Simulation mode makes your trainer respond to terrain — the resistance changes based on the gradient, just like riding outdoors. Combined with a GPX route, you can pre-ride any race course.

Switching to Simulation Mode

On the Ride tab, when your session is stopped, you’ll see an ERG / SIM toggle at the bottom of the screen. Tap SIM to switch.

You cannot switch modes while a session is recording. Stop and discard first.

What You See

Gradient display — large percentage at the top showing the current grade with a terrain descriptor:

Grade	Descriptor
Below -8%	DESCENDING FAST
-8% to -3%	DESCENDING
-3% to 1%	FLAT
1% to 5%	CLIMBING
5% to 10%	STEEP CLIMB
10%+	WALL

Animated cyclist — a visual rider that responds to your real cadence and speed, tilts with the gradient, and shows effort indicators on steep climbs
Data screens — same swipeable metric pages as ERG mode
Elevation profile — route elevation graph with your current position (when a GPX file is loaded)

Loading a GPX Route

In SIM mode with your session stopped, tap “Load GPX Route”
The iOS file picker opens — select a .gpx file from your device, iCloud Drive, or any connected file provider
Once loaded, you’ll see:
- Elevation profile — a graph of the route’s elevation with an orange dot showing your current position
- Route summary — total elevation gain/loss, max elevation, and average gradient
- Route name with “Change” and “X” buttons to swap or clear the route

The app supports GPX files with either track points (<trkpt>) or route points (<rtept>).

Virtual Gearing

Simulation mode includes a virtual drivetrain. You shift gears to control your virtual speed and the resulting trainer resistance.

Default setup:

Chainrings: 34/50T (compact double)
Cassette: 11-13-15-17-19-21-24-28-32-36 (10-speed)
Wheel: 2105mm circumference (700x25c)

How it works: Your cadence multiplied by the current gear ratio determines your virtual speed. That speed, combined with the current gradient, determines the resistance your trainer applies.

The gear display shows your current selection in “34x17” format (chainring x cog teeth).

Shifting with Zwift Ride Controller

Button	Action
Right Shift Up	Harder gear (smaller cog)
Right Shift Down	Easier gear (larger cog)
Left Shift Up	Big chainring
Left Shift Down	Small chainring
Right Hood	3 cogs harder at once
Left Hood	3 cogs easier at once

Each shift gives a light haptic tap. Without a Zwift Ride controller, set your preferred starting gear in Settings before riding.

Physics-Based Resistance

In Settings > Simulation Mode, there’s a “Physics-Based Resistance” toggle (on by default):

ON — the app calculates exact power required based on gradient, rider weight, bike weight, rolling resistance, and air drag. Gear shifts directly change the resistance you feel. This is the recommended setting.
OFF — the app sends the raw gradient directly to the trainer and lets its built-in firmware handle resistance. Some trainers may feel different in this mode.

GPX Smoothing

Raw GPX data from GPS devices contains noise — elevation stepping, GPS wander, and gradient spikes. Cyrace automatically cleans up your data when you load a file.

You can tune smoothing under Settings > GPX Smoothing:

Setting	Default	What It Does
Elevation Sigma	10	How aggressively elevation is smoothed (4–16). Higher = smoother gradients.
GPS Epsilon	3.0m	How much route path is simplified (2–5m). Higher = cleaner path.
Max Gradient	30%	Caps maximum gradient sent to trainer (20–30%). Match to your trainer’s limit.
Power Smoothing	0.4	Filters real-time power output (0.1–0.9). Lower = smoother resistance changes.

Important: Changes to Elevation Sigma, GPS Epsilon, and Max Gradient require you to reload the GPX file to take effect. Power Smoothing applies immediately.

Tuning Tips

Start with defaults and ride a section you know well
If gradients feel too spiky → increase Elevation Sigma by 2–4
If gradients feel too “flat” → decrease Elevation Sigma by 2–4
If you get sudden extreme resistance spikes → lower Max Gradient to match your trainer’s hardware limit (Wahoo KICKR: 20%, Tacx NEO: 25%)
Adjust Power Smoothing in real-time while riding to find your preference