If you put your workout in ERG mode, that means Zwift will control the trainers wattage regardless of RPM. That is a great way to stay of target wattage in Build Me Up (BMU). I did the plan twice last winter, and it greatly helped with FTP increase and cadence IRL.
Basically if you do 200W at 70RPM, and you up the RPM to 90, than your wattage will go up too (same gear + higher RPM = higher output power). Basically in that case if you should keep the same 200W but on 90RPM instead of 70RPM, you have two choices:
- in SIM mode (when Zwift doesn’t control your trainer) change to easier gear to balance out the higher RPM (so change to bigger cog at the rear)
- in ERG mode you have to do nothing. Zwift will tells to your trainer to give less resistance.
In the second case (ERG mode) you’ll see your wattage increase initially when you jkump from 70 to 90, but within 5-10 seconds (probably tad more, I’m not familiar with the Direto) you’ll see your watts “drop back” to 200W while keeping at 90RPM.
Please note that NO CYCLIST CAN KEEP exactly 90RPM. As your cadence varies between 87-93, you’ll see fluctuations in wattage because when your RPM drops below 90RPM, Zwift increases the resistance on your trainer, and when you spin up and overshoot to 93RPM, Zwift will decrease resistance (needs a few seconds to happen, as mentioned).
When somebody says that his trainer stays exact wattage when his cadence goes around 90RPM (in the example 87-93) that’s simply NOT TRUE, the trainer simply smoothing out the numbers what Zwift recieves. Typically Wahho trainers were notorious for that, but in the later firmware they changed them so they aren’t smoothing that much.
You can see the effect even during warm up (jump to 28th minute) in my video of BMU’s Amalgam workout in the video:
THe RPM and wattage reported by Power2Max NGEco power meter, and the smart trainer is a Neo 1st generation.