Erg Mode with a Stages SB20 Smart Bike

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In my last post, I described why Erg mode is not always optimal:  What is Erg Mode and Why Can it Suck?  In this post, I describe how the Erg mode works on a Stages SB20 Smart Bike.

 SB20 Erg Mode

The Stages SB20 bike is different than other smart bikes or a trainer. It uses crank-based power meters and not flywheel speed ⨉ resistance to determine power levels. This simplifies the power calculations but adds some Erg mode wrinkles.

A simple SB20 Erg mode flowchart looks like this:



The SB20 is measures cadence and power at the cranks rather than calculating power based on flywheel resistance and RPM.  

On paper, this method should provide a very responsive erg mode. The reality is: it is one of the slowest erg modes to stabilize after cadence and target power changes.


 





Why so Slow?

The Stages power meter cranks send readings to the SB20 processor once a second. Due to limited processing capability in the cranks, raw readings are transmitted. This data needs to be filtered (or smoothed) before it can be used by an Erg algorithm. 

If cadence or resistance changes rapidly, it can take up to 5 seconds to calculate a smoothed power reading.  When cadence and resistance change simultaneously - like in a sprint or at the end of a hard segment - it can take even longer to get a usable power reading.  

😇 Comment: Stages appears to have implemented some form of power averaging.  This works fine when data is relatively constant. Other curve fitting methods, like least squares regression and least absolute deviation work better when cadence and target power has large changes. This would help.  

Summary: The SB20 needs to filter (smooth) PM readings for a few seconds when cadence or target power changes (left chart). Other bikes and trainers simply read the flywheel speed (that is already smoothed thanks to physics) and do instantaneous power calculations (right chart). It is unlikely that the SB20's power meter driven Erg mode will ever be as fast other bikes. 

😱 Early versions of the SB20 could also do flywheel measurement. This feature was abandoned several years ago in favor of 100% power meter based measurements.

How to live with SB20 Erg Mode

1. Be happy that the SB20 rarely suffers from the Erg mode spiral of death.

2. Change cadence slowly to allow power meter calculations to keep up.

3. Simultaneous changes in cadence and power targets should be avoided. That's really hard to do.  Just recognize that stabilization will take longer when both happen at the same time.

4. Relax! When the SB20 erg mode it is hunting for a new power target (while showing high->low->high power values), resistance stays steady. Resistance goes up or down only when the bike has decided that a resistance change is needed. 

5. Consider using free ride (sim) or slope mode segments for shorter structured workout segments and all sprints. The SB20 takes up to 10 seconds to stabilize when switching to and from high intensity erg mode intervals. It takes less than a second do the same in a free ride or slope segment.

📣 Zwift workouts support free ride segments. You will need to change gears to hit power targets. TrainerDay.com, GoldenCheetah.org, BreakAway.app, and TrainerRoad.com all support slope mode structured workouts. Simulated hills can be used to automate gear changes and make structured workouts "handsfree". 


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