Bicycle trainer in ERG mode for a quieter training session on the older Wahoo Kickr
First some background. Per my understanding there are two modes that standard smart trainers support, ERG and Incline modes:
ERG mode is straight power control. If the smart trainer is set to 200 watts, then no matter how fast or slow you pedal the trainer will always keep the resistance as necessary to be 200 watts. Gear shifting makes no difference. So pick the smallest gear on your bike, so the trainer flywheel spins slower and makes less noise. In ERG mode you will often see numbers in Strava with ridiculous speeds and total miles per workout in comparison to the average wattage. This is because a person picked the biggest gear on their bike and was only pushing 100 watts. I don’t care how tiny you are you cannot do 30 mph at less than 100 watts.
Incline mode (called different things in different places) lets you set a fixed resistance usually set by incline %, weight, and a bunch of other stuff that would be calculated by your training app. The result is that like in real life if the back wheel moves faster, you are putting out more power given the same incline. So shifting will matter here.
The sound aspect of the information below supposedly only applies to the older trainers (Kickr models like my 1st gen). The newest models are supposed to be virtually silent. The observations on the freewheel effect should be consistent across any vintage.
I personally use a Wahoo Kickr in ERG mode, so I can control my power zones very specifically. I do not even have shifters on my trainer bike, as they are generally used to keep your pedaling within your cadence power band, which I can do by just keeping a set cadence on the trainer. I have it close to the lowest gear (smallest on front chainring and close to the biggest on the cassette in back) to make it as quiet as I can. As, per my understanding, the sound level is based on how fast the drive-train (pedals, chain, belt, flywheel) are going.
The one thing you will notice from this setup is the flywheel effect is much lower. If you lift weights, I am thinking a comparison of a cable machine (low speed flywheel) to free weights (high speed flywheel) is reasonably appropriate. The inertia of a fast lift with free weights makes it easier at the end of the movement. On a cable machine the effort required is more consistent over the movement. On the trainer, this effect is very much apparent when doing intervals. In a high gear, when the power drops from 120% to 60%, you will find it very difficult to reduce your pedal power without freewheeling the drive train. In a low gear these variations can be consistently controlled without freewheeling as the flywheel is spinning much slower, and the flywheel effect is less pronounced.
To find the right gear I start by setting the gear to the absolute lowest (which feels most like a pure resistance bike) then moved it up a gear at a time to ensure that I could keep pressure on the pedals throughout intervals, but the flywheel effect was just enough to make it feel more like riding a bike on the road. Given that the objective of the trainer is to give you a consistent pedal power effort and not like real cycling, where inertia of your body moving forward makes up for gaps in power of your pedal stroke. Also you generally don’t setup a trainer session profile to replicate go up a hill for 30 minutes then coast back down the other side without pedaling for 5 minutes… :)