Avarage watt comp.

Marlon -

As I mentioned before - I have a Tacx Flow and calibrate it often. Like you, I get different calibration figures and have seen anything from 0 to +8 on my unit.

The calibration process I use, is as the book describes. I cycling for 3 minutes (usually keeping the speed between 15-20mph), then run though the process.

My Powertap is accurate withing 1%. This is have checked with the stomp test. When riding at 230W on the flow my PT gives a figure of about 150W. If I push the wattage up to 330 on the Flow, I m lucky if I get 220 on the PT.

I used to believe my flow was accruate especially on the basis that this calibration process was implemented. In actually fact when I got my PT I got dis-heartened. While I used to think I was cycling at 220W for a heart rate of 60-65% in actual fact I was only pulling 145W.

Bottom line - get someone with a PT, SRM or Polar to check the accuracy of your unit.

It you need me to give you a figure for other wattages then I will be happy to.

Tom.

!!!!!!
So measuring the inaccuracy of the Tacx Flow, somebody with a Flow has to cycle with the same speed as I do, and calibrate the Tacx before cycling.

Let's say 30km/h on slope 0. If the Flow is super accurate we will get the same power output. if we compare the power output, we can see how inaccurate te Tacx is.

Anybody here who likes to compare his Flow?? The more people/data we have, the better we can estimate how accurate the Flow is.

!!!!!

Just follow this checklist (it's important thate verybody does exacly as described here, so we don't get different data). It will cost you about 10 minutes:

1.-You have a tacx Flow.

2.-Press the handle against the tire in a way the tire doesn't slip.

3.-Enough tire pressure, so the tire doesn't slip

4.- Make sure your drivetrain work smoothly

5.- Put on the Flow. Now you see the frequenty and Voltage. Write it down. Put the slope on 0.

6.- Cycle for 5 minutes at 30km/h. While cycling with a lower speed (25km/h or something) immidiately start the calibration. Cycle up to 30km/h. The Flow will say 'stop'. Stop cycling until your speed is 0. Remember the calibration number. Immidiately after setting the value start cycling 30km/h.

7.-Cycle for let's say 3 minutes 30km/h. Then let the flow computer show a the power and speed output only. Try to keep your speed constant for 1 minute. Don't change gear. Watch the power output.

8.-After 1 minute:

-write down the calibration number

and the data of the 1 minute test:

-estimate!! the mean speed you have seen
-give the highest and lowest value of the speed you have seen in that minute

-estimate!! the mean power you have seen
-give the highest and lowest value of the power you have seen in that minute

9. Turn the Flow computer off and on. Write down the frequenty and voltage.

10. Mail the data to me.

I will calculate how accurate the Tacx Flow is for you and post it here!!!!!

sounds like a lot of hassel to me! Besides the voltage will constantly vary.

I believe the bottom line is, assuming you are serious about training with power is to buy a PowerTap, SRM or Polar unit. The flow is (and never will be) accurate enough.

I can eliminate the variation of voltage by watching the voltage change in a one minute period several times.

10min. isn't much hassel if you awn't to know how accurate it is.

Some people don't have a PowerTap or SRM or son't want to cycle with such expensive and not-good-looking stuff..

We will see how accurate the flow is!!

We only need one other cyclist with a Flow to start calculating.

🙂

Quoted post said:

Originally posted by marlon1
I can eliminate the variation of voltage by watching the voltage change in a one minute period several times.

10min. isn't much hassel if you awn't to know how accurate it is.

Some people don't have a PowerTap or SRM or son't want to cycle with such expensive and not-good-looking stuff..


We will see how accurate the flow is!!

We only need one other cyclist with a Flow to start calculating.


🙂

i'm not sure i understand? how will this tell you how accurate it is? you'd need to compare it to a known standard.

ric

Comparing watts as described above (4 post above or something) tells us the the error in het power measurement.

it could be possible that the Flow has a systimatical error. In that case we need somebody with a flow and something like a powertap.

The rest is only maths. 🙂

I find it interesting that some report much variation with the power readings from their Tacx. Like I said, I'm using the Excel, and from day-to-day, I'm finding the readings to be very consistent. For example, I know that 100rpm in a certain gear gives a reading of right about 200w. That reading has not fluctuated. The voltage here is very steady, so perhaps that is the largest aggravating factor for some.

So long as someone who owns both a Flow and PT follows Marlon's steps, this should give Marlon an idea of how far off his Flow's numbers are. I realize that it won't be accurate enough for a scientific study Ric, but it should be close enough to be useful. I'd be interested in seeing the comparison myself.

Quoted post said:

Originally posted by Ted B
I find it interesting that some report much variation with the power readings from their Tacx. Like I said, I'm using the Excel, and from day-to-day, I'm finding the readings to be very consistent. For example, I know that 100rpm in a certain gear gives a reading of right about 200w. That reading has not fluctuated. The voltage here is very steady, so perhaps that is the largest aggravating factor for some.

my tacx units and/or the ones i've tested (and other trainer units) all report similar data. in other words, i can pedal at a constant X revs/min in a specific gear after doing a run down calibration and the trainer units report the same power value. however, when i compare it to a true power reading (e.g., SRM, PT, etc) i find that the true power (i.e., that reported by the PT/SRM etc) has decreased if i maintain a constant cadence. if i keep the true power constant the speed on the trainer increases.

the last time i checked this over a *long* training session (3-hrs) the power dropped from ~ 230 W to 190 W.

on the wattage list at topica, others have reported their finding with a Flow (with not great results)

ric

Quoted post said:

Originally posted by ricstern
...on the wattage list at topica, others have reported their finding with a Flow (with not great results)

Thanks for that bit of information Ric. Also, thanks for the Topica tip. Here are some interesting things I found there:

------------------------------------------------------------------
On Tue, 20 May 2003, Mark Adler wrote:

Thanks to all those who warned me about the Tacx (Grand Excel)
measurement error. Here are my results in watts:

Units warmed up for 10 minutes
Tacx roller pressure "calibrated" to coast down for 10 seconds at 25mph
and Slope = 0. 100psi.
(The PT observations are eyeballed averages as they jump around alot
more than the Tacx, as expected)

Slope PT Pro Tacx Tacx Error
0 120 100 -20
0 185 150 -35
0 245 200 -45
0 305 250 -55
9 150 150 0
9 205 200 -5
9 265 250 -15
9 315 300 -15

--------------------------------------------------------------------

I got the following undocumented "scaling" feature direct from Tacx.

I made sure the unit was fully warmed up, then adjusted the tyre pressure to
8bar & applied 2 full turns of roller contact pressure, from the point where
the tyre just touches the roller.

In "ergo" mode, riding my SRM equipped bike I set about adjusting the scale
factor until the Tacx & the SRM agreed at 280watts. I chose 280watts as this
is where I do my 2 x 20mins.

I found that even though the Tacx & the SRM agree @280, the Tacx reads
10watts low up until 280, then reads progressively higher above 280. ie when
I'm doing my Z5 @330, then the Tacx might read 350-360.

@280 I'm using 53/13, @330 I'm using 53/12

Cheers, Phil

You can optimise the scale factor.
Remove the plug underneath the Basic computer.
While replacing the plug, hold down the SET about 5 sec, release the SET &
press one time on the MODE key.
The scale factor will start to flash at bottom right.
Using the +/- key, you can set the scale factor between 80 and 120.
To return to the training display, press MODE.

Regards Tacx bv,
Paul v. Bemmelen.

I found each time I use the ergo, I have to hold down the MODE key when
plugging in the data cable, otherwise the scale factor defaults to 100. Mine
needs to be set @93, but I've calibrated others @86 @ 104........

-----------------------------------------------------------------------

I have both a Tacx Basic and a Power Tap Pro, and I regularly use them
together. In general, I have found it almost impossible to get
consistentcy out of the Tacx unit's wattage reading when conditions
change. Switching wheel speeds, cadence, upping wattage in ergo mode,
upping slope in slope mode, etc. all seem to affect the wattage reading
of the Tacx differently than the affect on the wattage reading of the
Ptap. For example, let's say I am running in ergo mode on the Tacx at
200 watts and 90 cadence. If I shift to a bigger gear (i.e. increasing
wheel speed) and keep cadence at 90, the Tacx is supposed to adjust
resistance so that the wattage stays at 200 (which is the purpose of
Ergo mode), but on the Power tap reading, I might see a 10 watt
increase. I simply gave up trying to somehow calibrate or adjust the
Tacx so that it gives accurate readings under differing conditions--I
just use my PTap with it so that I always have an accurate reading. In
general, I trust the accuracy of the wattage readings on my Ptap and
rarely trust the accuracy of the wattage readings on the Tacx. Don't
get me wrong--I love my Tacx. I especially love the micro adjustability
of the resistance ni ergo mode, which is great for step testing and
controlling slight increases in 2X20 power and other interval power.
Also, in general, I find that the Tacx wattage readings are within 10%
of the PTap's readings, if I keep the tire to roller pressure consistent
from ride to ride and if I keep my weight accurately programmed into the
Tacx unit.

---------------------------------------------------------------------

When I first get on the trainer, I'm usually spinning 160-170 watts.
The Tacx will display 115-125 for that power. 10 minutes later, the tap and
tacx are within 10 W.

And here's perhaps the most interesting post. Curiously enough, the Flow's calibration procedure only more or less ensures consistent readings from session to session, but has nothing to do with overall accuracy. Also interesting is that when calibrated in similar fashion the Excel's computer seems to read power readings more accurately than the Flow:

-----------------------------------------------------------------------

Was planning to do an experiment devised between myself and Roberts Chung and
Wells. This was planned to test the accuracy of the tacx flow/grand excel at
different flow calibration numbers, roller speeds, powers etc.

27 degree in my turbo area ( hottest day of the year outside ~22 degrees)
rear wheel 700c mavic cxp30 32 spoke on record 9 speed, continental 23mm tyre @
8 bar

I put the bike on and did the roller up a bit, put the flow head on and did a
roll down calibration result =0
15 minute warm up averaging ~200w
repeat roll down calibration result = -5
adjust roller pressure to get calibration to 0.
set the flow to 200w rode at 20.0mph, srm = 152
set flow to 270W rode at 20.0mph srm = 240
put the grand excel head unit on - both were set with the scaling factor at 100
and 69kg body weight.
excel set at 200W rode at 20.0mph srm = 202
excel @ 270w rode at 20.0mph srm = 270

Tried them both riding at 19 and 25mph and similar differences occurred.

with such a discrepancy I didn't think it worth proceeding with the experiment
as discussed so I decided to try to find what the calibration procedure on the
flow did ( if anything)

calibration = 0

speed power slope = 0
19.0 160
21.0 184
25.0 220

flow srm @ 20.0mph ergo mode
200 150
270 243

tightened roller onto wheel but didnt perform calibration procedure yet

calibration = ?

speed power slope = 0
19.0 189
21.0 214
25.0 259

flow srm @ 20.0mph ergo mode
200 175
270 265

then left roller pressure alone but did calibration procedure

calibration = +4

speed power slope = 0
19.0 189
21.0 210
25.0 256

flow srm @ 20.0mph ergo mode
200 171
270 264

now reset roller pressure until calibration showed 0. The wound pressure off the
roller but didnt perform calibration

calibration = ?

speed power slope = 0
19.0 160
21.0 180
25.0 214

flow srm @ 20.0mph ergo mode
200 145
270 243

then did the calibration procedure leaving the roller pressure alone
calibration = -4

speed power slope = 0
19.0 160
21.0 178
25.0 216

flow srm @ 20.0mph ergo mode
200 149
270 247

Looks like the calibration procedure doesn't alter the power vs speed
relationship at all. I think all it is telling you is the result of the roll
down test - it doesn't use this info for anything but you can use it to reset
the roller pressure to roughly what it was last time you used it. I.e. the
calibration procedure doesn't calibrate the tacx just by doing it - you use the
calibration procedure to calibrate it by getting the result the same each time.

With the flow calibration at 0 the Grand Excel head unit was spot on accurate
for power either in slope mode at 0 slope or on ergo mode at 20mph but the flow
wasnt.

Question is what is different between the head units?

Bob Tobin

ok But how do you ll know the SRM/PowerTap is accurate??? Maybe it isn't accurate ass well 🙂

Quoted post said:

Originally posted by marlon1
ok But how do you ll know the SRM/PowerTap is accurate??? Maybe it isn't accurate ass well 🙂

That's a good question of course, and one that i'm answering in my research.

However, the SRM has been validated as being accurate (assuming it's correctly calibrated), see research by Jones and Passfield.

currently the PT is being validated as precise as well.

The advantage the PT has over the SRM is that PT is always calibrated (which is different to the zero offset being correct) until at some point the calibration goes off, and then needs to be reset by the factory. the advantage with this is, is that once it needs calibrating the PT gives really silly data (e.g., riding easy at 1600 W) so it's obvious it needs servicing.

ric

Here's one more experiment specifically on the Tacx Flow:

http://home.comcast.net/~rwwells/Tacx/DOEFlow.html

Quoted post said:

Originally posted by Ted B
All else being equal (including frontal area), the 58kg rider would be able to accelerate faster and would be faster in the hills and anywhere gravity becomes a detractor. On the flats however, the two should be capable of comparable speed.

Hi Ted

Sorry but you don't seem to understand that additional body weight means a much more un-aerodynamic shape therefore very much slower on a flat road. This is fairly basic information.

Quoted post said:

Originally posted by Michuel
Hi Ted

Sorry but you don't seem to understand that additional body weight means a much more un-aerodynamic shape therefore very much slower on a flat road. This is fairly basic information.

Yes, I DO understand, but you missed my point.

My scenario was purely hypothetical, which is why I plainly stated "ALL ELSE BEING EQUAL". Think of both riders being twins, but one has an artificial pelvis made out of a dense substance. 😉

although we went from an "average watt comparison" thread to a how to tune your gear thread. It appears I can pull off about 3.36watts per kilo and survive for over 30 minutes.

I see others talking about sustaining 400 watts!?!?!? Dang!!! don't know that I could do that for more than 15 seconds... 🙂

<---just a plugger.

400watt/100kg=4watt a kg. 🙂

'I see others talking about sustaining 400 watts!?!?!? '
Where??? I didn't read about someone who can cycle 400watt??

But 400watt is much. For me it is an absurd 6.9watt/kg. I can tell you 400watt for 30minutes or something is impossible for me.

---------------

Does anyone know HOW the tacx flow calculated the power?

Does it measure the 'voltage' and 'amperes'?
voltage x amperes=power

Or does it measure braking force and speed?
force x speed=power

?

Marlon1,

On the first page the guy (matter of fact, I looked back, and it's you. 🙂 is talking about sutaining over 5watts per kilo. And congratulations man, it's no accident to be able to do that. It's possible I screwed up the math, and just for fun estimated that you might weigh 68-70 kilos, my math might be flawed and I guessed at your weight. :o

Even then 350 watts at 68-70 kilos is admirable to say the least.

275 watts at 80-81 kilos is where I sit wishin I was you... 😄