ERG mode feels impossible when your cadence falls, because the trainer holds the watts by raising resistance. Power is torque times pedalling speed, so at 250 W you push about 40 N·m at 60 rpm but only about 24 N·m at 100 rpm. TrainerRoad’s support site advises a constant cadence between 80 and 100 rpm unless the workout says otherwise, and ramping your cadence up slowly before an interval starts. If it still feels wrong, Zwift’s ordered checklist (firmware, calibration, connection) is below.
The arithmetic behind it
Torque at the pedals (N·m) equals power in watts divided by angular speed in radians per second, which is rpm times 2π divided by 60. The table is that division, rounded to two decimals; it is physics, not a trainer measurement.
| Target power | 60 rpm | 70 rpm | 80 rpm | 90 rpm | 100 rpm |
|---|---|---|---|---|---|
| 150 W | 23.87 N·m | 20.46 N·m | 17.90 N·m | 15.92 N·m | 14.32 N·m |
| 250 W | 39.79 N·m | 34.10 N·m | 29.84 N·m | 26.53 N·m | 23.87 N·m |
| 350 W | 55.70 N·m | 47.75 N·m | 41.78 N·m | 37.14 N·m | 33.42 N·m |
Read down any column to see what a harder target costs at a given cadence, and across a row to see what slowing down costs. At any power, 60 rpm needs about 67% more torque than 100 rpm (100 divided by 60). That is why a block that is manageable at 90 rpm can feel like a wall at 65: if you slow down, the trainer cannot lower the watts, so it raises the torque. Riders call the runaway version the “spiral of death”. It is a description of this effect, not a fault in the trainer.
What the apps say
- TrainerRoad: its page says the trainer lowers resistance when you raise cadence and raises it when you lower cadence, that changes of more than about 10 to 15 rpm are the noticeable ones, that best practice is a constant 80 to 100 rpm, and that you should slowly ramp cadence before an interval to the cadence you want to hold. It also describes Resistance mode and Standard mode, both of which do not hold a watt target automatically.
- Zwift: if you stop pedalling during a workout, Zwift disengages ERG until you hold the target effort for 5 to 10 seconds. Zwift also tells riders to stay in a neutral gear and avoid shifting during ERG.
Fix order
- Raise your cadence before the interval, not during. Use the table: move from 65 to 90 rpm and the torque falls by more than a quarter at the same watts.
- Stop shifting. Zwift says to remain in a neutral gear in ERG and avoid shifting; TrainerRoad says to set a mid-cassette gear and keep it, because changing gears can briefly make resistance uneven.
- Check the target. If the interval is hard at 90 rpm on a day when you are well rested, the problem is the target, not ERG; see FTP testing at home.
- Update trainer firmware through the maker’s app (Zwift lists trainer firmware first).
- Calibrate with a spindown through Zwift or the trainer’s app: calibration steps.
- Check the connection. Power, resistance and cadence should all use the same method, not a mix of ANT+ and Bluetooth, and no second app should hold the trainer. If it drops, use the disconnecting fix order.
- Avoid ANT+ protocol mixing. Zwift says not to pair power and controllable resistance to different ANT+ protocols.
- Reinstall the Companion app if you use it as a Bluetooth bridge.
Steps 4 to 8 follow Zwift’s “ERG Mode Troubleshooting” order (Zwift lists gearing last; it is step 2 here because it costs nothing to try). Step 1 comes from TrainerRoad’s cadence advice and the arithmetic above, and step 3 is ours.
If you still want out of it
TrainerRoad lists Resistance and Standard modes as alternatives that let you shift gears yourself. For races and group rides, free mode is the norm. The cooling fan guide helps with the other reason sessions feel harder indoors.
What this page cannot tell you
We did not find a vendor page that states a cadence at which the “spiral” starts, so none is given. Individual settings in each app (ramp rate, smoothing, flex) vary by app and version; look in the app’s own settings for the trainer.
How this page was researched
Compiled from TrainerRoad’s and Zwift’s support articles (Sources), read 2 October 2026, plus arithmetic shown above. SmartBikeWiki runs no tests.