Ergomo Pro or Power Tap SL 2.4?

nrhorwitz

Not attempting to discredit you. My experience has been different with Gita Bike and Nelson Frazier. He's been quick in answering my emails and helping out the customers I work with regarding Ergomo. I've been happy with his support.

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Robin Horwitz

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May 19-20, 2007 Seminar with Hunter Allen in Fremont, CA.

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johnb263

By Ergomo (Germany's) own admission, adjusting the k-factor alters the linearity of the unit's power reading... I've got a email reply as evidence.

Ergoman

Your experience may have been different from mine for many reasons. It may be that you do a lot more business with Gita Bike.

The fact remains that it took a full two weeks for Gita Bike to turn around a failed computer unit. I live just a few hundred miles away, I used Priority Mail and the mail takes one day. There were no intervening holidays. I made numerous calls in an attempt to confirm status of my warranty work, and none were returned. Worse yet, on the few instances when a real person answered, the response was "I don't know, I'll find out and call you right back." No one ever called back.

Ergoman

How much inaccuracy is introduced in hanging weights to do a calibration? How accurate is the weight? If you weigh it, how accurate is the scale? How close to level is the crank arm? How accurate is the measurement of the lever arm? How accurately is the weight placed? It wouldn't be too hard to introduce a 2% error in each of these steps. The product of all those errors could be significant. My point is that absolute accuracy is a very hard thing to attain, even in a lab. Finally, once you do all this weight hanging and testing, there's still the issue of how each meter measures, processes and records data.

It's impossible to attain absolute accuracy from a power meter...any power meter which is why it doesn't make much sense to judge the output of one by comparing it to another, particularly when the differences are small. Fortunately, consistent results are what's important. With the exception of sensitivity to temperature (which can be overcome by paying close attention to offset), the Ergomo has been very consistent for me. YMMV.

vadiver

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.

acoggan

None, at least directly.



In my case, validated against NBS-certified masses.



To better than 1 part in 1000.



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



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



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

Ergoman

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

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

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%.



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

Tom Anhalt

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

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

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

peterwright

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

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 ?