Rehearsing the race on a trainer

A pacing plan is a prediction until you pedal it. In ERG mode, at home, it becomes information.

Why rehearse it

A pacing plan says how many watts to hold, and for how long. On paper both numbers always look reasonable: the difference between a correct target and one that is twenty watts optimistic is invisible in a table and obvious ninety minutes into the race. Riding the plan is the only way to find out while changing it still costs nothing.

And it is not only about the legs. An hour in the aero position you will actually race in tells you whether the position holds; an hour with your own gels tells you whether the fuelling plan is realistic or merely arithmetically correct. Those are all things race day teaches you too late.

What ERG mode actually does

In ERG mode the trainer is given a target power and adjusts resistance to make you hold it: pedal slower and resistance rises, pedal faster and it drops, and the watts stay where they were set. It is the opposite of free mode, where you are the one chasing a number on the screen.

For a race rehearsal that is exactly what you want, because it removes the "how good am I at holding steady power" variable and leaves only the real question: does this effort, for this long, stand up? PeakPacing also reads the power range the trainer says it accepts and fits the targets to it, so an out-of-range value is not silently ignored by the trainer.

Or: the course, with its real gradient

The second mode is course simulation: the trainer is handed the parameters of the road — starting with the gradient of the stretch you are on — and computes the resistance itself. It genuinely gets harder when the profile goes up. On a treadmill the same command becomes the incline of the belt.

The two modes answer different questions: ERG answers "can I hold these watts for this long", simulation answers "how does the effort distribute over this profile". During a session you can switch between them, and to free watt or speed mode, without stopping.

If you would rather start from heart rate

There is a third way in: name a target heart rate instead of a power. The app adjusts the trainer's resistance to bring you to that heart rate and keep you there, with three phases (warm-up, approach, and holding inside a bpm band) and a correction for cardiac drift — the reason your heart rate creeps up at unchanged watts as the minutes pass. Any Bluetooth heart-rate strap works.

How to set a rehearsal up

1. Generate the race plan: GPX course, thresholds, forecast weather. The steps are described in how it works.
2. Look at the target watts of the sections that matter — the long climb, the headwind stretch, the closing kilometres.
3. Connect the trainer over Bluetooth and set that target in ERG: the trainer holds it, you pedal.
4. Ride it for a duration that means something. You do not need to simulate the whole race: the most exposed part is enough to expose a wrong target.
5. Use the rehearsal for fuelling too: the same gels, the same bottles, the same intake schedule the plan calls for.
6. If the target does not hold, lower the intensity in the plan and ride it again. It is the one time re-running the race costs nothing.

At the end of the session the virtual activity is recorded and can be uploaded to Intervals.icu and Strava as a TCX file, so the rehearsal stays in the log alongside the rest of the build-up.

What you need

• A smart trainer or treadmill compatible with Bluetooth FTMS and controllable in ERG mode.
• A browser with Web Bluetooth — Chrome, Edge or Opera, on desktop or Android — or the native iOS/Android build. Safari and Firefox do not implement Web Bluetooth.
• Optional: a Bluetooth heart-rate strap for the heart-rate session, and a Zwift Click remote to change the target without touching the screen.

If the race you are preparing for is a duathlon, the duathlon pacing page explains how the three legs are computed before you ride them.

Frequently asked questions about race rehearsals

Rehearse your plan