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PostPosted: Mon Apr 14, 2008 4:16 pm 
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Joined: Tue Feb 27, 2007 6:54 pm
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Location: Cleveland Hts., Ohio
Sorry to take my own thread off topic, but since weve got some minds working on this, why is my Powertap SL reading (as I have read everyone elses as well) 10 to 15 watts low when locked into a trainer? Compared to the Computrainer during a Conconi test my PT was 10 to 13 watts low the entire time.

Is this an affect of the Powertap being clamped by the trainer?

TIA,
Ray

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Posted: Mon Apr 14, 2008 4:16 pm 


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PostPosted: Mon Apr 14, 2008 4:39 pm 
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10 - 13 watts isnt bad and it's more important that it's consistent. The computrainer is just a different pick up and will stay pretty constantly in the same range difference with both the SRM and Tap and Ergomo (and, strangely enough the Polar).

My difference with an SRM was 4 and the Tap I had was basically dead on. It all depends on their initial set up. but they all track power well.

Switch that up and the same SRM actually read 5 higher than a friends computrainer, so honestly I think the + or - could be greater on the computrainer, but it's consistent. meaning your test might just as well have meant your computrainer was 10-13 watts high instead of the Tap being low.

Repeatability is pretty important and the computrainer (as long as you calibrate it doing the roll down each time) is pretty accurate/repeatable, just like the SRM and Tap. You probably don't see much drift between them.

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PostPosted: Mon Apr 14, 2008 5:12 pm 
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So we're agreed?

It's a cool toy, it's a nifty idea but to have sufficient information to provide reliable data over shorter times further development is required?


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PostPosted: Mon Apr 14, 2008 5:25 pm 
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I think we agree that the Ibike is a step down from the SRM and Tap but I don't think "toy" is completely true.

There are those that don't need the accuracy of the other units and will find the price difference enough of a reason to use the Ibike.

I'de spend the extra 190 bucks if I thought I needed accurate info for training, but that doesn't make me right or anyone that enjoys the Ibike wrong...

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PostPosted: Mon Apr 14, 2008 5:37 pm 
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PezTech wrote:
I think we agree that the Ibike is a step down from the SRM and Tap but I don't think "toy" is completely true.


I'd say my new bike was my new toy. Depeneds how you define toy!

Btw. I think my new bike might just be more...'colourful' than anything even you've built before Mr Pez! I'll put photos up once I get the Nokons and handlebars!


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PostPosted: Mon Apr 14, 2008 7:53 pm 
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Gotcha on "toy" :wink:

and this I gotta see on making a bike more "loud" than that last MotoGP bike... or the one being built weds... :twisted:

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PostPosted: Mon Apr 14, 2008 10:29 pm 
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Hmm so maybe I was going in a bit hard - but you should actually take that as a sign of respect, John. If it hadn't been somebody I expected to know better I'd probably have been a lot easier.

For the record I still stand by my points - a strain gauge system has far fewer and smaller errors than are possible with the system iBike uses. I've actually played with putting accelerometers on a bike, and it all gets rather difficult isolating all the different things going on - it seems they've done a superb job with what they have, but such a system is always going to be limited by sensor accuracy. Yes, it's a useful indicative tool, but I'm very unconvinced by the idea that it is (or can ever be made to be) an accurate and repeatable tool. I actually like the idea a lot, but you have to understand the limitations.

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PostPosted: Wed Apr 16, 2008 7:02 pm 
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IBike has a new PM out called the iAero... better vibration resistance and better CdA measurement apparently. Also uses data from another power meter (if you have one) to give real-time CdA info. Getting pricey though... now $800.

http://www.ibikeforums.com/viewtopic.ph ... t=181#p994


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PostPosted: Wed Apr 16, 2008 9:51 pm 
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rruff wrote:
IBike has a new PM out called the iAero... better vibration resistance and better CdA measurement apparently. Also uses data from another power meter (if you have one) to give real-time CdA info. Getting pricey though... now $800.

http://www.ibikeforums.com/viewtopic.ph ... t=181#p994


:lol:

When I first read this post, I thought you were being ironic because that is what some of us were arguing iBike is useful for... but it isn't a joke, rather a more effective use of the iBike... but at least it isn't being called a power meter, so we can stop bickering over semantics and start tweaking the hell out of the engine AND the chassis! :wink:


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PostPosted: Fri Apr 18, 2008 4:03 am 
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Take a look at the following graph. It clearly indicates that no 2 power meters will track exactly to each other. In this example, pick the one you want to believe is most accurate. The folks at Velocomp have a bike with 4 or so different power meters on it for the purpose of creating comparisons so people can judge for themselves. Also, the book Training and Racing with a Power Meter by Hunter Allen and Andrew Coggan has a chapter that is dedicated to discussing the pros and cons of the different power meters. Unfortunately, it was written before the existence of the iBike, but that is not the point. It describes how they each measure power, how often they measure, how often they record, how much internal smoothing is performed, etc. They all have their weaknesses. For example, the ergomo Pro only measures power from the left leg!


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PostPosted: Fri Apr 18, 2008 4:08 am 
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tlee-nice one. Not only is the SRM the most expensive, it makes you look the strongest!! j/k-I realize anyone or none may be the most accurate.

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PostPosted: Fri Apr 18, 2008 4:20 am 
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Now *that* is enlightening! So which one do you believe? Ergomo and SRM are the furthest apart. Interesting to see how well the iBike tracks - near the average. Except at a few points where it is remarkably less than the others. Since several are based on the same method of measurement (strain gauge), I wonder why there is such a discrepancy? Is it an accuracy vs precision problem? Calibration? Instrumentation? Drift? There are, indeed, problems with strain gauges.

John Swanson

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PostPosted: Fri Apr 18, 2008 5:42 am 
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It is stated that the ibike can have a tough computing data from sharp cornering. My guess is that the ibike discrepancies may be from this. It looks pretty accurate most of the time. This is one of the graphs I spoke of earlier. Tough to hold these machines to a standard of graphs in unknown circumstances though. It would be fun to do an independent comparison where we could really run them all through various terrains & conditions.

It's quite interesting that the wattage only peaked at a hair over 300w & what is the distance or time length of the ride? I'm sure that any of you who ride with a PM know that a 300w peak can be attained by even a beginner with ease.


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PostPosted: Fri Apr 18, 2008 12:00 pm 
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ScienceIsCool wrote:
Now *that* is enlightening! So which one do you believe? Ergomo and SRM are the furthest apart. Interesting to see how well the iBike tracks - near the average. Except at a few points where it is remarkably less than the others. Since several are based on the same method of measurement (strain gauge), I wonder why there is such a discrepancy? Is it an accuracy vs precision problem? Calibration? Instrumentation? Drift? There are, indeed, problems with strain gauges.

John Swanson


Note only the SRM and PT are comparable (Ergomo is crap and doesn't use strain gages, but topic for another thread)... the PT is always going to read lower by a couple % because it is measuring after the powertrain losses... they will track very close to each other... that chart is of limited use as it doesn't show time increment, and the wattage is very very low.

Yes iBike power "average" should be close if setup properly... if you are hobby cyclist and would like to know your average wattage it will be good enough.


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Posted: Fri Apr 18, 2008 12:00 pm 


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PostPosted: Fri Apr 18, 2008 1:16 pm 
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Exactly. The plot shows the strain-gauge-based meters to be in excellent agreement to the resolution of the plot. Drivetrain losses are relatively greater at lower power levels, so I believe the difference between the PT and SRM are real. The Ergomo probably isn't calibrated properly, or else the rider has a L-R imbalance. The iBike looks decent, but if you care about 10 watt differences, it's not so good. One can have to train extremely hard to get a 10 watt difference.


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