ergomo vs powertap comparison data

Hey

I've been happily training with an ergomo this winter for the last 4 months or so. Readings seemed realistic, reliable and very repeatable, all was well. Until I got a powertap SL, planning to move the ergomo onto my TT bike for the racing season and use the powertap on my training bike. I figured I might as well use them both on the training bike for a while to calibrate them against one another before moving the ergomo - after all I know the powertap is supposed to be a few watts lower than the ergomo due to drivetrain losses, and also I'm realistic and was prepared for the possibility of some non-linearity in the relationship between the two readings.

What I wasn't prepared for, and am rather upset by, is what seems to be a definite time-drift between the power meters. First of all the PT reads higher than the ergomo- surprising, but I could deal with it. It seemed about 10W higher. But over an hour's interval session it drifts up. I've attached a picture of a typical comparison - by the end of the last rep the PT was reading up to 50W higher at times than the ergomo!! Clearly this is too much to be tolerable. A session done at 'constant power' on the ergomo is going to be a very different session to one done at 'constant power' on the PT.

So what do I do? I have as far as I'm aware no way of determining which meter is the accurate one. My gut instinct is that the PT is reading high, but that could be just because I'm used to the numbers from the ergomo. Has anyone had a problem before with either unit drifting over the course of a session?

Any help much appreciated, am feeling pretty p1ssed off right now and considering going back to training with tarot cards and a heart rate monitor

Attached Thumbnails

 

PS forgot to mention, if any statistical genius out there wants some data to have a go at some serious analysis then don't hesitate to ask...

It would help if you posted the data files for the above ride. From the low-resolution screenshot, I see a little drift in the Ergomo as well, just not to the same extent.

You didn't happen to be on a fluid trainer?

Read the Power FAQ. You can do a static test of your PowerTap to measure its accuracy. There is no way to do anything like that with an Ergomo.

You can also do a long climb and compare your power data from both power meters to what analyticcycling reports. Have a friend with a calibrated SRM/tested PowerTap do the climb at the same time.

I have heard that PT does not work well on some trainers like the TACX Fortius or iMagic....

OK here's the data. No I was on a magnetic trainer. Why shouldn't the PT work well on some kinds of trainer though??

Note I was using the ergomo to try and hold a roughly constant power for this session, so it looks more consistent than the PT but of course it could have been the other way around if I'd used the PT to hold the 'constant' power...

I suppose I could run a static test for accuracy on the PT both at the beginning and the end of a long session and look for differences. But the other thing that worries me is a seeming non-linearity inbetween the two in that the PT reads lower at low power and higher at high power; no static test can look at that surely as it can only measure power at zero angular velocity...

Attached Files

comparison.txt (62.5 KB, 30 views)

I have a bad feeling about this being a dumb question but could someone point me to the power FAQ? - I can't seem to find it...

Exactly, maybe the link could be set-up as a sticky. It ain't at all obvious that it even exists or how to find it!

I'm sorry, I should have been more specific: Can you post the actual workout data files (be it WKO or PowerTap CSV) containing all the columns (cadence, speed, etc)?

About the trainer question...up until the 2007 model, Cyclops Fluid trainers have been notorious for getting harder as the fluid heats up. So if you keep a constant cadence and speed for those intervals, the power line in the graph will increase. For me there's a sharp increase in resistance after 3-4 minutes, then it continues to slowly increase for the next 20, before finally leveling off.

As far as the Tacx trainers, I think the issues they have with the PowerTap has to do with interference, not the resistance. Your magnetic trainer should be fine.

You may also want to check out the "Wattage" Google Group. A quick search on "ergomo powertap" brought up a few threads that seem relevant. You need to be a member to access the group but joining is easy. Just be sure to answer the question at the bottom of the "Join" page.

Here are a few threads that I picked out...

Ergomo and PT Sl
http://groups.google.com/group/watta...2c594170270a4d

Ergomo Offset Stability
http://groups.google.com/group/watta...6fee63229cd5f9

Comparing data from 2 different PMs
http://groups.google.com/group/watta...d68e5739cacee9

OK I've attached the wko files - had to rename them to .txt though to upload them so you'll need to change them back to wko before opening them.

I see what you're saying about the fluid trainers but I paid no attention to the speed during this (or any turbo) session - I just used the power reading to judge the intensity so I don't think it ought to matter. Yes if you look at the speed you'll see each rep got slightly quicker though so presumably something was heating up. My concern is whether something in the PT was also heating up and making it behave strangely

Attached Files

	ERG.txt (32.0 KB, 14 views)
	PT.txt (39.0 KB, 8 views)

The trainer can do whatever it wants, that won't impact the PT reading. Here are the possibilities that I see:

1) The PT gradually comes un-torque-zeroed over time. The way to test for this is to turn off auto-zero in the CPU, do another ride like this, but coast for 15 seconds every 5-10 minutes. Then in inspecting the file, see if torque always drops to zero.

2) The Ergomo drifts over time. I've never heard of this, but it is apretty new product...so who knows.

3) This is the most likely in my mind...as you fatigue you're becoming slightly right leg dominant, and the Ergomo is underreporting.

In general, if you've stomp tested a power tap and the torque is zeroed, there's quite a bit of evidence (studies over time, the product has been around for >6 years) that the PT is the one you ought to trust.

BTW, wattage FAQ: http://midweekclub.ca/powerFAQ.htm

You've got both meters on one bike, variables such as tire inflation and tension are constant and you're riding on a mag trainer which should be fairly consistent in terms of speed vs. power. It should be easy to determine which power meter is drifting. Just ride a steady speed for as long as it takes to see one meter or the other drift. Once you know which meter is drifting, you'll know which one to fix and/or believe.

If it's any consolation to you, the main benefit I see to a power meter is that the rider is never sure if he can believe it. This uncertainty leads to curiosity which leads to more riding to test various theories and calibrations. The additional riding results in better conditioning, which is the goal in the first place. I know that testing my Ergomo has given me at least 50% more indoor riding that I could normally tolerate this time of year.

During your install of the Ergomo, did you chase and face the bottom bracket of the bike?

Jim

Thanks for the files. It's the Ergomo that's drifting downward. The reason it doesn't look it from the graph is that you probably used your Ergomo for pacing so it's reported wattage is similar across intervals.

But if you look at speed, you'll see that your speed was faster for each subsequent interval. The only way to increase speed (without changing your mag trainer settings) is to increase wattage. That's why it looks like the PowerTap is drifting upward.

Ta da...mystery solved! I bet by the last interval you thought that maintaining the same wattage was getting pretty darn hard!!

Here are the interval stats:

Interval 1
Speed: 36.1 KPH (ave for both)
Ergomo: 316 Watts
PowerTap: 335 Watts

Interval 2
Speed: 37.15 KPH
Ergomo: 319
PowerTap: 342

Interval 3
Speed: 37.45 KPH
Ergomo: 321
PowerTap: 347

Interval 4
Speed: 38.15 KPH
Ergomo: 326
PowerTap: 357

You still should do the stomp test with the PowerTap and make sure it's accurate. Then you can use it to adjust the Ergomo as per the advice of the others on this list and the Wattage group.

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