SB20 - My Power Meter Cranks Failed - Now What?

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When your Stages SB20 smart bike is working normally, the built-in power meter cranks provide power and cadence data to apps like Zwift. Behind the scenes, they also provide the data that lets the SB20 regulate resistance when running in Erg mode.


Some riders also add 3rd party power meter pedals.  They can work very well.  

It surprises many people that Erg mode requires the built-in Stages power meters even when 3rd party power meters are in use.  

For more information on SB20 erg mode, see: Erg Mode with a Stages SB20 Smart Bike.

Question of the Day 📣

Question: If my Stages crank power meters fail and I have another set of 3rd party power meter pedals, can I still use my SB20 bike? 

The Short Answer: You can still do free rides and races in Zwift and other apps. You will not be able to use Erg mode. 

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Urban legend: Erg mode still works with 3rd Party Cranks 

Some people report that Erg mode still works when they use other power meter pedals like those from Favero or Garmin. It is easy to be fooled. Erg mode only works if your Stages PMs are still working. 

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Free Ride Mode

Let's start by reviewing how Free Ride and Erg mode works with the SB20.

Free ride mode (also called simulation or sim mode) is used for Zwift group rides, races and challenges. 

Free rides are actively controlled by Zwift (and similar apps). Speed and distance are calculated using mathematical models that account for riding surface, hill gradient and wind resistance. This creates an indoor ride that behaves much like outdoors.

For more details see: Smart Bikes, Zwift and Bike Computers. 

Erg Mode

Erg mode is primarily used in structured workouts to keep power at a desired level. 

Erg mode is a self-contained smart bike / trainer function. Zwift asks your Stages bike to start erg mode and target a specific power level (watts). The bike then continually varies resistance to keep you producing the desired number of watts as you pedal faster or slower.

For more details see: What is Erg Mode and Why Can it Suck? 

The app requesting Erg mode does not matter. The SB20, like all smart bikes and trainers, autonomously controls resistance without any further assistance.

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Some Examples

These examples use Zwift. Other apps work similarly.

Riding with SB20 for Resistance, Power and Cadence

Many people configure Zwift to use the Stages Bike for Resistance (Control), Power and Cadence. 

Summary (Everything works!)

1. Options set on the Zwift Paired Devices page:

    Resistance: Stages Bike
    Power Source: Stages Bike
    Cadence: Stages Bike

2. Zwift app sends wireless commands to the SB20 to vary Free Ride resistance or to enable Erg mode.

3. Power and cadence data is sent from the power meter cranks to the SB20 approximately once a second. 

4. When in erg mode, the SB20 uses crank power meter data to regulate resistance.

5. The SB20 relays power and cadence data from the cranks to Zwift approximately once a second.

Riding with Stages Power Meter for Power and Cadence

Some people use the Stages power meter cranks for Power and Cadence. 

Summary (Everything works about the same)

1. Options set on the Zwift Paired Devices page:

    Resistance: Stages Bike
    Power Source: Left Crank
    Cadence: Left Crank

2. Zwift app sends wireless commands to the SB20 to vary Free Ride resistance or to enable Erg mode (same as always).

3. Power and cadence data is sent from the power meter cranks to the SB20 approximately once a second. This data is used internally by the SB20 and is not relayed to Zwift.

4. When in erg mode, the SB20 uses crank power meter data to regulate resistance.

5. Power and cadence data is sent from the cranks directly to Zwift approximately once a second. This data is a duplicate of the data sent to the SB20 in Step 2.

Should you use the Stages Bike or the power meter cranks for power and cadence?  See How to Pick a Zwift Power Source. 

Riding with a 3rd Party Power Meter for Power and Cadence

Some people use their own power meter pedals for Power and Cadence. 

Summary (Everything works - But not like you might think)

1. Options set on the Zwift Paired Devices page:

    Resistance: Stages Bike
    Power Source: 3rd Party Power Meter
    Cadence: 3rd Party Power Meter

2. Zwift app sends wireless commands to the SB20 to vary Free Ride resistance or to enable Erg mode (same as always).

3. Power and cadence data is sent from the power meter cranks to the SB20 approximately once a second. This data is used internally by the SB20 and is not relayed to Zwift.

4. When in erg mode, the SB20 uses crank power meter data to regulate resistance. No data from the 3rd party power meters is used even though it is set as the Power Source in Zwift.

5. Power and cadence data is sent from the 3rd party power meters to Zwift approximately once a second.

Note how data from the Stages and 3rd party power meters are both used even though only your 3rd party power meters are set as the Power Source in Zwift. Erg effort will be high if the Stages power meters read low compared to the 3rd party power meters. Erg effort will be too low if the Stages power meters read high. 

Riding with a Malfunctioning Stages Power Meter

How do 3rd party power meter pedals work when the Stages power meter cranks are not working?  

Summary (Free riding works. Erg mode does NOT work)

1. Options set on the Zwift Paired Devices page:

    Resistance: Stages Bike
    Power Source: 3rd Party Power Meter
    Cadence: 3rd Party Power Meter

2. Zwift app sends wireless commands to the SB20 to vary Free Ride resistance or to enable Erg mode (same as always).

3. ❌ Power and cadence data cannot be sent from the power meter cranks to the SB20. 

4. ❌ No data from the power meter cranks means: Erg mode will not start.  When requested by Zwift to switch to Erg mode, the SB20 stays in free ride / sim mode. Zwift will think your bike is in Erg mode. It's not. The power readings you see (if any depending on the app) are based on the last free ride / sim settings. Don't be fooled. Erg mode is not working.  

5. Power and cadence data is sent from the 3rd party power meters to Zwift approximately once a second. Free riding and racing events work correctly.

In short: When your Stages power meters are not functioning and you are connected to a set of 3rd party power meters, Free Ride works normally but Erg mode is unavailable.

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