ERG mode vs simulation mode: what your trainer is actually doing

ERG mode: the trainer holds the number

In ERG mode the app sends a target power and the trainer continuously adjusts resistance so that your output matches it. Pedal faster and resistance drops; pedal slower and it rises. The product of force and cadence stays put.

This is what makes structured workouts possible without watching the screen. A 4 × 4 minute session at 105 % of FTP is four blocks where the trainer does the arithmetic and you supply the legs. Over Bluetooth this is the FTMS Set Target Power command; the Cadexa app clamps every target between 30 W and the lower of 800 W or twice your FTP so a typo in a workout cannot hurt you.

ERG has two well-known quirks:

• The spiral of death. If your cadence falls under a high target, the trainer raises resistance to keep the watts up, which slows you further. Most apps and trainers now ease off at very low cadence; still, shift to an easier gear before a hard interval so the flywheel spins quickly.
• Gear choice changes feel, not watts. A big gear spins the flywheel faster and feels more like a flat road; a small gear feels like a climb. The power is the same either way.

Simulation mode: the trainer mimics a road

In simulation mode the app sends a gradient plus rolling and air resistance coefficients, and the trainer sets a resistance that corresponds to riding that road at your current speed. How hard it is depends on how hard you push. Shift gears as you would outdoors.

The physics is simple: resistance equals the sum of the gravity component (mass × g × gradient), rolling resistance (coefficient × mass × g) and aerodynamic drag (½ × air density × drag area × speed²). The trainer cannot measure your mass or your position on the bike, so the app sends assumptions. Cadexa, for example, uses a single rider model to compute both the virtual speed and the parameters sent to the trainer, so the two never disagree. The FTMS specification’s example wind coefficient of 0.51 kg/m implies a drag area of about 0.83 m², roughly twice a real rider on the hoods, which is why some apps feel like a permanent headwind.

Simulation mode also exposes the trainer’s limits. A trainer rated for 16 % will simply flatten anything steeper, and a lighter flywheel will feel less like coasting when the gradient turns negative. Only motor-driven units can actively spin the flywheel to simulate a descent.

Which one for which job

A third option: resistance mode

Most trainers also accept a fixed resistance level (FTMS Set Target Resistance Level). It behaves like a dumb trainer with a dial: a constant brake, speed and gearing decide the watts. Useful for sprints and for apps that want full manual control.

How Cadexa uses both

Cadexa runs structured workouts in ERG mode and imported routes in simulation mode. In route rides the gradient sent to the trainer is ramped no faster than 1 % per second uphill and 3 % per second downhill, so a 0 to 10 % transition never arrives between two pedal strokes, and an effort setting scales the trainer’s feel without scaling the virtual speed, which keeps times honest.