We’ve changed the formula for this trainer twice now based on feedback from users so I thought I’d write a little bit on why we’ve changed it and how good we think the new formula is.
The first version was based on a ride I did with my Powertap. Other users with power meters reported that they were getting numbers much higher from their power meter than what VirtualPower was displaying.
So we changed the formula to one published by Kurt Kinetic here: Kurt Calibration chart. It matched up pretty well with a sample ride from a user using a Quarq power meter. You can see in the chart below how the purple and blue curves are pretty close up to about 17 MPH, which is how I was fooled into thinking it was a good fit.
However, people commented that the power they were seeing for faster speeds seemed way too high. So I plotted out a few of the curves and it became pretty apparent that the formula from Kurt was way off above about 17 MPH.
Someone pointed me to the CycleOps website and their power graph for the trainer. The resolution is pretty low but by sampling some points I was able to derive a formula that matched their curve pretty closely. And as one more source, I plotted the formula in the SportTracks Trainer Power plugin. The curve I came up with was really close to the SportTrack plugin.
So last night we updated the formula to match the SportTracks plugin. We created a new profile called (Beta) CycleOps Fluid 2 v2 and left the old one. New users will see it as (deprecated) CycleOps Fluid 2 and users that already have that trainer will still see (Beta) CycleOps Fluid 2. In a future release, we’ll fix it to update the name.
Update (2/19/13): We got rid of the deprecated one so now there is only one. Look for (Beta) CycleOps Fluid 2
Here are the watts for given speeds
Model | 5 MPH | 10 MPH | 15 MPH | 20 MPH | 25 MPH | 30 MPH | 35 MPH | |
(Beta) CycleOps Fluid 2 v2 | 43 | 91 | 161 | 259 | 393 | 569 | 794 | |
Fluid 2:Powertap ride | -2 | 37 | 112 | 221 | 361 | 531 | 726 | |
(Beta) CycleOps Fluid 2 –deprecated | 10 | 54 | 169 | 388 | 748 | 1283 | 2028 | |
Fluid 2:Quarq ride | 45 | 76 | 176 | 319 | 477 | 624 | 735 |
I’m confident that the new formula is better, but I’m not confident that you’ll get accurate power numbers with the Fluid 2. If you are consistent with your set up (same tires, PSI, and trainer tension) and base your FTP on the results from an 8 minute or 20 minute test using the new trainer profile, I think you can still use it to train effectively with TrainerRoad.
We’ve also had reports that the new formula shows higher numbers than the iBike, but I believe the iBike is essentially doing what we are doing (converting speed to power) so I don’t want to try to calibrate to that. I haven’t found the formula that the iBike uses to calculate speed on indoor trainers or anything on how accurate it is.
We have aspirations to create something like this to do some calibrations of our own, which should help us verify our formulas. But even that won’t help if the power curve varies from trainer to trainer or external variables like tire, PSI, and tension on tire have such a large effect on the curve that they can’t be canceled out with consistent set up. Notice how the Powertap ride curve is pretty much shaped like the new formula but the curve from the Quarq ride is shaped quite differently so even a simple offset wouldn’t help there.
If you have any theories on how to get more accuracy for this trainer or explanations on why we aren’t seeing more consistent results, post them to the comments.
We’ll continue to work towards getting power data as accurate as possible with VirtualPower on TrainerRoad.
Thanks to blueraidermike and IronCroptop for your help and feedback on this issue.
-Reid
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Doesn’t the power curve change as the fluid warms? How is this taken into account?
In our tests we threw out the first 10 minutes and derived the formula from data when the trainer was warmed up. But if it changes temperature quickly with bigger efforts or cools significantly while it is being ridden and those changes have a big effect on the power curve I don’t know how we could account for that without more data about the temperature of the trainer. We also don’t know the variance from trainer to trainer coming off the production line and whether the power curve changes over time or use.
Scott, the fluid2 might be full of shit. We have one and are going to do some testing.
We are working on a roll down calibration that would take in account heat fluctuation. The way we want it is during your workout you would spin up to like 20 mph and let it roll down. We’d time how long that took and then apply a offset to take in account heat.
We aren’t sure if ANT+ is accurate enough to do this..we are still trying. But if it does work, we should be able to correct for heat.
That’s cool. I just know from doing another cycle program on the trainer where I’d plug the speeds into a spreadsheet to find the power (I’m sure similarly to what TrainerRoad is doing instantaneously) that the creator of the program said to make sure the fluid was warmed up for 10 mins or so to get accurate results. That’s why I ask. I’m glad you guys are taking that into account as well. And if you can’t work it out because of too many variables, that’s okay too because TR is a pretty bad-ass program!
Would you recommend retesting FTP if I update my power source from the old Fluid 2 formula to the new one? My previous FTP was determined during the 8 min. test.
If you tell me which ride it is, I’ll look at it and give you your ftp from the ride using the new formula
Here’s the link to the ride: http://www.trainerroad.com/cycling/rides/1915
Thanks a lot.
Try setting your FTP to 189 using the new formula. The new avg power for the first 8 min interval would be 210.
For anybody else, let me know what your test ride is and I’ll convert the avg power for your test interval into what it would be for the new formula and calculate your FTP from that. In the case of the 8 min test, it is 90%.
Reid – your formula for the fluid 2 must be working – it is dead on with my quarq. Thanks again. Skip
Svschott – What is your tire psi? and how tight is the wheel on the trainer? thanks
looks like someone else tried to do the same thing. http://thebikegeek.blogspot.com/2009/12/while-we-wait-for-better-and-better.html
numbers look like they largely match up
I thought the virtual power form for the fluid2 was:
((0.0115*(a*a*a)) – ((0.0137)*(a*a)) + ((8.9788)*(a)))
Where a= speed in mph. Can you post the new form?
This revised profile aligns with Cyclops published power curve.
Out of interest are you using the same curve for the JetFluid Pro or did you trace the graph from the JetFuid’s manual as well?
http://www.cycleops.com/pdfManuals/18456D_JetFluid%20Manual.pdf
I would also like to know if the JetFluid curve is based on any of these…
Have you thought about the losses that attribute to both power measurements? PowerTap is in the Hub, where Quartz is in the Crank? So there are going to be obvious differences between these two. I haven’t done any research on how much, but there would definitely be a bit. I’d be interested in seeing the statistical information on Power Measurements ie: Accuracy, Resolution, and Precision. It might vary greatly between power measuring devices.
Something else I never thought about.The trainer probably has losses too.. I wonder if the production of the trainers is consistent. Does cycleops have any information on the consistency of their manufacturing?
Has there been any further update on this? The Fluid 2 is a very popular trainer, and I’m curious if there’s been any consensus on how accurate (or not) the power numbers are. I realize ultimately it doesn’t matter so long as the setup is consistent, but I’d still like to know.
So I bought a Fuild 2 in April 2013, and have started using it.
I was excited by the Virtual Power option in TR.
I’ve been reading though about how it needs to be warm before the power readings are accurate.
Can anyone tell me how warm?
ie: in terms of minutes @ a certain speed (in a room at 70F lets say)
Or just warm to the touch etc?
The reason I ask is because I recently did the Rubber Glove FTP test by Sufferfest, and I could have sworn the last half of the test felt alot harder than the first, even though was getting more tired etc.
I’ve also read about how the Fuild 2 needs to be warm before the Virtual Power is accurate.
Any thoughts?
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.
Hope this helps.
Exactly my experience, too, David. But I have always wondered if there might be another step or just a gradual increase with temperature after the one at ~5 minutes that might be harder to detect.
Hi, I see the Fluid 2 also has a choice “Cycleops Fuid 2 (Polar Power). Believe I read somewhere on TR support that options with brackets after would be the best choice. No mention of the Polar Power curve here so just wondering which you would currently recommend, the Beta version or the Polar version. I’m not particularly needing precise figures but would like the best curve to ensure I’m getting the most benefit from your excellent workouts and plans.
Power (watts)= 10.66 + 5.478 * Speed(mph) + 0.168 * Speed^2 + 0.009 * Speed^3
This is the power curve formula for the Cycleops Fluid 2 Trainer
This is from the sports tracks Plugin.
It is very similar to your speeds to power so might help you tweek it slighty
My FluidII is about 11 years old. I think my power numbers are a bit too low actually. My FTP says 173 with my 20minute test range being 184. However, my flat, out and back time trial time suggests a power of around 212, so my FTP would have been closer to 200. For me to pedal along at 15mph on the fluidII is extremely difficult. Yet on the road that would be total recovery.
What’s more what is suggested at a tempo power output on TR, only puts my heart rate in the endurance range.
Any thoughts on that?
thanks
I have the exact same experience with my Fluid2 bought in 2004.
I’m having a similar experience. I tried the 8 min ftp test, and in order to keep the 100 watts, my speed was 30 km/h or 19 mph What cycle ops fluid 2 profile should we use?