What is Erg Mode and Why Can it Suck?

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Free Riding is Easy

Let's start with the relationship between resistance, cadence and power (watts) when doing an indoor free ride (sometimes called sim mode).




Free riding follows a simple formula: 

Power in watts = How hard you push ⨉ How fast you pedal 

Your indoor bike offers constant resistance when you stay in the same physical or virtual gear. Pedal at a constant RPM and you get a consistent number of watts. If you then pedal 10% faster, you produce 10% more watts.  Pedal 10% slower, you produce 10% fewer watts.

📣 Free Riding Summary: When simulated riding conditions stay the same, pedalling faster produces more watts. Pedalling slower produces fewer watts.

And don't forget: There is one way to go faster in Zwift.  Produce more watts 💪.   

If only indoor riding was always this simple...

Erg Mode

When doing structured workouts, Erg mode is popular. It allows you to produce a desired number of watts whether you pedal fast or slow. It's very useful for zone workouts where you want to avoid going too hard or too easy. In Erg mode, you set a desired power target, and just ride. Your indoor bike automatically adjusts resistance to keep you producing the same number of watts.

How it works


Your bike provides variable resistance by effectively changing gears with its electronic brake to keep you at a given target power level. 

Pedal at a constant RPM and you get a constant pedal resistance. Pedal 10% faster, and the bike will provide 10% less resistance.  Pedal 10% slower, the bike will provide 10% more resistance.

The Erg mode formula can be described as:

How the hard the bike resists = Target watts ÷ How fast you pedal

📣 Erg mode summary: Pedalling faster causes resistance to be reduced. Pedalling slower causes resistance to be increased.

Changes in Power Targets

Erg mode also accommodates the changes in power targets that routinely occur in structured workouts.  


When target power changes, the bike raises or lowers resistance to keep you producing the desired number of watts.  The process of determining the correct resistance is iterative. It takes a few pedal revolutions for resistance to stabilize. 


 

Simplified Algorithm to Explain Erg Mode 

These flowcharts illustrate how most smart bikes and trainers keep power constant as cadence and power targets change.


1. Your bike/trainer reads flywheel RPM and resistance to calculate power. It increases or reduces resistance to maintain a power target.  

💥 The first algorithm has a serious issue: If you suddenly pedal slower, the resistance can increase to a point where you can no longer move the pedals. This is often called the Erg mode spiral of death.

2. The second algorithm addresses low RPM issues. It basically enforces: If cadence gets too low,  resistance is not increased.

🔬Notes:

  • Actual Erg algorithms are more complicated to deal with data dropouts and sensor inaccuracies.  
  • Changes in RPM, cadence and power targets are handled differently by each brand of indoor bike. Some can stabilize in a second or two. Others can take 10 seconds and more.

Alternatives to Erg Mode

Consider using free ride (sim) or slope mode segments for shorter structured workout segments and all sprints. 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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I also have some comments on the Erg mode implemented by my Stages SB20 Smart Bike.  It is different than all other indoor bikes and trainers.  See: Erg Mode with a Stages SB20 Smart Bike


Comments

  1. dglenncaseyJanuary 13, 2024 at 4:58 PM

    Hi Craig. I've tried leaving this on FB, but it keeps getting removed. Anyway, My question(s) is does the ERG mode not work when connecting with ANT+? Or below a certain wattage? ERG mode used to work in BT, but that stopped working months ago. Now I can only get a halfway decent connection through ANT+, but no ERG mode.

    ReplyDelete
    Replies
    1. CraigJanuary 13, 2024 at 7:18 PM

      What app and device are you using to run Erg mode? Erg mode is enabled with a command sent from the controlling app (Zwift, TrainerRoad, etc.) telling the SB20 to run Erg mode at a given target power. Once enabled, erg mode runs autonomously on the SB20, until it receives a new command to change modes. BT and ANT+ use different command sequences so a problem with the controlling app could be the issue - probably because your exact configuration wasn't tested with the SB20. Can you try a different app on a different computer or device?

      Delete
    2. dglenncaseyJanuary 13, 2024 at 8:17 PM

      I'm connecting directly to Zwift through the ANT+ channel. Connecting the left crank power and the FE-C connection as the controllable. The FE-C connection is the only option that shows up when connecting the cranks through ANT+.

      Delete
    3. CraigJanuary 13, 2024 at 9:34 PM

      Thanks for confirming that you are use Zwift and only see the SB20 as a FE-C service (on the Paired Devices page). You should also see SB20's BT LE (BLE) FMTS service on the same page.

      Are you using a PC or other device? Can you try adding Zwift to your phone and then connecting from that app? I suspect you will be able to the SB20's BLE FMTS service. If so, this says something has happened on your original device.

      Delete

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