If you have ever tried to hold exactly 250 watts by watching a number and easing on and off the pedals, you know it is a losing game. You overshoot on the downstroke, drift under when you stop concentrating, and by the end of a five-minute block your “steady” effort is a sawtooth. ERG mode removes that job entirely. You set the target once and the trainer holds it.
What ERG mode actually does
A smart trainer has a motor or a controllable magnetic brake, and it can read how hard you are pedalling. In ERG mode the app sends the trainer a single instruction: hold this many watts. The trainer then adjusts its own resistance, many times a second, to keep your power on that target no matter how fast or slow you spin.
Pedal faster and it eases off. Grind slower and it pushes back. The wattage stays flat because the machine, not you, is doing the correcting. That is the whole trick, and it is why a structured workout ridden in ERG looks like a clean staircase of power blocks instead of a fuzzy cloud.
The name is old. “ERG” comes from ergometer, the lab machine that holds a fixed workload so physiologists can measure oxygen use at a known power. A modern smart trainer in ERG mode is the same idea in your spare room.
Why it matters for structured training
A structured workout is a list of targets with durations: ten minutes at 150 watts to warm up, then four intervals of five minutes at 300 with three minutes easy between them. The point of the session is the dose — time spent in a specific zone. If your power wanders, the dose is wrong, and the adaptation you were training for gets diluted.
ERG mode makes the dose exact. When the app steps from the warm-up block to the first interval, it sends the new target and the trainer ramps the resistance up to meet it within a couple of seconds. You feel the floor come up under your legs. Hold your cadence and the watts are simply there. There is nothing to chase.
That also frees your attention. Instead of staring at a power number and micro-adjusting, you can watch your cadence, your heart rate, your breathing, or nothing at all. On a hard VO2max interval that mental space is worth real watts.
How it feels on the bike
The first ERG ride surprises most people in two ways.
The first is how quickly the resistance responds. Change gear on the road and the effort shifts. In ERG, gearing does almost nothing to the wattage: shift to a harder gear and the trainer quietly reduces resistance to keep the target, so your legs feel the same. Cadence is your only real lever, and even that only moves the feel, not the number.
The second is the recovery blocks. When the workout drops to 120 watts for a rest, the resistance vanishes and the pedals go light. After a brutal interval that can feel like the chain fell off. It is normal. The trainer is holding the low target just as faithfully as it held the high one.
The spiral of death, and how to stay out of it
ERG has one failure mode worth naming, because everyone meets it eventually. Near the end of a hard interval, if you let your cadence fall too far, the trainer has to raise resistance to keep the watts. The extra resistance drops your cadence further, which makes it raise resistance again, and within a few pedal strokes you are bogged down at 40 rpm and about to stall. Riders call it the spiral of death.
The fix is simple once you know it. As you fatigue, spin lighter and faster, not slower and harder. Keep your cadence up around 85 to 95 and the trainer has room to work. If a block is genuinely beyond you today, drop the intensity a notch rather than fighting the pedals into a stall.
ERG versus resistance versus grade
ERG is one of three ways a smart trainer can behave, and each has a job.
- ERG holds a fixed wattage. This is the mode for intervals and any workout where the target is a power number.
- Resistance sets a fixed brake level. Your power then follows your effort and gearing, like riding a single fixed hill. It is the mode for a free spin, a sprint, or a warm-up where you do not want the trainer chasing a target.
- Simulation, or grade, makes the trainer mimic a road gradient. A 6% climb feels like a 6% climb, and your gears matter again. This is what virtual-gear systems and route-based apps use.
Good training software lets you switch between them mid-ride. Structured work runs in ERG; when you want to open up a sprint at the end, you drop to resistance so the trainer stops capping your power.
What you need for ERG to work
ERG needs a trainer that can accept control commands, which means a smart trainer, not a classic wheel-on unit with a fixed resistance curve. Almost every current smart trainer speaks the Bluetooth FTMS standard, and any app that supports FTMS can put a compatible trainer into ERG mode. If your trainer pairs as a controllable device rather than just a power meter, you have ERG.
In CrankPilot, ERG is the default the moment you start a workout. The app requests control of the trainer, sends the target for the current interval, and updates that target every time the session steps to a new block. You can nudge the whole workout up or down in 5% steps without leaving the ride screen, and on a Zwift Ride the controller paddles do the same. When the interval ends, the next target goes out on its own.
That is the payoff of ERG mode: you pick a workout, and the machine rides the numbers so you can spend your attention on the effort.