Stages SB20 - How to Pick a Zwift Power Source

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Real Power Meters

The Stages SB20 Smart Bike is unique. It uses two crank-based power meters to send power measurements to apps like Zwift. Most other bikes and trainers guess at power based on how much electronic braking is being applied at any given moment (this method is good when a bike/trainer leaves the factory - not so good as components wear).

How to Connect an SB20

Zwift can connect to the Left Crank or the Stages Bike (SB20) itself as a Power Source:





Which should you pick?








Zwift Connections

Here's how crank power meter data (power and cadence) is sent to Zwift, the Stages Cycling app and other devices like a Garmin computer:


Bluetooth, ANT+ and WiFi connections are all used. 

Here are some additional details:

1. The right crank gets its own data (raw force and cadence information) and sends an update to the left pedal once a second with power and cadence metrics.

2. The left crank collects the right crank data, adds to its own data and sends consolidated information with L:R power and cadence metrics to the Stages Bike (or any other connected app) once a second.  

3. FWIW: The Stage Cycling app connects to the SB20 to get power data and relay commands like zero reset.


When using Zwift, you have two Power Source options. Here's how they differ:

4a. Connect to the left crank to get the same data sent above in #2. That's power and cadence metrics, broadcast once a second.

4b. (recommended) The SB20 collects data from both pedals, and adds other bike related data like calculated speed and distance. Zwift gets all this data in a single once-a-second update, by connecting directly to the SB20. It is also easier to pair since you don't need to start pedalling to "wake up" the left crank.  

NOTE: Connecting to the Stages Bike as a Zwift Power Source had issues 2 years ago. The SB20 had difficulty collecting and sending all required data in its once-a-second updates. Occasionally crank data was "stale" (from one or two seconds ago). This problem has since been corrected. It is a coin-toss whether a crank-to-Zwift (4a) or SB20-to-Zwift (4b) connection is more reliable 🤷‍♀️.

Missing Data

Is there any real advantage connecting directly to the left crank? There is one.

5. The data provided directly from the left crank (4a) is more complete. It includes additional force and crank angle data that can be used by apps like Intervals.icu or a Garmin computer to show pedalling dynamics (power efficiency) data. If you get power data via the Stages Bike (4b), this additional information is stripped out.

Data Latency and Accuracy

Prior to 2022, using the SB20 as a Power Source often produced a slower reacting Zwift avatar.
Is that still true? Let's find out.

Free Ride Test

In this simple test, a Garmin computer is used to collect power data from the left crank (5 in the diagram). At the same time, Zwift controls the test simulation and collects power data from the Stages Bike (4b in the diagram). Heart rate readings are used to synchronize the recordings from both sources. Then I just free ride in Zwift for 4 minutes, reacting to terrain changes while changing gears and cadence at random.

Here is the data (.csv) from the test ride. 


Average power from both data sources: 

Average Left Power

Average SB20 Power

234

233


Here's a comparison of the once-a-second updates from each:


I expected data to be exactly the same. It's not. Increases and decreases vary slightly in time. High and low power readings are similar but not always identical. Either way, none of the variations are significant. The overall averages after 4 minutes differ by just 0.4%.  

My guess: The once-a-second readings for Power (watts) and cadence (RPM) are sampled at slightly different times. For example, the left crank connection could transmit data every second starting at ss.mmm 00.000 while SB20 data transmissions could be offset by 600 ms at 00.600 (i.e. data is sampled 600 ms apart). Some values are higher. Some are lower. The averages are essentially the same.

Is this a conclusive test?

Connecting to the Stages Bike as a Zwift Power Source looks good. My tests are far from conclusive.  I'm am an outlier who believes in restarting Zwift and the SB20 before each riding session to reset all related software and wireless connections. Riders who don't do this, could get different results.

Summary (Finally 😇)

  • Riders can confidently to connect the Stages Bike as a Zwift Power Source (cadence and resistance too). 
  • A small number of riders who use different workout apps like TrainerDay and TrainerRoad, or want additional metrics like pedalling dynamics could benefit from using the left crank as a Power Source.
  • In my opinion, restarting your workout apps and the SB20 before each riding session is a good idea, no matter which type of power connection you use.


Extra: Bug when Connecting to Left Crank

There is a longstanding bug when connecting Zwift to the SB20's left crank.  Here are the details:







Comments

  1. scottyucsbJanuary 1, 2024 at 12:37 PM

    This is a great post, thanks. In the last few months of 2023 I have switched my Zwift connection (for races) to the left crank. Previously and still for free-ride, group work outs, and work outs that can benefit from a controllable PM, I use stages bike. I started doing zracing and my team was calling me out in TTT for pulling through too fast... Subsequently I came to acknowledge I was benefitting from "sticky watts." Switching to left crank connection fixed this for me. What I found was: during climbs in particular, I could shift into a harder gear, stand for a 10-15 second effort, return to sitting, shift down, and see a sharp peak in power along with a 5-10 second sustained effort. This does not seem to be (as) possible connecting to left crank arm. I doubt this will ever get me in actual trouble (low Cat b, senior), but is not sporting. I am much better able to pull through in TTT as well (although staying with the team is harder).

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