The short answer to this question is simple: Neither. Use ERG mode in your Tacx or Wahoo app and follow the VODs or Live Sessions ;)

If you're on Zwift or TrainerRoad for a weekend session or for some reason can't access a VOD, read on for thoughts from Matt on ERG vs. Free Ride:

Technology is here and in the endurance training space, it has certainly accelerated over the last few years.  One of the most interesting and useful additions for a time-starved athlete (like you) is the explosion of the smart-trainer and various supporting app options out there (Zwift, Trainerroad, Wahoo, Sufferfest etc).

The ability to deliver and execute high-value indoor sessions not only provides a focused training session without traffic, but can dramatically reduce the amount of time commitment for the sport. How many busy and time-starved athletes really have two-to-three hours mid-week to ride their bike?  There is no doubt that the trainer+app combo is a net positive, and why we embrace structured workouts delivered to your home and even plan to produce Purple Patch classes to your trainer and home over the coming year or so.

With this said, we still find ourselves in a learning curve of how best to apply and utilize the trainer+app.  When first becoming popular, the common cry was that, finally, we could dial in specificity. The combo provided an ability to hit the exact wattage to create a stimulus, as though we are robots. It took a few years, but now we realize that it is complete nonsense, as we cannot possibly predict physiological shifts and identify what is the exact fixed wattage we should ride on any given day.  As we learn more and more of the correlation between power (the measure of work done), heart rate (metabolic stress) and perceived effort (how it feels), it becomes apparent that not all sessions are equal, and different types of work might be best executed with a different trainer set up.

The main options of riding on a trainer can be labeled ERG mode and FREE-RIDE.  Most smart trainers generally provide both options to set up your rides.  Let’s look at ERG mode versus FREE-RIDE.

ERG MODE:  Set it and forget it.  This mode fixes the resistance against the rider, no matter what cadence they are riding.  For structured workouts, this means that the pre-programmed resistance (wattage) is set, and no matter what cadence the rider goes to, the trainer will adjust to keep the rider exactly at the wattage.  This is great for the auto-pilot riding, as the trainer is doing all the ‘thinking.’ The challenge is that the only measurement the trainer is managing is the power - it doesn’t care what the associated heart rate stress or perceived effort is.

FREE-RIDE:  In the FREE-RIDE mode, the rider will use gears and cadence to generate the power (load) just as one would outside.  With apps such as ZWIFT that mimic terrain and grades, this becomes similar in course management to outside riding. The benefits are obvious to this style of riding, but it does lose the ‘set it and forget it’ gift, and also is tougher to find a true steady-state load/resistance for extended intervals.  If you are riding on a ‘terrain’ focused app, such as ZWIFT, and ‘ride’ upon a 90 second descend, you will be unable to maintain power or requested cadence.

For your Purple Patch riding, both modes are of value and should be utilized.  When utilizing a trainer, there are some sessions that are beneficial to have a fixed load relative to prescription (then managed with the awareness of stress and shift the percentage (%) of prescription to manage harder or easier load as appropriate). Other sessions simply cannot be effectively nailed within ERG mode and are preferable to be executed in a FREE-RIDE format.

At the upcoming training center, we have complete rides and sessions that hit a combination of both ERG mode, incidentally, with a 3% swing either side of prescription to allow give, and also while retaining the need to shift gears, but also FREE RIDE.  That is our special sauce and magic in the center, but not easy to do at home yet.

To go deeper, here is a good article from ZWIFT Insider on shifting between ERG mode and FREE RIDE mode.  You will likely find it beneficial. 

Below is a sprinkle of sessions that I like to see in both modes:

ERG MODE:

1. Extended steady-state intervals at a fixed cadence (RPM)
2. Most SE work (including pyramids)
3. Specific Intervals at Z3 to low Z5

FREE-RIDE:

1. Max Effort and Z5+ Intervals
2. Endurance Ride over variable terrain (including integrated SE)
3. Recovery Ride
4. PREP Rides

I don’t get overly fixed on making life hard, but the single session I believe can only be effective in FREE RIDE are the short-intervals at Z5 or Max effort.

I hope that helps.

Matt