ERG mode is a smart trainer setting in which the trainer holds the power target an app sends it, adding resistance as your cadence falls and taking it away as your cadence rises. The Bluetooth Fitness Machine Service standard, published as version 1.0 on 14 February 2017, carries that target as a Set Target Power command in whole watts.
What is ERG mode on a smart trainer?
ERG is short for ergometer. Wahoo's support page for the KICKR, CORE and SNAP lists four control modes, Passive, Target Power (ERG), Resistance and Simulation, and describes ERG as the mode where you set a power target and the trainer adjusts its resistance to keep you there regardless of cadence or speed. Pedal slower and it adds resistance, pedal faster and it bleeds resistance off.
Underneath, your app is writing to the trainer's Fitness Machine Control Point. In the Bluetooth standard the Set Target Power command, op code 0x05, carries one number, the target in watts at a resolution of 1 W, and an app has to request control first, op code 0x00, before the trainer will accept it. That one number is why ERG feels so precise and why it can go wrong, because the trainer knows your target and nothing about your legs.
The precision is real on the trainer that has been tested in a journal. Morais and colleagues had ten regional-level cyclists, average age 45.6, ride a Tacx Neo 2T against Garmin Vector 3 pedals from 100 W to 270 W, and both devices repeated their readings with coefficients of variation between 0.03% and 0.15%, with no significant difference at any stage. What ERG cannot fix is a wrong target, which comes from your FTP, so an honest number matters more indoors than anywhere. Our guide to what FTP is covers the tests.
What you gain is time at a fixed intensity. A 3 x 12 minute sweet spot session in ERG delivers 36 minutes at the target, including the last block where you would otherwise let a few watts slip. What you lose is pacing practice, because a rider who only trains in ERG never has to hold a number themselves.
What is slope mode and how does an app switch to it?
Slope mode, which Wahoo calls Simulation, sets a gradient instead of a power. Wahoo's app changes the nominal grade in 0.5% steps and the trainer works out the resistance from your weight, so your power comes from your gears and cadence. Resistance mode is the third option, a fixed brake level from 0 to 100% that ignores speed and cadence.
In the Bluetooth standard the switch is a different command. Set Indoor Bike Simulation Parameters, op code 0x11, carries wind speed in metres per second, grade in percent to 0.01%, a rolling resistance coefficient and a wind resistance coefficient. The specification has no command that means leave ERG, so an app releases a trainer by sending it a simulation to ride instead.
WattPlan's player works the same way. For every step with a target it sends the trainer a power in watts and re-sends it every tick, and for a free-ride step it sends one simulation command for a flat road, grade 0 and no wind, then leaves your gears to set the effort.
Should you use ERG or slope mode for each workout?
| Session | Mode | Why, with the numbers |
|---|---|---|
| Sweet spot and threshold intervals | ERG | Minutes at the target are the point, and the trainer holds it to 1 W |
| Steady endurance | Either | The target band is wide, and slope feels more like riding |
| VO2max intervals of 3 to 5 minutes | Either, with care | ERG keeps pulling the watts as your cadence falls, which is where the spiral starts |
| Sprints and efforts under a minute | Slope | ERG exists to stop power moving, and a sprint has to move |
| Ramp test | ERG, in absolute watts | The trainer sets each step, 100 W plus 20 W a minute in WattPlan, so the test can't be paced |
| 20-minute FTP test | Slope or free ride | The effort must be free to exceed any target, so WattPlan's 20-minute test block releases the trainer |
| Pacing practice | Slope | The skill is holding the number yourself |
Our view at WattPlan is that ERG belongs on sweet spot and threshold intervals, where a session is measured in minutes at a target, and that any test of what you can do has to be ridden with the trainer released. Sweet spot training gains most from ERG, because the whole value of the session is in the time at the target.
What is the ERG spiral of death and how do you escape it?
Wahoo's page gives the mechanism in a sentence, decreasing your cadence too much may leave the trainer applying more resistance than you can pedal through. You tire, your cadence drops, the trainer adds resistance to keep the watts at target, the heavier pedals slow you further, and within a few seconds you are grinding to a stop. It is a feedback loop with one input, your power, and the trainer cannot tell fatigue from a gear change.
To avoid it, keep your cadence up before the trainer reacts. Wahoo suggests the small chainring and a cog in the middle of the cassette, for a quiet chainline, and that gear also leaves you a shift in either direction. Pick your cadence at the start of the interval and hold it, because the trainer's job is the watts and yours is the rpm. If the target itself is too high, lower it rather than fighting it. WattPlan's player has an intensity control that moves every target in 5% steps, from 50% to 150% of the session, and keeps sending the adjusted target until the step ends, so the way out of a spiral is yours to take.
To escape one that has started, ease off the pedals for a few seconds, let the resistance drop, then spin back up to your normal cadence before you push again. If it happens twice in one session the target is wrong for today, so drop the intensity and finish the set.
When should you not use ERG mode?
Anything under a minute, the 20-minute FTP test, and any session where the point is to pace yourself, such as a rehearsal for a sportive climb, because ERG does the pacing for you.
ERG also changes what your speed and distance mean. Wahoo's page explains that in ERG the trainer holds the target at any wheel speed, so the speed recorded swings with the gear and cadence you choose and may not be accurate, and it offers an ERG Mode Speed Simulation setting that broadcasts a speed derived from your power instead. That is one reason indoor and outdoor power and speed don't line up, and our smart trainer guide covers what to compare instead.
Should you do an FTP test in ERG mode?
Not the 20-minute test. In WattPlan the test's effort block is a free ride, where the trainer lets go and you set the effort with your gears, and your FTP is 95% of the average you hold. A ramp test is the one test ERG suits, because each step is a number the trainer sets. WattPlan's ramp starts at 100 W and adds 20 W every minute, in absolute watts so a wrong FTP can't cap it, and the ride ends itself when your power stays more than 15 W under the target for 10 seconds. Your FTP is 75% of your best minute. The FTP calculator turns either result into zones, and if the test opens a build to a sportive, our event training plans cover UK events.
What to do this week
- Check which mode your app sends for each step, and ride sweet spot and threshold blocks in ERG.
- Set up in the small chainring and a middle cog, pick a cadence in the first 30 seconds of each interval and hold it.
- Ride sprints, race rehearsals and the 20-minute test with the trainer released.
- If a spiral starts, ease off for a few seconds, spin back up, and turn the intensity down if it happens twice.
- Retest your FTP only with the trainer released, then set your zones with the FTP calculator.
What we don't know yet
No peer-reviewed trial we found compares training adaptations in ERG with slope mode, so the session-by-mode table is coaching judgement rather than measured outcomes. The Bluetooth standard defines the commands, not how a trainer responds to them, so how fast a trainer reacts to a falling cadence and what it does in a stall are the manufacturer's choices and change with firmware. The accuracy study covered one trainer model, ten riders and 100 W to 270 W, which says nothing about sprint power or other trainers.
Sources
- Fitness Machine Service 1.0, Bluetooth SIG
- Trainer control modes for KICKR, CORE, SNAP, or BIKE in the Wahoo app, Wahoo Fitness Support
- The Accuracy and Reliability of the Power Measurements of the TACX Neo 2T Smart Trainer and Its Agreement against the Garmin Vector 3 Pedals, Journal of Functional Morphology and Kinesiology