Zwift Training Modes: SIM vs. ERG Explained for Optimal Cycling

What is the difference between SIM and ERG in Zwift?

In Zwift, SIM (Simulation) mode replicates the dynamic resistance of outdoor riding based on virtual terrain, requiring rider input for gear changes, while ERG (Ergometer) mode automatically adjusts resistance to maintain a precise, pre-set power output, removing the need for gear shifting and allowing focused interval training.

Understanding Indoor Cycling Training Modes

Modern smart trainers, when paired with virtual cycling platforms like Zwift, offer sophisticated control over your indoor cycling experience. This advanced functionality allows for highly specific training protocols and immersive virtual rides. At the core of this control are two primary resistance modes: SIM and ERG. Understanding the distinct characteristics and applications of each is fundamental for optimizing your indoor training and achieving specific fitness goals.

SIM Mode: Simulating the Open Road

SIM mode, short for Simulation mode, is designed to mimic the variable resistance you would encounter during outdoor cycling. When engaged, your smart trainer dynamically adjusts its resistance based on the virtual terrain, gradient changes, and even factors like drafting effects within the Zwift environment.

• How it Works: In SIM mode, the trainer's resistance fluctuates in real-time to reflect the virtual world. If you encounter an uphill climb, the resistance increases; on a downhill, it decreases. Factors like your avatar's speed, virtual wind resistance, and your in-game weight are all considered to provide a realistic feel.
• Key Characteristics:
  • Dynamic Resistance: Resistance is constantly changing, mirroring the virtual environment.
  • Gear Shifting Required: Just like outdoors, you must actively shift gears on your bike to manage your cadence and power output, especially on varied terrain.
  • Mimics Outdoor Riding: Provides a more immersive and realistic cycling experience, developing skills related to pacing, gear selection, and responding to terrain changes.
  • Focus on Bike Handling & Pacing: Encourages strategic thinking about effort distribution and interaction within the virtual world.
• Best Use Cases:
  • Group Rides and Races: Essential for navigating the peloton, responding to attacks, and climbing effectively.
  • Free Riding and Exploration: Ideal for simply riding around Zwift's worlds, enjoying the scenery, and maintaining a steady pace.
  • Developing Pacing Skills: Teaches riders how to manage effort over varied terrain and durations.
  • Practicing Gear Selection: Refines the ability to choose the optimal gear for different gradients and speeds.

ERG Mode: Precision Power Control

ERG mode, short for Ergometer mode, is a highly effective tool for structured, power-based training. Unlike SIM mode, ERG mode takes control of your power output, ensuring you hit precise wattage targets regardless of your cadence or gear selection.

• How it Works: When a workout in Zwift (or another training platform) specifies a target power, ERG mode instructs your smart trainer to automatically adjust the resistance to match that target. If your cadence drops, the trainer will increase resistance to maintain the power; if your cadence increases, it will decrease resistance.
• Key Characteristics:
  • Constant Power Output: The trainer automatically maintains the prescribed wattage, making it impossible to "miss" your power targets.
  • No Gear Shifting Required: While you can shift gears, it has minimal to no effect on your power output in ERG mode, as the trainer will simply compensate. Your focus shifts entirely to cadence.
  • Cadence as Primary Variable: Your chosen cadence directly influences the perceived effort for a given power target. A lower cadence at the same power will feel harder due to higher torque.
  • Focus on Hitting Specific Targets: Ideal for structured interval training where precise power zones are critical for physiological adaptation.
• Best Use Cases:
  • Structured Workouts: Perfect for interval training, tempo rides, sweet spot training, and high-intensity efforts where specific power targets must be maintained.
  • FTP Tests: Ensures consistent effort during critical power assessments.
  • Targeted Physiological Adaptations: Facilitates training specific energy systems by precisely controlling the workload.
  • Maximizing Training Efficiency: Allows riders to focus solely on their effort and cadence without worrying about terrain or gear selection.

Direct Comparison: SIM vs. ERG

Choosing the Right Mode for Your Training Goals

The choice between SIM and ERG mode is not about one being inherently "better" than the other; rather, it's about selecting the most appropriate tool for your specific training objective.

• When to Use SIM Mode:

  • To replicate the feeling of outdoor riding: If you miss the dynamic nature of the road.
  • For social rides or virtual races: Where strategy, drafting, and navigating the peloton are key.
  • To practice real-world cycling skills: Like efficient gear changes, pacing climbs, and maintaining speed on flats.
  • When you want a less constrained, more interactive experience.

• When to Use ERG Mode:

  • For precise interval training: When hitting specific power numbers for specific durations is paramount for adaptation.
  • During Functional Threshold Power (FTP) tests: To ensure consistent, maximal effort without external distractions.
  • When you want to focus purely on power output and cadence: Removing the mental load of gear selection and terrain.
  • To maximize time efficiency: Especially for short, intense workouts where every second at target power counts.

Advanced Considerations and Best Practices

• "Spiral of Death" in ERG Mode: A common phenomenon where a rider's cadence drops too low in ERG mode, causing the trainer to increase resistance dramatically to maintain power, making it incredibly hard to pedal and often leading to a complete stop. To avoid this, try to maintain a consistent, higher cadence (e.g., 85-95 RPM) and anticipate upcoming power changes in your workout.
• Cadence Control in ERG Mode: While power is fixed, your cadence is your primary variable for perceived effort. A higher cadence at the same power will feel easier (less muscular force, higher cardiovascular demand), while a lower cadence will feel harder (more muscular force, higher torque). Experiment to find what works best for different intervals.
• Warm-up and Cool-down: Both modes can be effective for these phases. SIM mode allows for a more relaxed, self-paced warm-up, while ERG mode can be used for structured warm-ups with specific power ramp-ups.
• Hybrid Training: Many cyclists benefit from incorporating both modes into their training week. Use ERG for focused, structured interval sessions, and SIM for group rides, races, or longer endurance rides to develop real-world cycling skills and enjoy the virtual environment.

Conclusion: Optimizing Your Indoor Cycling Experience

Both SIM and ERG modes are invaluable tools in the modern cyclist's arsenal. SIM mode offers realism and skill development, preparing you for the dynamic challenges of outdoor or virtual racing. ERG mode provides unparalleled precision for power-based training, ensuring every watt counts towards your physiological adaptations. By understanding their differences and strategically applying them to your training plan, you can unlock the full potential of your indoor cycling setup and significantly enhance your fitness.