This applies to Zwifting for interval training on a 1x MTB - might also be useful to some for road bikes.
Been experimenting with my 1x 29er on my Elite Suito for a few months. Frustrating without some lessons learned, especially when trying to ride intervals. Both my son and I struggled with this to the point of nearly giving up. We found two keys:
If your intervals are above about 350 watts, ensure your controllable trainer is paired using FE-C. Otherwise zwift can’t create resistance changes, and you will basically just have to make your own power - Zwift can’t tell your trainer to add resistance without FE-C control - may seem obvious to those who’ve already learned the lesson… but our pairings were all FE-C except for the controllable trainer (which was just ANT+) - and that doomed us until I tried FE-C. (This also explains why you don’t feel resistance changes on climbs).
If your trainer gets unpaired - be sure everyone knows how to repair (use FE-C).
If your intervals are below 350W, and you don’t want to feel gradient changes, then don’t use FE-C, or use the next lesson with FE-C (we just use FE-C all the time):
Before riding intervals, go to settings and slide the Trainer Difficulty slider all the way left (off). This will reduce the trainer torque changes dragging your legs to a stop.
How you begin each interval also matters - if you begin the interval (where power goes way up) at too low of a cadence, the torque change can drag your legs to a stop. Sometimes a downshift can help, but often it won’t… so begin the interval above 80 cadence and attack into it as you pass through the translucent wall - just like you’d begin a sprint - at the start point, go hard into the interval.
The greater your change of power from the rest to the interval, the harder you have to attack in order to counteract the torque change in the trainer - the trainer will apply the brake to ‘force’ you to ride at a set power - this is ‘Erg’ mode.
The companion app is very helpful for intervals - you can increase or decrease the power level by 10 percent using the ± keys on the companion app. If the torque drags you down, you can try reducing the interval rather than just abandoning it.
VO2 Max intervals are very difficult in Erg mode - because the intervals won’t follow your physiology - ie, a sprint at max that fades over time. You can try building a series of short intervals that ‘fade’ from high power to more naturally mimic reality.
If you build intervals using the slider, the minimum rest period is 30 seconds. We use 15 second rest periods for some blocks - which you can get by manually clicking on the rest period and typing into it.
Remember that your FTP matters - do the FTP test often enough to reflect your actual fitness – and remember that an FTP test is actually an interval - so it’s a workout!
Hope these help.