Ergomo Pro or Power Tap SL 2.4?

peterwright

Ok - I buy the DT losses greater at lower power..

But lets assume I ride a 280 AP TT with PT & Ergomo - as I would expect to do in this scenario - then I still do not buy a .98 NP for 3 hours due to higher "low" power readings. So we are using AP to estimate FTP but NP in a race scenario to validate - which should stack up fine.

If DT losses lead to this scenario with Ergomo then why do we not see it reported with SRM - this is a device that seems to track consistently at high and low powers ?

acoggan

Drivetrain friction can't explain the distinct non-linearity that I observed between the Ergomo Pro that I tested about a year ago and my SRM. (Neither could a leg imbalance, since I obtained essentially identical results pedaling with just my left leg.)



Obviously you're not a scientist, 'cause if you were, you wouldn't cite sources that aren't readily accessible by all.

Tom Anhalt

If both devices are calibrated properly, I WOULDN'T expect the AP for a TT to be the same for both devices. I would expect the PT to record a lower AP. If they are identical, then one or the other device is "off".



People who run PTs with SRMs do observe the drivetrain losses....however, I haven't seen any other attempts to quantify it over a wide range of power levels and with a look at old vs. new drivetrains.

Tom Anhalt

True...and that's an important clue in this whole discussion. I was merely pointing out that there may be an additional factor to consider in Peter's Ergomo vs. PT comparison.




Well...obviously you're not an engineer, or you would've figured out a way around that issue already

acoggan

I've done this numerous times.

acoggan

Not worth the trouble.

Tom Anhalt

Did you make sure that the lower power runs were done with continuous pedaling? Besides, I've never seen the data

Tom Anhalt

Spoken like a true scientist

acoggan

No need to coast on the Velodyne.



It's not very exciting...basically every time I've done an SRM-vs.-PowerTap (or SRM-vs.-Velodyne) comparison, I get an excellent correlation with a slope not significantly different from unity, but an intercept that is significantly different from zero. IOW, the frictional losses in the drivetrain are statistically independent of the power output, which in turn means that efficiency increases with increasing power (as you'd expect).

Tom Anhalt

Could you define "a slope not significantly different from unity" a little better?

Here's what I found:

Old Chain
PT Watts = (1.0543 * Polar Watts) - 24.181W, CV = 2.4%, R^2 = 0.9762

New Chain
PT Watts = (1.0219 * Polar Watts) - 8.5696W, CV = 1.8%, R^2 = 0.9937


Thinking about this some more, if you consider that the "state" of the chain at any given time is somewhere between the 2 "extremes" I separated out, then the best fit to use overall would be the one for the full data set. The CV will be larger since there isn't any "middle" data...but the way it stands now, the fit is:

PT Watts = (Polar Watts * 0.997) - 5.7W, CV = 2.6%, R^2 = 0.9839

That looks like a pretty constant ~5-6W offset, and the CV looks reasonable considering the claimed % accuracy of the 2 devices...and that's why I've set my FTP for the PT at 5W lower than my Polar FTP.

peterwright

My point exactly - At factory calibrated K Factor, the two do read very closely at FTP and greater - to within a few %age points -so FTP would be very close when tested. We then have a situation where the Ergomo reads ~3% high @ FTP and ~12-14% at lower powers. Whatever we attribute it to this does not lessen the fact that the Ergomo is seriously compromised as a training tool as a result of this non linearity.

I am not referrring to DT losses but to non linearity - this "quirk" seems to be Ergomo specific. In fact we have run compariosons with PT & SRM and found they track very well across all power bands.

Ergoman

Isn't the purpose of using a powermeter to provide a reliable benchmark against which training can be planned and conducted and against which progress can be gauged? Assuming that a powermeter shows realistic and consistent readings is it really important for it to be in total agreement with any other power meter? Once an individual develops a database of power info with one type of power meter and uses this power info for training does it really matter if another power meter might give slightly different results?

It appears to me that power meter users spend an inordinate amount of time comparing one meter to another, calibrating, testing, and in general obsessing about absolute accuracy, when, in fact, absolute accuracy is unobtainable and not even that important. The things that are important are cost, consistency, reliability, and functionality.

The reason I list temperature sensitivity of my Ergomo installation as one of the major cons is that temperature sensitivity is the one main thing that contributes to inconsistency. I don't care if my powermeter shows my FTP at 280 watts or 320 watts, so long as it shows the same results for the same effort every time and so long as it functions reliably.

peterwright

Ergoman - to a certain degree you are correct. From a training perspective, repeatability is paramount.

There are a few reasons why this is not enough IMO;

1. If I pay this sort of money for a PM then should I not expect it to be accurate and able to be benchmarked ?

2. I have one client who, after a reinstall, has seen the power on his Ergomo drop by 60w

3. This same client rides with a team mate that has an Ergomo - his reads 50w higher...

4. As a coach, I do a lot of inter rider comparison - many of my clients ride the same races and as such an accurate reading is critical

5. Again, from a coaching perspective, the comparison of various power/duration figures is essential in developing a rider.

If this was an Ibike we were talking about then I would maybe live with these issues but a device costing this much... leg imbalance, temp sensitivity, installation issues, non linearity, inability to benchmark against calibrated devices -are these really acceptable compromises ?

Pelotonium

I'm not sure why non-linearity is a show stopper. Aren't the Ergomo manual k-factor calibrations designed to linearise the power output?

As long as the relationship between "power-at-the-crank" and "power-at-the-hub" is monotonic, it should be fairly easy to map one to the other, shouldn't it?

And I would certainly be shocked if the relationship wasn't monotonic.