Unachievable watts during workout

I have a Wahoo Kickr Core, and I just did my first Zwift workout (Jon’s Mix) with ERG mode enabled. I found that the wattage levels were not possible. The first workout segment went up to 280 watts, which required all of my weight on one pedal to inch it forward. The next segment went up to 480 watts–totally impossible unless I weighed 900 lbs. I couldn’t budget the pedal at all.

I’m sure this has been asked before (and I did search the forums first before asking), but what’s wrong here? How do I make the wattage for workouts reasonable?

Workouts are based on % of your FTP. In Zwift worlds screen click on the workouts, in there at the bottom right it should show what Zwift has your FTP set to. Check to make sure it isn’t something crazy.

Have you done any of the FTP tests within Zwift? If not I’d suggest doing one before workouts as stated above the workouts use your FTP to set the power limits.

1 Like

My FTP was set to 187, which is a little high–I think 168 is more accurate (based on an FTP assessment in The Sufferfest). But it wasn’t crazy high–not enough to explain the impossibility of a 480 watt output.

Seems about right (power wise) for the workout. It has a few anaerobic sections that are over 200% FTP. Might be your aerobic system is stronger than your anaerobic and working on sprints and strength training can help out there.

Also in workouts when in ERG mode make sure to keep your cadence up (90rpm or more) as when the trainer applies more resistance and your cadence dips if you don’t recover the trainer will keep applying more power causing your cadence to continue decreasing. It’s know as the spiral of death. If you know you have a high power section coming up it’s a good idea to increase your cadence by at least 10rpm about 5-10 seconds before the new segment.

Workout based on your initial FTP of 187

Workout by % of FTP

1 Like

I’ll try it with increased cadence next time now that I know it’s coming. But this workout is supposed to be physically possible, right? It really doesn’t seem like it. Nothing in the world could have moved a pedal in the 480 watts segment. I had to pause the workout just to start riding again.

That’s probably because of the ERG mode “death spiral” that the others referred to.

It takes a bit of getting used to because it’s the opposite of what you feel riding outside. Outside, if you reduce your cadence then it’s easier to pedal, and similarly increasing cadence makes it harder. But in ERG mode, if you reduce cadence then the trainer increases resistance so that you’re producing the same power (otherwise lower cadence means less power).

So as you feel resistance increasing, there’s a natural tendency to decrease cadence. But if you do that the resistance will increase further, and you can get into the “death spiral” and your cadence keeps reducing and the resistance increases to the point that you can’t turn the pedals.

So keep a steady cadence the whole time. Don’t change gear. Start off on the small chainring at the front, and roughly the middle of the cassette at the rear - that typically makes it a bit easier to deal with the changes of resistance.

Good luck!

4 Likes

Thanks for this feedback. I will give it another try!

Thanks for posting such an interesting question. Made me stop and think about how ERG mode works.

The formula for Power is Kilowatts = Torque (Newton Meters) x Speed ( RPM ) / 9.5488

Assuming the Speed (pedalling cadence RPM) is 100, to generate 400 Watts (0.4 Kw) you would need to apply 5.8 Newton Meters of force to the pedals. Assuming a crank length of 170 mm, that means you’d need to apply roughly 22.4 Newtons of force to each pedal. The application of the force gets a bit tricky, since it involves accelerations, but its roughly equal to putting 22 KG of weight on the pedal when it’s in the drive position. That’s at a cadence of 100 RPM.

Drop the cadence down to 50, and the amount of force necessary to generate 400 watts doubles. Now you need to put almost 45 kg of force on each pedal. Go down further, to 5 RPM, and you’d need to apply 400 kg of force. Truly, as you said, an impossible amount of force for a human to apply. (In the mechanical engineering world, we’d call that a stalled motor… :sunglasses:)

I’m sure there is some lag and fuzzy logic built into the ERG system in Zwift and the way it works on Wahoo trainers. Keep the cadence high if you want to make the big watts, especially in ERG mode.

2 Likes

I’m not very adept at physics but isn’t Nm to kg ratio at about 10 to 1?

Once more confirming the death spiral. My FTP is 220 and I can sustain 340 for a minute in a workout - but it’s much easier when keeping the cadence high at 100+.

Many workouts even suggest high cadence. However I did a workout at 55 cadence once… And while I could do it, my knees weren’t happy. I can go down to 60-65 and no more, too much stress on the knees.

I really appreciate this mathematical breakdown!