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Aerodynamics and power
 

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KerryIrons
Joined: 12 Jan 2004
Posts: 3038
Location: Midland, MI

3/17/20 8:07 AM

Aerodynamics and power

In the latest issue of Velo News, Lennard Zinn makes the following statement:

"Aerodynamic drag increases exponentially with speed. It doesn't take twice as much power to go twice as fast relative to the air; it takes for or more times as much power."

While both parts of this statement are correct, the second is wildly inaccurate. Aero DRAG is proportional to the square of the speed, but the POWER required is drag times speed, so it is cubic. So saying "four or more" when you should have said "more than eight" is inexcusable to me. How is it possible in this day and age that someone like Zinn could get this so wrong? I expect this kind of nonsense from Bicycling magazine, but not Velo News and not Lennard Zinn.

Note: "more than eight" is because doubling speed means 8 times more power to overcome aero forces and double the power to overcome rolling resistance (tires, bearings, chain losses, etc.).

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Sparky
Joined: 08 Dec 2003
Posts: 17650
Location: Portland, OR

3/17/20 9:42 AM

Intern?

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walter
Joined: 11 Jan 2004
Posts: 4389
Location: metro-motown-area

3/18/20 8:10 AM

zinn has a funny relationship with science and facts...often mixing gut-level intuition into his assertions - often close, but often not *quite* right. they have an element of "truthiness" to them.

i attribute it to zinn not having a formal STEM education -- he's like an "interior decorator" vs an "interior designer" ...the former is a housewife/househusband with a flair for style, whereas the latter actually has an advanced education and a BS level degree or higher.

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dan emery
Joined: 11 Jan 2004
Posts: 6339
Location: Maine

3/18/20 8:25 AM

Zinn education

Actually I believe Zinn has a BS in Physics.

I am not a scientist, but many years ago I read an article by him that struck me as so wacky that I stopped paying attention to anything he writes. I may well be wrong.

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dddd
Joined: 11 Jan 2004
Posts: 3292
Location: NorCal

3/23/20 11:36 PM

Sometimes a writer will put something on paper a little too late in the day?

Missing a multiple of two along the way is easy to do, especially when there are more than a couple of them.

I've seen where automotive tech writers (quoting engine builders) have similarly conflated piston acceleration with increases in piston speed (feet-per-second), and with increased RPM, but piston acceleration actually increases with the square of RPM.
I was left to figure out that one for myself.

Also with chainring size, where drivetrain power-transfer stiffness increases/decreases with the square of chainring size.

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KerryIrons
Joined: 12 Jan 2004
Posts: 3038
Location: Midland, MI

3/25/20 8:49 AM

What?


quote:
drivetrain power-transfer stiffness


Please explain what this means?

As a side note, I sent an email to VeloNews pointing out the factual error in the article. Have not heard back yet.

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dddd
Joined: 11 Jan 2004
Posts: 3292
Location: NorCal

3/25/20 4:54 PM

A lot of new terms come from marketing, but some seem to describe an actual quantitative, measurable quantity.

"Power transfer" came to mean resistance of flex due to pedaling and handlebar forces that absorb motion, and thus energy.

A portion of the system flex is in the drivetrain, and shows up at the pedals as absorbed crank rotation motion (and energy), as opposed to the frame's bottom bracket trying to move out of the way of pedaling forces. Drivetrain flex would include compressive flex of the right chainstay as the chainstay is within the load path of the drivetrain.

A smaller chainring increases chain and chainstay tension and compression, respectively, to the first power.
And that combined tension- and compression-induced flex allows a first-power (or linear) bit of take-up motion of the chain on the chainring, in response to any pedal force increase.
Since the smaller chainring also increases any resultant crank rotation due to any given amount of chain movement, the flex felt at the pedal becomes a squared or second-order inverse function of the chainring size.
This is pretty much just geek-talk, but does explain why using small chainrings makes the drivetrain power transfer feel disproportionally squishy (to the extent that one notices it at all).
The extra elastic energy absorbed by the drivetrain is also an inverse-squared function of the chainring size, but most of which would seem to be recoverable as each pedal stroke peters out.

I would also have to include the rear axle and the bb spindle as components that are part of this second-order "power-transfer" function of chainring size , but the actual crank arm is not included.
I would also not include the rear wheel's torsional flex, as I am comparing the flex difference using a different size chainring but keeping the same gearing ratio with a similarly-smaller rear cog ("compact" approach).

Certain Italian components (wheels) marketing language appears to combine and at times confuse various flex and mass-related parameters such as this under different terms such as "change of rhythm" (acceleration?) and "power transfer", without distinguishing the applicable parameters in defined/measurable terms. Such is marketing I guess.

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