Accuracy of turbo power readings

phreak

I was talking to a few people about their turbo and a few mentioned that their turbo (a Tacx Flow) tended to over exagerate wattage quite considerably.

As that's my turbo I'd be interested to hear others opinions. I've input my data into this online calculator - http://bikecalculator.com/veloUS.html - and it returns much the same data as I'm getting from my trainer.

phil s

Forget online calculators, forget turbos, the only decent way to measure your wattage is to buy or hire a powermeter. I know of a guy who thought he could bag out 400w for an hour based on a Tacx. He was probably 70-80w out.

ut_och_cykla

However much one can rely on readouts on a flow you will always be able to see improvements if you make sure set up is the same every time!
My impression is that may Flow ties in well enough (for me) with expected results out of doors and also with the much more refined set up on which I have done 2 max VO2 tests. There was a USA blogg about how much Flows varied but i cant find it now....

phreak

The online calculator I refer to gives an expected speed for a given power output, and this chimes reasonably well with the speed my bike computer is giving me on the turbo.

borisface

Its not so much whether the power readings are accurate its whether they are consistent that is the key. A tacx flow might be 100w out but if its always 100w out then its consistent and therefore creates a reliable and repeatable environment. The problem with the tacx as I understand is that compared to a power meter based on strain guages such as SRM or powertap it varies depending on things like how warm the day is or whether the tacx has warmed up. That said I also know of people with imagics who have been able to calibrate it to within a wattt of two to their powertaps.
FWIW I have an elite real tour that measures power in a similar way to the tacx, for the same 50 minute workouts, recent ave powers for me have been
powertap 211w Elite 227w
powertap 256w Elite 291w
So the % differences are different whch leads me to think that the elite is not consistent.
In realtime there is no great correlation between the power readings for the elite and the powertap, other than they both go up when you push harder. But sometimes one is 50 watts higher sometimes the other is10 watts higher it seems a bit random. What I don't know however is how often the elite takes a sample of teh power produced for display with a p/t you can chose the display rate that could explain the realtime differences.
What might be worth doing is borrowing/hiring a powerta against which to calibrate the flow.

Anonymous

phreak wrote:

The online calculator I refer to gives an expected speed for a given power output, and this chimes reasonably well with the speed my bike computer is giving me on the turbo.

Is that the same one underestimates power at a given speed quite considerably?

According to bike calculator I should be doing 22 minute qo mile TTs for the power given on my Powertap compared with the actual 26 minutes it took me!

SBezza

My flow is pretty consistent, I doubt it is accurate though. You just need to make sure the tyre pressure is the same each time, the roller pressure is the same, and that is is calibrated before every session.

Also don't use it in the ergo mode, these readings can be all over the place.

Tom Butcher

My Tacx was a fairly consistent 10-15 watts under the reading of a powertap hub between 200-400 watts - I didn't test it beyond this range.

The turbo seemed to take longer to ramp up the watts though - for example a 5 second sprint the powertap would have a peak power way in excess of the turbo readout and it was easier to keep the turbo in a fairly narrow wattage band than it was the powertap. I didn't download the data from the turbo to the PC this was just done by eye.

The turbo is a Grand Excel and is at least 5-6 years old so I thought overall not a bad result - certainly good enough to work with.