Ergomo Pro or Power Tap SL 2.4?

[acoggan]

How much inaccuracy is introduced in hanging weights to do a calibration?

None, at least directly.

How accurate is the weight?

In my case, validated against NBS-certified masses.

If you weigh it, how accurate is the scale?

To better than 1 part in 1000.

How close to level is the crank arm?

Since the length of the lever arm barely changes as the crank rises or falls slightly above level, it doesn't really matter.

How accurate is the measurement of the lever arm?

Now I think you're grasping at straws...

How accurately is the weight placed?

It isn't hard to assure that the center-of-mass hangs directly below the pedal spindle.

 

[Ergoman]

I do many rides where I start out early in the morning when the Air temp is in the low to mid 70s F and return when the temp is in the mid to upper 90s. (my guess is the road temp ranges from low 70s to 110 or so just under the BB) How does this teperature affect the readings.

I would guess that the temperature effect on offset is dependent on the type of frame and BB shell and the accuracy of the facing of the BB shell. If the frame and BB shell are steel, I would suspect that the effect on offset with temperature change would be different from the effect on a carbon frame with an aluminum BB shell insert. A properly faced BB shell would also show a different result from one improperly faced. Again, this is just a guess.

In my case, I get about a 1 unit offset change for a temperature change of about 5 degrees, and one unit of offset change will cause about a 5 watt power change. (These numbers were gathered very informally, and could be significantly in error.) I ride a carbon frame with an aluminum BB insert, and I suspect that the facing on my BB shell is not as good as it could be. Someday I'll get it refaced, and hopefully the temperature effect will be diminished.

 

[Ergoman]

In my case, validated against NBS-certified masses.

Are you claiming that any powermeter can be calibrated to be 100% accurate?

Would you not recommend the Ergomo as a useful tool for power measurement?

 

[acoggan]

Are you claiming that any powermeter can be calibrated to be 100% accurate?

No, but those that are based on strain gages (i.e., SRM and PowerTap) can certainly live up to their specs. For example, between my wife and I we have 2 SRMs, 1 PowerTap, and 2 Velodynes, and once you factor in expected power losses in the drivetrain, all agree to w/in <2%.

Would you not recommend the Ergomo as a useful tool for power measurement?

Not based on my experience with one (which predates their "new-and-improved" installation instructions).

 

[Tom Anhalt]

I have been trialling an Ergomo for the last 1000km alongside my regular calibrated PT SL's.

I run a power based coaching business and have been looking at offering Ergomo.

In short - I have been unable to get the Ergomo to track the PT in a linear fashion despite numerous K Factor tweaks. Offset is steady, but I can either achieve tracking at power >FTP or at power <FTP but not both.
Differential is as much as 28w in a race with an NP of 256w - so pretty significant (and is generally ~10-12%)

Hmmm...perhaps you are observing the effects of variability in drivetrain losses with power level? Based on the following, I personally wouldn't expect the power to track exactly between the 2 devices.

http://biketechreview.com/phpBB2/viewtopic.php?t=1370

 

[peterwright]

Hmmm...perhaps you are observing the effects of variability in drivetrain losses with power level? Based on the following, I personally wouldn't expect the power to track exactly between the 2 devices.

http://biketechreview.com/phpBB2/viewtopic.php?t=1370

So what are the implications for a rider that may have been using an Ergomo at the recent race I rode where NP = 278w for 3 hours as opposed to the same rider with PT and an NP of 256W - pretty significant difference (in this case .98IF vs .91IF)

FTP estimate is pretty much the same using either device so that would not change - and riding at .98IF is not feasible for 3 hours ..

Whatever we attribute it to it still represents a fairly major drawback for the Ergomo as an accurate and repeatable device.

I attribute a lot of what I am seeing to the Ergomo's tendency to read high at very low wattages - I often see 130-150w when barely turning the cranks over (say turning them over when coasting)

 

[Tom Anhalt]

So what are the implications for a rider that may have been using an Ergomo at the recent race I rode where NP = 278w for 3 hours as opposed to the same rider with PT and an NP of 256W - pretty significant difference (in this case .98IF vs .91IF)

FTP estimate is pretty much the same using either device so that would not change - and riding at .98IF is not feasible for 3 hours ..

That's the thing...the FTP SHOULDN'T be the same with the 2 devices. I have my FTP for my PT set at 5W lower than with my Polar. The Ergomo would see even more drivetrain losses, so I'd expect the FTP difference to be even greater.

Whatever we attribute it to it still represents a fairly major drawback for the Ergomo as an accurate and repeatable device.

I attribute a lot of what I am seeing to the Ergomo's tendency to read high at very low wattages - I often see 130-150w when barely turning the cranks over (say turning them over when coasting)

Well...you're making the assumption that the PT is the "gold standard" here. It's true that it will be telling you almost EXACTLY what power is making it to the ground...but depending on the state of your drivetrain, it may not be entirely consistently telling you what's coming out of the "engine". Make sense?

130-150W while soft pedaling? Hmmm...it measure BB twist right? Stop pushing down so hard with your right leg while it's on the way up

 

[peterwright]

Hmmm...perhaps you are observing the effects of variability in drivetrain losses with power level? Based on the following, I personally wouldn't expect the power to track exactly between the 2 devices.

http://biketechreview.com/phpBB2/viewtopic.php?t=1370

Tom - would you not expect to see the two devices track at lower power (if the Erg was accurate) and then the drivetrain loss see the Ergomo read higher at greater powers ?

Erg is consistently higher at lower power and the differential is smaller at >FTP

I would love to get a feel for how this might be resolved as I think it is a great device - but I fear that whatever the reason, it is related to the design of the Ergomo and as such will be tough to put right.

 

[peterwright]

That's the thing...the FTP SHOULDN'T be the same with the 2 devices. I have my FTP for my PT set at 5W lower than with my Polar. The Ergomo would see even more drivetrain losses, so I'd expect the FTP difference to be even greater.



Well...you're making the assumption that the PT is the "gold standard" here. It's true that it will be telling you almost EXACTLY what power is making it to the ground...but depending on the state of your drivetrain, it may not be entirely consistently telling you what's coming out of the "engine". Make sense?

130-150W while soft pedaling? Hmmm...it measure BB twist right? Stop pushing down so hard with your right leg while it's on the way up

Understood - but lets look at a scenario where I ride my Ergomo and estimate or measure FTP by one of the established methods (which would give a simialr reading to the PT) and then I ride a race like the one I mention that gives me an IF of 0.98 - it would lead me to raise FTP (or certainly seriously question it)

So I can ride 60 mins at 280w NP and 3 hours at 276w NP - due to the Ergomo not reading DT losses ?

 

[Tom Anhalt]

Erg is consistently higher at lower power and the differential is smaller at >FTP

Actually, THAT'S exactly what I'd expect to see. According to a drivetrain study published in the IHPVA journal (Spicer, et. al.), drivetrain efficiency is proportional to chain tension. The higher the chain tension (i.e. higher power for a given gear and cadence), the higher the efficiency...and therefore the devices will read closer together.

I observed the same thing with my PT vs. Polar rides...larger differences (both absolute and percentage-wise) between the 2 devices at lower average powers.

Also, don't forget that using NP to estimate FTP is just that...an estimate. I believe Andy himself insists that it's no more accurate than within 5%...

 

[peterwright]

Actually, THAT'S exactly what I'd expect to see. According to a drivetrain study published in the IHPVA journal (Spicer, et. al.), drivetrain efficiency is proportional to chain tension. The higher the chain tension (i.e. higher power for a given gear and cadence), the higher the efficiency...and therefore the devices will read closer together.

I observed the same thing with my PT vs. Polar rides...larger differences (both absolute and percentage-wise) between the 2 devices at lower average powers.

Also, don't forget that using NP to estimate FTP is just that...an estimate. I believe Andy himself insists that it's no more accurate than within 5%...

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]

Hmmm...perhaps you are observing the effects of variability in drivetrain losses with power level?

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.)

Based on the following, I personally wouldn't expect the power to track exactly between the 2 devices.

http://biketechreview.com/phpBB2/viewtopic.php?t=1370

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

 

[Tom Anhalt]

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 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".

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 ?

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]

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.)

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.

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

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

 

[acoggan]

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

I've done this numerous times.

 

[acoggan]

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

Not worth the trouble.

 

[Tom Anhalt]

I've done this numerous times.

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

 

[Tom Anhalt]

Not worth the trouble.

Spoken like a true scientist

 

[acoggan]

Did you make sure that the lower power runs were done with continuous pedaling?

No need to coast on the Velodyne.

Besides, I've never seen the data

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]

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).

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.