Smart Bikes, Zwift and Bike Computers are all WRONG

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My SB20 smart bike comes with an app that shows power, distance, cadence, L:R balance and current virtual gear. You can also use a bike computer like a Garmin 540/840/1040 to show similar data. When you use either at the same time as Zwift, you see different numbers. Who is correct?


Answer: They are ALL CORRECT and ALL WRONG!  How can that be?  

Let's find out.


Before getting started, it's a good idea to agree on basic parameters.

Note: In this discussion, I use the term Indoor bike for smart bikes and smart trainers along with their related apps.

RESISTANCE AND POWER

1. RESISTANCE - An indoor bike creates resistance that makes it harder or easier to pedal. 

2. POWER - The harder you push against the indoor bike's resistance (measured in newton metres) multiplied by how fast you pedal (RPM) defines the POWER you are producing (specified in watts per second or just watts). POWER is a measured number and is EXACTLY the same if you are riding with Zwift, using your indoor bike's app or just monitoring your indoor workouts (like I always do) with a Garmin computer.

SPEED AND DISTANCE

Speed and distance are calculated using a mathematical model (a formula) that simulates rolling resistance, weight, riding surface, hill gradient and wind resistance. Zwift, Rouvy, FulGaz, MyWhoosh and others all calculate a different speed and distance for the exact same POWER being produced by you.

WHY DIFFERENT NUMBERS?

Zwift (and other apps) simulate their own virtual world based on a proprietary model. The indoor bike does the same. Its model is much simpler, so let's start with that.

INDOOR BIKE

An indoor bike simulates a world with constant numbers for rolling resistance, riding surface, hill gradient (flat) and wind resistance (fixed) plus the weight that you input into the related app. As you produce more or less POWER, while also changing virtual gears (i.e., further modifying resistance), the indoor bike uses its formula to decide how fast you are going. 

Aside: The indoor bike calculates speed and distance. These numbers are included with the Bluetooth and Ant+ data streams that the indoor bike transmits once a second while you are riding (even in Zwift - more on this later).

BIKE COMPUTER

A bike computer records workout data. When indoors, this data is provided by your indoor bike. It reflects the  power, cadence, speed and distance calculated by the indoor bike.

ZWIFT 

Zwift simulates its own world with constantly changing values for rolling resistance, riding surface, hill gradient (% incline) and wind resistance (including rider and pack dynamics), plus the weight that you input into the Zwift app. Most of these parameters are reflected in the Zwift avatar that visualizes the simulated Zwift world. The result is always the same: Zwift does a calculation and then sends a command to your indoor bike telling it to increase or decrease resistance to simulate conditions in its world. You respond by increasing or decreasing your effort by pushing harder/softer and/or pedalling faster/slower. Zwift uses the resulting POWER number (sent once a second from the indoor bike to Zwift) to calculate how fast you are going in its simulation. 

Note: Zwift ignores speed and distance data calculated by the indoor bike and always uses its own calculated values.

WHY ARE INDOOR BIKES, BIKE COMPUTERS AND ZWIFT DIFFERENT?

Zwift knows it is simulating something like a hill - let's say a 12% incline - when you are producing something like 300W. Its formula says: you are traveling at 10 km/h at this moment in the Zwift world. 

At the exact same time, the indoor bike and bike computer knows nothing about the Zwift simulation. All they see: You are producing 300W. The indoor bike and bike computer assumes you are riding on a flat road with other parameters constant. This will create speed more like 40 km/h. 

All calculated speeds (and computed distances) are equally correct. They are just reflecting different simulated terrains.

Aside: Zwift, indoor bikes and bike computers could share more data about their simulated terrains. The BT FTMS and Ant+ specs don't mandate that this information be shared, so it isn't. 

WHY YOU SHOULDN'T CARE

Modelled speed and distance numbers don't tell you much. For a given power output Zwift might say I am travelling 30 km/h, Stages will say I am going 26 km/m. Meanwhile on my road bike, I will be travelling 32 km/h. 

Do yourself a favour: Stop caring about speed and distance. Focus on WATTS and DURATION for training, comparison and planning. Bike computers, plus apps like Strava and Intervals.icu, calculate Training Stress Score (TSS). Garmin describes it as: TSS measures how much stress is put on the body from a ride. It is calculated using hard efforts (Normalized Power or NP), Intensity Factor (IF), and ride duration. 



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