Zwift CP20 Test: Fluctuating Resistance in Minute 1

Did the Zwift CP20 test and in the first minute the turbo resistance fluctuated and this caused considerable variation in cadence as I tried to maintain target power - unsuccessfully as the graphic shows the swings in power achieved. After a minute things settled down and power and cadence levelled off to much more stable values. I use smart turbo (Elite Direto) and had warmed up properly so I don’t think this issue was caused by myself not maintaining steady power in the first minute.

I know that for the CP20 test Zwift automatically turns off Erg mode and at first I wondered whether these swings were caused by differences in the gradient on the course. I doubt that this is correct as power and cadence were so steady for the rest of the test.Seemed more likely that the issue was in some way caused by interaction between Zwift disabling Erg mode and the turbo. Anyone else seen this and any comments?

I’m not sure I’m following what you’re saying.

I assume by CP20, you’re talking about one of the FTP tests? I’ve done several of them with my Direto and never had any problems with resistance fluctuations.

When the 20 min test begins, ERG is disabled. Generally speaking, it’s a good idea to keep in your small chainring while in the ERG portion before the test - then shift up to the big ring and start spinning up the gear in the final seconds before the ‘test portion’ begins. That way, things are fairly settled cadence and power-wise as soon as the 20 min test time starts.

If you’re experiencing issues with wild cadence swings and corresponding power/resistance problems? I’d (almost) be willing to bet that you are trying to use the Direto’s built-in cadence ‘sensor’.

If so, stop and buy a dedicated external cadence sensor. Direto doesn’t really have a direct way of measuring cadence. The RPM value it spits out is based on fluctuations in power output measured by the OTS as you move through your pedal stroke. It is notoriously inaccurate - especially at low-resistance levels (as you might experience immediately after ERG drops off if you’re then spinning quickly in a low gear). It tends to be more stable and accurate (or, at least, less problematic) as resistancce increases - but it can still jump around at times. For this reason, it’s basically useless in workouts with a prescribed cadence range - and I could also see it causing the problem you experienced,

If you are already using an external cadence sensor (e.g. wahoo, Garmin, or similar)? Then I don’t know what to tell you!

Thanks for the comments. Yes for CP20 read FTP.

Was already in big ring when test started.

Not sure that the Direto cadence “sensor” will be the explanation, but I might be wrong. I had assumed that when Zwift deactivates Erg mode no further signals would be sent to the turbo hence the resistance at the crank would remain constant and changes in power will be entirely down to the rider varying the cadence and gear (assuming constant force on the pedal). Presumably if Erg is off, then Zwift would not try to maintain power at a given level as it would if Erg were on for a workout specifying a target power. So if Erg is off, I would guess that turbo resistance would not change in response to changes in cadence/gear- it would not be possible to do a reliable FTP test if Zwift were varying the turbo resistance. What I found odd was that in the first minute of the test the resistance would drop for a bit and as a consequence I would have to increase cadence in an attempt to maintain my target power. In the graphic cadence is shown in yellow, power in pink. If the crank resistance were constant then increasing cadence should result in increasing power (assuming neither the gear nor the force applied to the pedal had changed). However the graphic shows that the power dropped when the cadence increased which I would imagine can only mean a drop in turbo resistance.

Interested to hear that you have not had any problems like this, which was one of the reasons to post this in case this was a recognised problem.