Ergomo Validity shot down....

[Roadie_scum]

Here

OK. Sorry, sorry.

Really: this is the link for the initial write-up

It's the first thing you get when you type ergomo, validity and reproducibility into google. The full paper should be available through your local university library at some stage this month.

 

[doctorb]

Gentlemen,

Having just read the 2 page published report, Ergomo's validity has not been shot down.

If one assumes the SRM is correct (and that is quite an assumption), in several cases the Powertap's deviation from the SRM is greater than the Ergomo's.

To quote the paper, "ERGOMO overestimated the PO by 5% compared to the SRM." "POWERTAP underestimated the PO by 6% compared to the SRM."

From a scientific perspective, more than one version of each meter should be tested to determine reproducibility. I personally would have liked to see the test run on a Computrainer for an additional comparison.

I have no bias as I use both the Powertap 2.4 and the Ergomo.

Barry

 

[Bob Edberry]

Gentlemen,

Having just read the 2 page published report, Ergomo's validity has not been shot down.

If one assumes the SRM is correct (and that is quite an assumption), in several cases the Powertap's deviation from the SRM is greater than the Ergomo's.

To quote the paper, "ERGOMO overestimated the PO by 5% compared to the SRM." "POWERTAP underestimated the PO by 6% compared to the SRM."

From a scientific perspective, more than one version of each meter should be tested to determine reproducibility. I personally would have liked to see the test run on a Computrainer for an additional comparison.

I have no bias as I use both the Powertap 2.4 and the Ergomo.

Barry

Reminder: The Powertap should always measure under the SRM and ergomo because of drive train loss. 7-10 watts is the usual amount from what I have read. So, if PT is reading 250 watts, then SRM should be reading about 257-260watts. 6% of 260watts is 15 watts, so we are talking about 5 watts then. Which is 2.5%, which is what PT claims isn't it?

I too would have liked to see them do the test on multiple athletes. It's unfortunate that this study was done using one athlete only. Could have been that he was right leg dominant. Who knows. Across 10 athletes or more would have been much better.

What concerns me most is that it the + % changed drastically throughout the different intensities.

BE

 

[yawg]

(though I wouldn't actually recommend the usual stomp procedure)

Robert,

What is your procedure for the stomp test?

Thanks,

greg

 

[jbvcoaching]

I have an Ergomo Pro and a SRM Dura-Ace, why have I not noticed the difference in the power numbers between the two?

It's very hard to perceive a difference of less than 10% between two powermeters that can't be ridden concurrently. Here's an example, in 2005 one of my PT hubs came back from service reading 7-8% low. I didn't realize it for 4-5 weeks, until I saw the pattern that all my "bad" days happened to be on that wheel. At that point I ran both wheels on a bike with an SRM, then compared downloaded files to see how each compared with the SRM, and came up with the 7-8% number.

Saris confirmed my diagnosis when I sent it back, and of course they fixed it.

I've been racing since '86, coaching professionally since 2001, and using powermeters since 2001, and it took me 4-5 weeks to even figure out one of them might be off. Then of course I still had to get an SRM equipped bike and download files to determine the magnitude.

So, unless you're running your Ergomo and your SRM each on a trainer-based PM (Velotron, Computrainer, etc) using the same rear wheel/tire/air pressure, and then doawnloading and comparing files, I would not expect you to notice small differences between the two tools.

Whether or not you care about differences less than 10%, again depends on what you use a PM for (per my previous post).

 

[RChung]

What is your procedure for the stomp test?

The classic description is here: http://www.midweekclub.ca/powerFAQ.htm#Q23

However, I would slightly amend that procedure. It's kinda hard to balance on a pedal while the crank is exactly horizontal, so this is what I do:

1. Use a known weight (like barbell plates).
2. Clamp the bike into a trainer but back off the roller pressure so the rear wheel turns freely. Hang the weights from one of the pedals. Place the PT into torque measuring mode. Backup the rear wheel very slowly until the torque reaches a maximum. That will occur when the crank is horizontal.
3. To check linearity, repeat for a couple of different cogs. Doing it this way is easier than changing the weights for the same cog.

 

[Bruce Diesel]

The classic description is here: http://www.midweekclub.ca/powerFAQ.htm#Q23

However, I would slightly amend that procedure. It's kinda hard to balance on a pedal while the crank is exactly horizontal, so this is what I do:

1. Use a known weight (like barbell plates).
2. Clamp the bike into a trainer but back off the roller pressure so the rear wheel turns freely. Hang the weights from one of the pedals. Place the PT into torque measuring mode. Backup the rear wheel very slowly until the torque reaches a maximum. That will occur when the crank is horizontal.
3. To check linearity, repeat for a couple of different cogs. Doing it this way is easier than changing the weights for the same cog.

Nice procedure, simple and accurate!

 

[JTE83]

Shouldn't the ergomo read higher watts than an SRM because it accounts for power lost in the BB itself whereas the SRM measures the strain / calculates the power between the Chain and the crankarm thus BB loss is not measured by the SRM?

 

[beerco]

Shouldn't the ergomo read higher watts than an SRM because it accounts for power lost in the BB itself whereas the SRM measures the strain / calculates the power between the Chain and the crankarm thus BB loss is not measured by the SRM?

And just how much power do you suppose is lost in the BB

 

[JTE83]

And just how much power do you suppose is lost in the BB

I wouldn't be surprised if 4 to 8 watts is lost by a BB. Saw a lot of friction in my Octalink Ultegra BBs.

 

[jbvcoaching]

I wouldn't be surprised if 4 to 8 watts is lost by a BB. Saw a lot of friction in my Octalink Ultegra BBs.

How did you measure that, and what were the results?

 

[jbvcoaching]

1. Use a known weight (like barbell plates).

Just to elaborate on this one, inexpensive barbell plates (not certified for weightifting competition) can be off by enough to skew your test results. A more useful approach for cyclists can be to buy an accurate scale ("postal" scales come as low as $20), then just use any old combination of **** (water jugs, tools, etc) to rig up the exact weight you want.

The advantage of this is you can test your PM using different weights, even if you just buy the one tool. Plus, if you're a weight weenie (or if one of your cats has a weight problem and you need to weigh her regularly, not that that's why I bought one ) you can use the scale for all that as well.

 

[yawg]

John,

What's the minimum weight needed to get an accurate result?

TIA.

greg

 

[JTE83]

How did you measure that, and what were the results?

I was just guessing at these watt numbers. I do love low friction stuff like low friction hubs and BBs if they exist. Went ceramic for a few BBs and hub bearing but they still have friction...

 

[jbvcoaching]

John,

What's the minimum weight needed to get an accurate result?

TIA.

greg

Good question...I'm not really sure, but when I used to calibrate SRMs I tried to use weights from 10-50kg. The heavier setups required multiple people assisting.

I think Bob Tobin and Andy Coggan have some posts on the wattage list as to the range you'd want to use to verify a PT...perhaps R. Chung has an opinion?

 

[Roadie_scum]

I was just guessing at these watt numbers. I do love low friction stuff like low friction hubs and BBs if they exist. Went ceramic for a few BBs and hub bearing but they still have friction...

Your guesses are wrong. Ceramic bearings are a waste of money in most applications. Not that they don't make a difference, but that the difference is not cost-effective because it is a very small order change for quite a bit of dollars.

Buy some nice wheels and a good set of tires and learn to tuck up on your bike. I just saved you about 100 times the watts. If you really want I'll sell you a bunch of small balls for $20 each as well.

 

[ric_stern/RST]

Good question...I'm not really sure, but when I used to calibrate SRMs I tried to use weights from 10-50kg. The heavier setups required multiple people assisting.

I think Bob Tobin and Andy Coggan have some posts on the wattage list as to the range you'd want to use to verify a PT...perhaps R. Chung has an opinion?

Not that I'm any of those mentioned, but i've used masses of ~5 to ~25 kg. I always used multiple masses and multiple positions on both sides of the cranks.

Ric

 

[beerco]

John,

What's the minimum weight needed to get an accurate result?

TIA.

greg

I would say you'd want a weights which would give you equivalent peak and average torque to riding at threshold in a common gear at your standard cadence. You could then shift gears to get a couple more data points above and below.

 

[RChung]

...perhaps R. Chung has an opinion?

Perhaps? Dude, I try never to let the absence of information restrict the breadth of my opinions (just the forcefulness with which I defend them).

I do something closely related to what Andy Birko suggested, but I looked at the actual torque distribution. I don't try to mimic the peak torque -- 80th percentile was good enough, and I can get that with a 20kg load. I check linearity by using the same 20kg load over several different gear ratios.

 

[Speedskater]

So, I´m back from the Eurobike trade show, where I had the opportunity to speak to the head of service of Ergomo. He was not aware of the publication mentioned by the OP, but when I confronted him with the results, we had a good discussion. First he insisted that it is questionable that the the SRM is accepted to be absolutely precise, just because its on the market for 20 years. Second he said, they has bikes with SRM, PT and Ergomo installed and SRM and Ergomo are showing the same values. The same is true for the Ergomo and a Cyclus ergometer (This ergometer was designed in the ex-GDR and is accepted to be very precise). But he also responded to the left-leg-question. When three riders are riding the same wattage on a Cyclus ergometer, the wattage measured by the Ergomo can differ by as much as 10 W from person to person. More than by leg force inequalities this is based on pedalling technique, i.e. how much the leg on the upstroke pushes against the other one. He said this still was within the claimed accuracy of the system. Regarding the publication he questioned whether the system was installed properly, since it´s not that easy, as he admitted. Ergomo is working on an installation that is less prone to mistakes than currently.

The Ergoracer 3 software that should be downloadable for one week now, is shipped with all new systems. They did not put it on the download section since they wanted some feedback first. I got one on CD, so if there is some interest, I can put it online as torrent file.