Fwiw… I have two fluid trainers and used them with power meter. Kinetic Road Machine is rock stable. Cycleops one got a very clearly perceivable ramp, it just chandes resistance by quite a bit maybe 10-15 minutes in. Not that it matters when using external power meter. But using virtual power I’d definitely do a spindown test 15 min into the workout on cycleops one. Kinetic, probably won’t matter much.
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While I haven’t had to test many dumb fluid trainers in recent years, all my past testing said that virtually everyone stabilized by about 12-15 mins. Some a bit faster around 8-10 mins, but all by 15 mins. Keeping in mind that the intensity (and thus speed) of the warm-up would impact it.
Ironically, the trainer (non-dumb) that I found over years upon years of testing (and multiple units) that had the longest warm-up time was the CompuTrainer. It mostly stabilized around the 15 minute marker, but I saw many cases where it took into about 18-20 mins to finally reach stabilization.
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Sort of. During the ramp test, from 9:24 till failure I was in the same gear. To make each step i’d up the cadence by 1-3 rpm. By the end of the step i’d lose that cadence increase. I went 11 min in the same gear, almost the same cadence, and the ramp test matched the trainers power slope. FYI, I have had 4 different Cyclopes fluid 2’s over the last 4 years (lifetime warranty), they all have had different slops and warm up different. This particular one is the worst. It will continue to increase resistance on 90 min rides.
Nope - its the metal that heats up, decreasing the clearance the fluid is forced thru.
Bingo. Crazy right?
Or perhaps my understanding of thermal drift is wrong. Or maybe it goes both ways?..
My take away, if you train on a Cyclopes fluid two you really need a power meter. I used VP for a few months when I first got the F2, and it was pointless.
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Fluid 2 user here. Have always experienced the huge jump in resistance after a certain period of time. And resistance decreases back down during rest intervals. So virtual power was virtually 
useless and caused a lot of angst before I realized I wasn’t just dying halfway into an interval. Went to P1 pedals once I realized this.
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Yeah I spent like 20 minutes on a rabbit hole of Google last night just for my own curioisty, and the takeaway seemed to be: The Fluid 2 behaves really weirdly compared to a lot of other fluid trainers. Not necessarily ‘badly,’ just atypically. Lot of people also reported a sudden jump in resistance. And yeah, that’s opposite of most trainers which become a bit easier as they warm up and the fluid thins out. Like you said, it seems like a good trainer to have a PM on, for sure. One of my friends just got setup with a mag trainer on virtual power. She’s also having a bunch of problems. That said, I’m generally really happy with my Kurt, and I like fluid in general. My upgrade path will be power pedals first, then a DD smart trainer.
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I’ve got a Tacx Satori Smart, which is still a dumb trainer but with an inRide type sensor. It’s a mag trainer but it also sees a thermal drift across workouts and intervals. I used to be able to crush the ends of workouts and now I realize after getting a staged that it’s because at the end I was no longer doing the prescribed wattages. It also cools down very fast so doing something like 6x3min intervals I experience the drift over each one. If I stayed in the same gear my cadence would probably rise about 10rpm from beginning to end of each interval.
On my Elite Volano, I can definitely tell that it is a tad more sensitive than that. Depending on the average power over the last few minutes, I’d get a slight drift. Usually, that means I need to up my cadence by 3-8 rpm depending on the intensity. In case the average power stays constant (e. g. 30 on-30 off or when doing long, sweet spot or threshold efforts), the resistance remains more or less consistent throughout the interval, though.
I have the same trainer and I use a staged non drive side power meter and my experience is exactly the same as yours. As the fluid heats up it gradually increases resistance. It tends to decrease resistance back to “normal” after a few minutes or if I back pedals for a second or two.
I’m using a fluid 2 as well - for kicks and giggles I put a temperature probe on it. It has serious temperature drift. I realised this after I did a 3x9 minute threshold ride on a peleton bike - I held the same speed for all three intervals and my heart rate was consistent for all three. By comparison on my fluid 2 by the third interval I’m spinning 2 or 3 pedal rpm slower and my heart rate is up 3 to 5 BPM. The fluid 2 will be about 60 degrees at the start and will warm up to 90 to 105 by the end. Even an increase of 10 to 15 degrees from 85 to 100 will result in greater resistance. It’s too bad they don’t tell you it isn’t steady. Are mag trainers more consistent?