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Is that what the 6W figure relates to, or is it normalized to the 200W given in the chart?
My guess is it is normalized to 200W so we have a common point for comparison but you would have to ask Jim since im not an expert on all the formulas involved.
The speed data his calculations use were from a different speed sensor mounted on my chainstay that used a magnet on the rear wheel. I think he said the magnetless one wasn't accurate enough. So his speed data may be slightly different than mine.
Right now, I would say the easiest way to get a sense of CdA is try out BestBikeSplit. You'll quickly get a sense of what a change of 0.007 means for you and your chosen course.
Case in point.
- local monthly TT course (hilly 24km out and back)
- reduce my calibrated CdA by 0.007
- hold everything else constant
A .005 reduction to CdA at 48kph is almost 7 watts, But it takes a rider ~400 watts to go that speed with a starting CdA of .25. A rider capable of pushing 200 watts is going to be looking at a savings of a bit less than 3 watts.
As others have said when someone says something saves X you have to look at the specific conditions and speeds in which they were tested. CdA completely eliminates the guess work as it is a relative constant and scale pretty linearly.
There are CdA figures, near the bottom. Though I find Watts more useful.
Yeah, I get it. But at least for me differential Watts are a more accessible mechanism for understanding the magnitude of effect.
If he had just reported the CdA differences of 0.006 (which is roughly what it was), I'd have to spend some time converting that to something else.
Reporting that it saved 6W @ 200W is immediately accessible.
Maybe you've learned to have a more intuitive sense of CdA, but I think you'd be in the minority there. We spend all day looking at power meters. Except Jim, and a few others. I do my own Chung testing, but even then I like to look at Watt differential at the speeds I'm going.[/quote]
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Seems like a big delta. My garage wind tunnel work moons ago suggests a smaller difference between brakes:
Garage Wind Tunnel Data!
in order to get 3W at 30mph with my setup, I had to do this:
Furthermore, I noticed the phrasing of "transparency" in the tri rig article linked above. A long time ago, a simkins brake was tested using the same field testing approach and the raw power meter data files were published. That was a nice touch by Matt.
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These articles have to be consumable to the average athlete, not those who like to pour over all the data, which I respect, but most people would stop reading real quick if I put all the data in the article itself.
As for the CdA's, well, I have to swallow my pride and admit I was the third test rider and, at 6'2", 240lbs, my CdA is a bit embarrassing. We're actually going to have some fun with that and keep recording my drag numbers both on the velodrome and in the tunnel as I lose about 50-60lbs. That will be a fun article...How Weight Really Matters! My next test will be in about 3 weeks and down about 20lbs.
Jim Manton / ERO Sports
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