen!
Note that virtual speed and distance aren't measurable without a virtual course.
My speed is different from another speed and cadence sensor
A common reason for small discrepancies in average speed between Fliiiight and other speed sensors is Fliiiight measures wheel speed at the wheel rim, whereas most Speed/Cadence sensors measure it at the tire.
Fliiiight also samples speed far more frequently than a standard sensor which only updates once per wheel revolution, leading to small discrepancies in instantaneous speed reading between the two sensors.
Why the different approach?
Measuring speed at the wheel rim
Speed and cadence sensors typically measure speed at the tire because they are meant to measure your speed outdoors.
Fliiiight trainers measure speed at the rim because it's the rim passing through the magnetic field that creates the resistance, so this is most relevant for the power calculation.
It's also simpler to learn your wheel rim circumference than calculate how your rim, tire and tire pressure affect the outside circumference of your wheel. Make sure to update this value in the 4iiii app if you have a wheel size other than 700c / 29".
Sampling speed more frequently
In order to create virtual inertia, Fliiiight needs to sample speed far more frequently than once per wheel revolution- it needs to detect and predict speed variation within each wheel rotation. To achieve this, Fliiiight detects every spoke, using your speed index to determine how many spokes are in a wheel rotation.
For the sake of accuracy, we don't recommend pairing a separate speed and cadence sensor as the source of your speed reading in apps.
I'm don't see my speed in my training app
- Some ERG mode apps choose not to display speed by default, due to its lack of relevance in indoor training.
- Apps designed to track outdoor activities may use GPS-based speed. In this case, since you are stationary they should report the speed as zero. Extremely low GPS speeds may also be reported due to minor errors from the GPS. The 4iiii app only displays speed based on GPS.
Continue troubleshooting only if your speed previously populate in the same app, or if the app stops reporting speed and power. See I've stopped getting updated power, cadence, and/or speed readings in my app.
Signs Fliiiight is not getting a good speed reading
Fliiiight's virtual inertia system requires a very high-resolution speed reading; It updates with every spoke, not just once per wheel revolution, in order to control wheel speed variations within each pedal stroke. If the sensor isn't getting a clear read, the first sign is virtual inertia disengaging.
- Virtual Inertia disengages. You may feel like you are riding through sand, or like your pedals are slipping once per wheel revolution or once every several wheel revolutions.
- The trainer does the initial wheel touch when it turns on but then doesn't engage the magnets to create resistance.
- The 4iiii app indicates it is having difficulty detecting the wheel.
See also: Troubleshoot Power readings
GPS based speed
Apps designed to track outdoor activities may use GPS-based speed. In this case, since you are stationary they should report the speed as zero. Extremely low GPS speeds may also be reported due to minor errors in GPS tracking. The 4iiii iOS app only displays speed based on GPS.