Dumb Fluid Trainer Thermal Drift

We all know fluid trainers drift in resistance as they heat up, but does anyone have any idea approximately how much drift one might see across the length of a workout (in watts)?

I use a Travel Trac Comp Fluid and VirtualPower and I’m never sure if things start feeling easier around 20-30 minutes because my legs are finally fully warmed or because my trainer is getting hotter and losing resistance as I bring the power up.

I know on my fluid trainer if i do some SS intervals around 20 mins long, I end up doing +2-5 RPM more to hold the same power at the end of the interval.

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Why would that be important? Do you have a real power meter or are you working off of virtual power? I have an Elite Volano dumb fluid trainer, and it takes about 5-10 minutes for it to warm up. That’s usually the duration of the warm-up, so during the intervals the resistance curve is very stable for that trainer.

Mine is with a real PM

Then if I may ask, what is the issue? Is it that you don’t like that your gear/cadence shifts slightly until your trainer is warm? (I also use a proper power meter, so power is always accurate.

For me there is no issue at all with the change in RPM etc. I’m more than happy paying £80 for a cheap trainer. I’m just giving a response to the OP’s thread.

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Without knowing, we can only hope that TR used an appropriate warm up time before capturing their power / speed data that forms the foundation of their Virtual Power Curve for any trainer.

Most trainers will have different results from cold to full operation temp. So, if TR gave good warm up, then you are getting the most likely correlation to them when you get over the cold period.

Again, if true, it may mean the data in the early part of the workout is more likely to be different, and not ‘accurate’ than the data later in the workout once the trainer is hot.

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That cadence drift is a decent number to know, although gearing ratio will change how that all works out. I was just curious what kind of drift others see after the trainer is warmed up. The first 10-15 minutes are clearly different, but I’m never sure how much it drifts after the fluid is relatively warm and I’m in the main bulk of the workout. If that’s typically ~10W or less…whatever. If it’s like 30-40W, then I should maybe start riding over target a bit later in intervals or workouts to maintain the desired zone. A power meter is on the list for me for this year, but until then, I’m living the Virtual Power life.

I guess another way of stating the question would have been “How stable is the resistance on a typical dumb trainer once warmed up?”

That is highly trainer dependent, but I would suspect pretty stable overall. Once you get your typical warm up done, it is “hot” and should hold around that resistance level. Some trainers may tend to drift a bit more if the workout has lengthy highs or lows.

The Kinetic Road Machined claimed to have a temp neutral fluid, but my use with a PowerTap hub showed the need to change gearing over time to hit the same power target. Other trainers may be more or less susceptible to drift, with the particular fluid and trainer design in use.

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Could you test your specific trainer for us and let us know?

You can get on your trainer at ambient temp then start pedalling at a constant rpm in a constant gear to achieve a constant power output and hold that rpm while watching the power output drop over time (as the trainer heats up). Once you notice that the power output is no longer changing you can stop.

Let us know what the wattage difference is and even post the cadence vs power output trend for us.

Not sure he can really do that. He seems to be using Virtual Power only, with no additional power meter.

So he really has nothing to quantitative compare. And I highly doubt that TR has input time sensitive power data into the VP control. It is all dictated by wheel speed.

The only thing that will change if he starts with and maintains a gear and cadence, is the RPE. It is potentially useful, but also highly subjective. That’s because wheel speed is a direct product of the cadence input and the bike gearing. And wheel speed, multiplied via the TR VP curve is the way it calculates the VP value.

Suffice to say, that there is most likely a change in the resistance with heat build up, but without a real power meter, there’s not much to learn beyond that.

This is highly anecdotal, but my experience on my Cyclops Fluid 2 is that between 5-10 minutes the trainer will slip into significantly more resistance. It usually kicks in on the rest interval after the warm up. So I’ll be running at 135w and then I’ll feel the resistance increase and if I were to keep the same cadence and gearing, I’d now be doing 185w. Two curious things about my trainer are that the resistance doesn’t seem to slope, it seems to have a ‘cold’ resistance and a ‘hot’ resistance and that transition happens very quickly, and secondly that when it gets hot the change is an increase in resistance. I would have expected the fluid to become less viscous with heat and lower the resistance. Again, I doubt my experience is translatable to other people’s trainers.

I also run a Cyclops Fluid 2. Here’s a link to an example 3x20 workout. Most of the change happens during warmup. But you can see on the first interval my trainer is still warming up. It drops me from about 95 to about 86 on the first interval. The trainer does cool off in the rest periods. For the second and third interval you still see some drop. 90 - 85 and 89 - 85 respectively.

This is pretty common behavior I see in all my workouts.

https://www.trainerroad.com/career/slis/rides/64944716-round-bald

Once it is warmed up, the resistance level is extremely stable on my Elite Volano. I did not notice any substantial changes after the unit has warmed up. In case of my trainer, the fan is driven by the flywheel, so fan and flywheel speed are locked. I suspect this means that the unit has a relatively stable steady state temperature, which depends more or less on your average power over a longer period of time (because all the metal parts absorb a lot of heat, and it takes time for them to appreciably change temperature).

You’re totally correct. It’s impossible without a power meter. I’m curious now though to test my kurt kinetic with my power pedals.

I use the kurt kinetic. I notice, to hold the same power, a 1-2rpm increase later in the workout. However, if I am doing intervals w longer breaks, 7+minutes, it seems to go back to the original rpm.

not exact, but 1 rpm increase is around 7-8 watts difference.

I know this is an old thread, but just recently had an interesting example of thermal drift.
This is 11 min’s of a ramp test, Fluid 2 trainer, Quarq power meter. I shifted once, but shifted back down in a few seconds - so same gear for the 11 min.

The ramp test matched the trainer’s power curve +/- 2 rpm.
133 watt drift in 11 min.


To mark under ‘interesting’ rather than useful probably, but I’m on rollers with a power meter and this morning during the warmup had to go from 85 to 92 cadence in 5 minutes to keep the same power as the tyres warmed up. Adding this into the modeling of top of the fluid in the trainer (and of course using an average as different tyres would give different resistances and warm up differently) would be a challenge.

I’d recommend whatever the cheapest power meter you can get if you want to know the real answer to what you’re putting out, as anything else will be just a best guess.

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I’m very confused by this.

Are you saying that you got on the trainer, pedals at a flat cadence, never shifted, and that the trainer, over the course of 11min, increased reported watts by 133?

Because one, typically when they heat up, the fluid viscosity decreases (becomes thinner.) You would expect less resistance as it heats up, not more.

Two, are you saying that the increase just happened to perfectly match the ramp? That makes little sense.

I am probably just interpreting your point wrong, but I’m very confused. Thermal drift typically means the trainer becomes a little bit easier once it warms up.

EDIT: Interesting… I found this thread on Slowtwitch discussing the warmup period of the Fluid 2. For one, it does seem like that trainer gets harder as it warms, not easier. Two, people are discussing that it’s a super dramatic switch, not gradual. Thirdly, that it’s on the order of 30-45 W.

So what’s the takeaway here? I have no idea. What an odd behaving trainer.

EDIT 2: Oh but wait, there’s more. @Jonathan wrote about the Fluid 2 equation back in 2011. They were talking about virtual power (not relevant since you’re using actual PM), but the comments are really interesting:

Erwin, when I am warming up at a (zone 2) effort I can “feel” my fluid 2 trainer make that jump from not being warmed up to, yep it is warm now! To me it feels as if I were to just shift into the next gear. I would think that it would be the same for all the fluid trainer 2’s. This seems to happen right at the 5 minute mark every time no matter my speed of the warm up.

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I have a Jetblack Whisperdrive (dumb direct drive magnetic resistance) which I use for TR with a Quarq power meter. My speed increases quite a bit from the beginning to the end of a session for the same power.

You can see speed and cadence trend upwards within an interval. Im never on the trainer for more than an hour, but even the last interval in an hour session will show about the same steady increase as earlier intervals

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