Indoor riding on a phone comes down to three numbers, the watts you are producing, the cadence you are turning, and your heart rate. Each comes from a sensor, and each sensor talks Bluetooth in a particular way. Once you know which device is responsible for which number, pairing stops being a guessing game.
This guide covers the three roles a sensor can play, what a smart bike does differently, ERG mode, dropouts, and the small habits that keep everything connected.
The three roles
A workout app does not much care what a device is called. It cares what the device can do. Bluetooth describes that with services, and for cycling there are three that matter.
Trainer
A smart trainer or smart bike that the app can control. The Bluetooth service is FTMS, the Fitness Machine Service. Over it the app reads power, cadence and speed from the trainer, and writes back to it, telling it to hold this many watts or to simulate a road. If a device supports FTMS, connect it as the Trainer. This is the link that makes ERG mode work.
Power
A source of watts. The service is the Cycling Power Service, and it is what a crank, pedal or hub power meter broadcasts. Most smart trainers broadcast it too, alongside FTMS, so the trainer's own strain gauge can act as a power meter for any app that asks. Connect a device as Power when you want its watts to be the ones you look at. Cadence usually arrives on the same service.
Heart rate
A chest strap, or a watch broadcasting its optical reading, over the Heart Rate Service. It supplies one number and nothing else. Some trainers can relay a paired strap's reading over their own link, but it is simpler to connect the strap directly.
Which sensor gives you watts, cadence and heart rate?
- Watts. The Trainer or the Power device. If both are connected the app has two sources, and it helps to know which one is on screen.
- Cadence. The Trainer or the Power device, whichever reports it. Many trainers estimate cadence from the power signal, while a crank or pedal meter measures it.
- Speed and distance. The Trainer. Indoors these are derived numbers and only a rough guide.
- Heart rate. The Heart rate device.
One device, two roles
A smart bike such as a KICKR Bike, or a direct-drive trainer, advertises FTMS and the Cycling Power Service on the same Bluetooth link. Your phone holds one connection to the device, and a well-built app can use that one connection for more than one role. So you can add the same bike as Trainer and again as Power. It will accept ERG targets and supply the power reading over a single link. Nothing is duplicated and nothing fights.
When to add a separate power meter
If you own a crank or pedal power meter, connect it as Power for indoor rides. Two reasons.
First, consistency. That meter is the one on your bike outdoors. If the same device supplies the indoor numbers, your zones, personal bests and Load all come from one source, and a 10-watt gap between trainer and meter stops muddying the picture.
Second, calibration. Trainers drift with temperature and belt tension. A pedal or crank meter has its own zero-offset routine and is usually easier to keep honest.
Whether an app prefers the Power device's watts over the trainer's when both are reporting varies. Check on the first ride that the number on screen matches the meter, not the trainer. If it does not, connect the trainer as Trainer only and leave the meter as the sole source of power.
One thing does not change. ERG mode still runs on the trainer's own reading. The trainer holds the target using its own strain gauge, and your meter reports what it sees. A small gap between the two is normal.
Should heart rate come from a chest strap or a watch?
Wrist optical sensors are fine for a walk. Indoors on a bike they struggle. Your wrist is bent on the bars, the watch strap is loose, and sweat gets between the sensor and the skin. The reading often lags behind changes in effort and can drop out on hard efforts.
A chest strap measures the electrical signal directly. It is quicker, steadier, and it stays put. Indoors, where heart rate is one of your two ways of judging how hard an effort actually was, use the strap.
ERG mode, plainly
In ERG mode the trainer holds the target power. Whatever cadence you turn, the resistance adjusts so the watts stay where the workout says. Your job is to hold a steady cadence and let the trainer do the rest.
Two things follow. If your cadence drops, the trainer adds resistance to keep the watts up, and the pedals get heavier still. Keep the cadence steady through intervals and lift it slightly before a hard block starts. On recovery segments the target falls and the pedals go light. Keep turning them, because the trainer needs you pedalling to hold any target at all.
ERG targets are sent over the FTMS link, so they only work when the device is connected as Trainer. A power meter alone can measure watts. It cannot set them. There is more in ERG mode explained.
Dropouts happen
Bluetooth is a short-range radio in a room full of interference. Every setup drops a sensor now and then. The question is what the app does about it.
A decent app marks the sensor as lost, keeps recording, and tries to reconnect on its own. The ride file is never interrupted. You get a gap in one channel rather than a lost workout. If the automatic attempt fails, a tap on the sensor should bring it back without going through the scan again.
If one sensor drops repeatedly, check the basics below before blaming the app.
Pairing hygiene
- One app at a time. A sensor holds one Bluetooth connection. If a head unit or another app has already taken it, yours cannot. Close the other one first.
- Bluetooth on, and any permission the phone asks for on the first scan granted. Without it, scanning finds nothing.
- Keep the phone near the bike. Two metres is fine, the other side of the room is not.
- Replace the strap battery when readings get patchy. A coin cell lasts months, and the first sign it is going is dropouts, not a dead strap.
- Wet the strap electrodes before you start. Dry skin can mean a patchy signal for the first few minutes.
- Zero the power meter after moving the bike, and let the trainer warm up for ten minutes before a spin-down.
How WattPlan handles this
The workout player has three chips at the top, labelled Trainer, HR and Power. One physical device can hold more than one of those roles over a single Bluetooth link, so a smart bike can be added as both Trainer and Power. ERG targets go to the trainer over FTMS. If a sensor drops, its chip shows the loss, recording carries on, and the player makes one automatic reconnect attempt before leaving the chip as a tap-to-reconnect. When a power meter and a trainer are both connected, the meter's watts are the ones shown and recorded, and the Power chip is marked WATTS. WattPlan uses Bluetooth only, so a trainer needs FTMS support to take ERG targets.
Sources
- Fitness Machine Service 1.0, Bluetooth SIG