Turn an ESP32 into a Bluetooth bridge that lets Zwift and your Garmin use the same smart trainer at the same time.
Many trainers accept only one Bluetooth connection at a time, so you pick: ERG mode in the app, or power and speed on your Garmin. TrainerBridge connects to the trainer once and re-publishes it as a new device that several apps and bike computers can pair with at once. Apps keep full control (ERG, simulation grade), and Garmin gets power, speed and distance without GPS or extra sensors.
make deps upload # pinned ESP32 core + NimBLE, then build and flash
Tested on a standard ESP32 DevKit with the Decathlon Van Rysel D100. Other FTMS trainers should work: pair them by advertised name or MAC address.
Riding indoors with an app and a bike computer usually means compromises:
- the trainer drops the Garmin as soon as Zwift connects, or the other way round
- no speed or distance on the Garmin indoors, because there's no GPS and no wheel sensor
- ride data split between the app and your Garmin history
- a separate power meter or speed sensor just to record the ride twice
TrainerBridge sits in between and gives everyone what they need from one trainer connection.
| Apps | Zwift, MyWhoosh, TrainerDay and any FTMS app see a controllable trainer: ERG target power, simulation grade, start/stop, routed to the real trainer |
| Garmin | Edge and watches pair it as a Smart Trainer, or as separate Power and Speed sensors |
| Speed & distance | Physics-based virtual speed on a flat road (CdA, rolling resistance, 85 kg rider + bike), with flywheel coasting so Garmin doesn't auto-pause |
| Multi-client | App and Garmin connected together; each gets the same telemetry, and control responses go only to the client that sent the command |
| Recovery | Trainer power-cycled? Clients get 0 W immediately, and the bridge keeps scanning and reconnects on its own |
| Status LED | Blinks while searching, solid once the trainer is connected. No serial monitor needed |
| Diagnostics | Serial console with live status, power calibration and manual ERG commands |
You need an ESP32 (the classic ESP32, e.g. a DevKit V1), a USB cable and Arduino CLI (or the Arduino IDE with ESP32 Arduino core 3.3.12 and NimBLE-Arduino 2.5.1).
git clone https://github.com/antomanc/TrainerBridge && cd TrainerBridge
make deps # install the pinned core and library (once)
make upload PORT=/dev/ttyUSB0 # build with all warnings and flash
make monitor # optional: serial console at 115200 baudTurn on the trainer: the LED blinks while the bridge looks for it and stays on once connected. Then pair your devices with TrainerBridge:
Garmin Edge or watch: Settings > Sensors > Add Sensor > Smart Trainer, pick TrainerBridge. You get power, speed and distance over one connection. If asked for a wheel size, use 2096 mm. (Alternatively, add it as a Speed/Cadence sensor and a Power sensor.)
Zwift, MyWhoosh, TrainerDay: in the pairing screen, choose TrainerBridge for both Controllable and Power Source.
Everything lives in TrainerBridge/Config.h. Defaults work for a
Van Rysel D100. The settings you are most likely to change:
| Setting | Default | |
|---|---|---|
TARGET_NAME_CONTAINS |
"RYSEL" |
Case-insensitive part of the trainer's Bluetooth name |
TARGET_MAC |
"" |
Exact trainer MAC. Takes priority over the name; leave both empty to use the first FTMS trainer found |
DEVICE_NAME |
"TrainerBridge" |
Name apps and Garmin see |
DEFAULT_POWER_SCALE |
1.00 |
Multiply the trainer's watts, if it reads high or low |
ENABLE_VIRTUAL_SPEED |
true |
Physics-based speed and distance |
ENABLE_CSC_SERVICE |
true |
Speed sensor service for Garmin |
VIRTUAL_WHEEL_CIRCUMFERENCE_M |
2.096 |
Match the wheel size set on your Garmin (700x25c) |
STATUS_LED_PIN |
LED_BUILTIN or 2 |
Status LED GPIO; STATUS_LED_ACTIVE_HIGH sets polarity |
DEBUG_LOG |
0 |
Set to 1 for verbose serial logging while debugging |
Rider mass, CdA, rolling resistance and coasting are in the same file. → How it works explains the model and the Bluetooth services.
Open the serial port at 115200 baud (make monitor):
status Connection, power, virtual speed, distance and control queue state
scale 1.05 Change the power calibration until reboot
p 200 Send an ERG target of 200 W to the trainer (testing)
adv / noadv Start or stop advertising to apps and Garmin
- One trainer connection, many clients. The ESP32 is the only device that talks to the trainer, so apps and Garmin never fight over it.
- The trainer has the last word. ERG and simulation commands are queued, sent one at a time, and the app gets the trainer's real answer, not an optimistic one. Commands that time out or hit a disconnect fail right away instead of leaving the app waiting.
- Answers go to whoever asked. Control responses are sent only to the client that sent the command, so the Garmin never sees an app's traffic (Garmin drops sensors that send unexpected indications).
- Never blocks the Bluetooth stack. Writes to the trainer happen in the main loop, not in Bluetooth callbacks. State shared between the two is locked.
- Fails safe. If the trainer disappears, every client sees 0 W immediately, not a frozen number.
make test # host tests: the real sketch against Arduino/NimBLE test doubles (g++ only)
make check # tests with AddressSanitizer + UBSan, then the real firmware buildThe host tests run the actual firmware code through scanning, reconnecting 20 times, two clients at once, and FTMS success, failure, timeout and late-response cases. They can't test the radio, so hardware changes still need a ride on a real trainer: → docs/testing.md. See CONTRIBUTING.md and CHANGELOG.md.
GPL-3.0 © 2026 Antonio Mancini
TrainerBridge is an independent project, not affiliated with or endorsed by Garmin, Decathlon, Van Rysel, Zwift, MyWhoosh or TrainerDay.