ERG Mode vs SIM Mode: A Decision Tree for the Two Modes of Smart Trainers

The Fundamental Difference Between the Two Modes

How ERG Mode Works

You set 250W, and the trainer adjusts resistance inversely based on your cadence:

• Cadence 90rpm → Moderate resistance
• Cadence 70rpm → Heavier resistance
• Cadence 110rpm → Lighter resistance

Result: Power is always 250W.

Workouts Suited for ERG Mode

• VO2max Intervals: 5×5min @ 110% FTP, no need to watch the power meter
• Threshold: 2×20min @ 95% FTP
• Sweet Spot: 3×15min @ 88-93% FTP
• Any structured workout that “locks power”

The ERG Mode Trap: Spiral of Death

When you fatigue and your cadence drops below 60rpm, the trainer cranks up the resistance relentlessly. The harder you pedal, the more tired you get, your cadence keeps falling, and eventually you can’t turn the pedals at all. Solutions:

1. Actively shift to an easier gear or exit ERG when cadence drops to 70rpm
2. Accept a one-step power drop between intervals (251 → 230W)
3. Set a Cadence Floor (supported by some software)

How SIM Mode Works

It simulates the gradient of the virtual world, you control the gears and power:

• 10% uphill → Resistance spikes sharply; you choose to stand and pedal or shift down
• -5% downhill → Resistance is nearly zero
• Flat road → Maintain cruising pace

Scenarios Suited for SIM Mode

• Zwift races (ZRL, ToW, community races)
• Group rides
• Route exploration (Alpe du Zwift, Ventoux)
• Explosive power training (Sprint, Attack)

Decision Tree

What's today's goal?
├─ Structured workout (VO2/Threshold/SST)
│  └─ ERG Mode
├─ Race / Group ride / Route exploration
│  └─ SIM Mode (Trainer Difficulty 50%)
├─ Recovery Ride
│  └─ ERG Mode (locked at 50-60% FTP)
└─ Sprint / Attack training
   └─ SIM Mode (Trainer Difficulty 100%)

Choosing Trainer Difficulty

Zwift defaults to 50%, meaning it halves the resistance of virtual gradients:

• 100%: Realistic; a 10% grade will be hard to turn over
• 50% (default): Comfortable; a 10% grade feels like 5%
• 0%: Completely flat; purely for cruising

Recommendation: Use 50% for races, 50-70% for training, and 100% for pure exploration.

Choosing the right mode can make a 20-30% difference in workout effectiveness.

Related Reading

• Indoor Trainer Intervals: The Difference Between ERG Mode and Resistance Mode
• Indoor Trainer Power Training: Proper Use of ERG Mode and Common Pitfalls
• The Physics of Smart Trainers: Flywheel Inertia, Gradient Simulation, and Differences in Power-Matching Modes
• Indoor Trainer Climbing Simulation: Climbing Workouts with ERG Mode vs. Slope Mode