2x20 FTP interval resistance too low

I have a Tacx Neo Smart that I’ve been using with Zwift for the past 9 months with no issues, for both workouts (in erg mode) and just ride.

Yesterday, I rode the 2x15 FTP interval workout and everything worked fine.  During the FTP interval, resistance (erg mode) was “normal” so I would hit FTP level in my normal cadence (upper 70’s to mid 80’s).

Today, I rode the 2x20 FTP workout.  Everything was fine during the warmup but during the FTP interval, resistance was set very low so that I had to pedal 100-110 rpm to reach FTP level.   That was no fun and I stopped after 10 minutes.

Does the 2x20 FTP normally set the resistance super low to require a high cadence?  It seems strange that 2x15 FTP was fine and 2x20 FTP was not.

Is there something I can do to get it to give me resistance such that I can hit FTP with my normal cadence?

Thanks.

Zwift should definitely be controlling it such that your smart trainer holds you at the appropriate wattage.   You’re controlling over bluetooth?   We’ll test your configuration here at Zwift HQ today and see if we can reproduce the issue.

Thanks, Jon.  Yes, I’m controlling over Bluetooth to my Macbook Air.  I also have Mobile Link running.  But the devices are bound to the Mac, not the iPhone.  I’m happy to provide additional info or whatever you would need.  Here is a screen shot of the Strava analysis screen for the first 20 minute FTP interval.  You can see cadence in the >100 range.
I did a short Just Ride after ending the workout to see if things were behaving properly again, and they were.   Thanks, Brad

 

Oh, I should say that I quit after the first 10 minutes of the 20 minute interval.

 

From that graph you posted it does look like ERG was working since it looks very steady.

When ERG mode is enabled you should be able to pedal at whatever speed you want in almost any gear you want and the wattage will adjust over a few seconds to match the interval.   Are you sure if you slowed to 90rpm it wouldn’t have adjusted over the next 5-10 seconds ?  

My recollection is that I had to pedal at that speed to keep the watts up.  If I slowed down, watts would drop.  Indeed as the interval went on, it felt like resistance decreased further and my cadence trended up to maintain the watt level. You can see that in the graph – increasing rpm, steady wattage.

I know right after the warmup, at the beginning of the interval, resistance dropped quite significantly and I had to greatly increase rpm to get to FTP wattage.

But I can’t say I’m sure that if I slowed to 90 rpm after a few minutes that it would have adjusted.  I don’t think it did but I can’t tell you with 100% certainly after the ride.  I know that I didn’t have any such issues yesterday with the 2x15 FTP.

Jon, I was bugged by what happened earlier today with the 2x20 FTP workout so I re-did the 2x20 FTP workout just now to see if I could repro the problem.

The resistance kicked in as normal during the interval.  It was definitely very different from the effort earlier today where the resistance was very light and non responsive to reduced cadence.

Now you can see that with normal cadence I was at FTP.  And when I gave it an FTP bias, resistance increased slightly (or more rpm needed).

After this second effort, I can confidently say that in the first effort, if I slowed to normal rpm, it did not increase the resistance.   In this second effort, I played around a little with cadence to see if resistance responded, and it did.

If the problem happens again, is there some data I should capture for you?

Thanks, Brad