Tacx Flow ...

blue_paul

Hello everyone...

I have bought a Tacx Flow trainer (which I think they dub an 'ergo trainer'). It has a power reading facility. Tacx's manual and web site is written in near cryptic english (who did they get to translate!!), but as far as I can tell they claim the accuracy of the Flow’s power facility is +/- 5%, so long as you use the ‘self calibration’ feature every ride.

Now, I happen to think this is not true. There seem to be a wide range of reports on the net casting doubts on the Flow’s accuracy. In my experience, when in the ‘ergo’ mode (this is where it creates a resistance that is constantly adjusted so that your power output is the same, whatever your gear or wheel speed) I am sure that in big gears at high RPM the effort is greater than for a lower gear with lower wheels speed.

So I mean, I seriously doubt the accuracy of the brake unit, and its ability to correctly ‘micro adjust’ the resistance in order to demand a constant power from the rider.

Yesterday I completed a 330watt short interval session. An interval in a 53 by 15 gear at a ‘fixed’ load requiring 330watts was harder that the same in 53 by 16 (with a slower wheel speed and thus increased braking from the unit to generate 330watts worth resistance). I could judge this pretty accurately as I data log my pulse, and the plotted charts showed a consistent rise in pulse for the 53by16 intervals (and it felt harder too!!).

I also wonder if it is supposedly measuring power AT the brake unit, and thus not accounting (of course) for drive train resistance, rotating wheel aero resistance, all bearing resistance and the friction between the tyre and flywheel.

SO.... I should add around 30 watts for these extra resistances ... so if my Flow is overestimating power, then maybe its final reading is a good estimate of what power my legs are actually generating ‘up stream’!

My 1 hour power on the Flow is 300watts ... but with this question of accuracy I can’t be sure.

Suspecting the Flow’s accuracy is not great, I try and keep as many things constant between training sessions as possible... same gear, cadence etc..
Anybody had experience of the Flow, or even used a Power Tap whilst on a Flow and thus be in a position to comment on its accuracy?

SolarEnergy

Power readout on the Flow isn't accurate, it overestimates. It is greatly affected by the gears you choose, and by the cadence you hold.

But my conclusions are that, the more you generate power from the bike itself (e.g. increasing cadence, increasing gears), the easier it is to hold a given power level. IOW, with my Flow, riding 300w using 53.19 would be harder than riding 300w using 53.17, at the same cadence.

I don't care, I looove it.

blue_paul

Yes I moade a mistake in my original post. I meant to say that riding in a bigger gear at a given cadence and 'Flow power setting' felt easier than a smaller gear .. i.e. 53 by 15 feels easier than 53 by 16.

So it would appear that the brake unit is not correctly calibrated to the power readings given.

Assumin a cadence of 95rpm and a Flow power of 300watts - which gear do you think results in an 'accurate' power reading .. i.e one that actually reflects the power that your legs are making.

53 by 17? Or would you have to drop to the small chainring?

I agree - the device works well, and is particularly good at keeping a steady resistive load in 'ergo' mode .. just a damn shame about this inaccuracy!

SolarEnergy

This would be hard to answer without a reliable source of comparaison. My guess would be, an ez gear, will make that 300w feel harder. But even with 53.26, I don't think the 300 will be accurate.

And with the Flow, accuracy is not a concern for me. That toy was an introduction to power training, power tap hub is on its way. As soon as I hit the road, in April, I will be using PT SL.

What I really like about the flow, is the road like feeling, and the ergo mode. So even with its innacurracy, I am a very satisfied camper with this toy.

bigbevans

Out of interest I whacked my PT on a Flow on Thursday. When PT was showing 300w Flow was showing 400w.