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I am greatly unmoved by this article

b987654 said:
http://www.ccbracing.com/feature/2004/january/mcallumaero.html

I don't know how to paste in the page, but it seems to have good info.
I'm not sure that this article is really worthwhile reading. There are many glaring ommisions and inconsistancies.

Firstly, where does the data for the wheel aerodynamic power come from? Does it show absolute power, or relative power? The graph shows a range of 40-50 (I assume the units are Watts) for the 32 spoke box wheels, and 0-10 for the disk wheels. I'm pretty sure that no wheel has zero drag, so perhaps this is supposed to represent relative power (i.e the 32 spoke box wheels take 40-50 Watts more than the disk wheels)? At any rate, the numbers are way too high - much higher than I've seen published elsewhere. As a comparison, go to the AnalyticCycling page for Forces on Rider and enter a speed of 12.1 m/s (27 mph), which is the same as indicated in the CCB web page. The AnalyticCycling analysis uses the wheel drag coefficients for 36 spoke box rims (see the page on wheel drag coefficients), and the results of the Forces on Rider analysis show that the drag force on the wheels is 280.7 gmf, or 10% of the total drag. Well, 280.7 gmf of drag at 12.1 m/s is only 33.3 Watts total, so no aerowheels would be able produce a drag reduction of 40 watts. Entering the drag coefficient of disk wheels (0.0364) into the formula reduces the drag on the wheels to 208.1 (24.7 W at 12.1 m/s), or a power savings of only 8.6 Watts (not 40 Watts).

Also the statement in the CCB web page that disk wheels can increase speed by 2 mph has never been demonstrated by any measured test anywhere. Most tests indicate speed increases are more on the order of 0.5 - 0.75 mph.

Finally, the conclusions of this paper, that various commercially available wheels (Mavic Ksyriums, Rolf Sestrieres, Zipp 303s, etc.) are based on highly subjective and unmeasured rider impressions.

I find nothing of great value in this web page.
 

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Mark McM said:
I'm not sure that this article is really worthwhile reading. There are many glaring ommisions and inconsistancies.
Not to mention outright errors.
"The easiest way to visualize this phenomenon is to run your hand palm first through the surface of still water and observe the wake."
What is being observed in this test depends too much on interfacial effects and the density difference between water and air to reveal anything about the drag on a body or wake behind it in a single fluid phase.

"Aerodynamic drag can be measured in grams of resistance or watts, and for the purpose of making comparisons, I’ll be using watts to simplify things since most cyclists’ measure performance in this manner."
Except drag is a force and watts are a measure of power. To say drag can be measured in watts shows the author doesn't understand this difference and raises all kinds of questions about how he might be interpreting results.
 

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asgelle said:
To say drag can be measured in watts shows the author doesn't understand this difference and raises all kinds of questions about how he might be interpreting results.
Actually he is right... drag is converted to watts simply by multiplying it by the velocity... and I think dealing in watts makes more sense, also.
 

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rruff said:
Actually he is right... drag is converted to watts simply by multiplying it by the velocity... and I think dealing in watts makes more sense, also.
Converted to, yes; equal to, no. Things work out pretty much the same whether someone chooses to work in terms of force or power, but if the two are mixed, there results are meaningless. In the referenced article, the author drops the distinction at the start, so there's no telling how he might have compared force and power data later. By the same token, grams are a unit of mass and I'm not sure how the author gets from there to either force or power.
 

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asgelle said:
By the same token, grams are a unit of mass and I'm not sure how the author gets from there to either force or power.
It may be stupid but very common in this business... I guess in wind tunnels they use gram scales to measure the drag. Multiply grams by standard gravity to get force. Multiply force by velocity to get power. Personally, I like to see aero drag (of bikes) presented as W @ mph... but nobody listens to me...

Actually, I'd like CdA even better since it isn't velocity dependant. Ever see anybody present aero wheel data as CdA?
 

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rruff said:
It may be stupid but very common in this business... I guess in wind tunnels they use gram scales to measure the drag. Multiply grams by standard gravity to get force. Multiply force by velocity to get power. Personally, I like to see aero drag (of bikes) presented as W @ mph... but nobody listens to me...

Actually, I'd like CdA even better since it isn't velocity dependant. Ever see anybody present aero wheel data as CdA?
Yup. The gram thing kinda rubs me the wrong way, since it's a pretty contrived way of ending up with a unit that doesn't really makes sense in the given application. Force (newtons, lbs) is better, but rruf is right Cd is great (my preferred) and CdA is alright, too. Having emerged from the physics world, correct units were hammered into my brain. Grams just ain't right.

Unlike rruf, I'd prefer W @ mps (as in meters per second) over W @ mph, cuz I think things go all pear shaped when you start mixing systems of units....I'm still waiting for the petition to ban all English and Engineering units to come my way so I can sign it. MKS or die, I say.
 

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rruff said:
. Multiply grams by standard gravity to get force. Multiply force by velocity to get power.
I know that's the correct way to do it. It just isn't clear that the author knows that or even is aware of the difference between mass, force and power. It's the writer's responsibility to convince the reader not to throw stuff out there which may or may not be correct and then let the reader figure it out. The simple fact is the statement "Aerodynamic drag can be measured in grams of resistance or watts" is just plain wrong since watts have units of power.
 

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SI is good...

alienator said:
I'm still waiting for the petition to ban all English and Engineering units to come my way so I can sign it. MKS or die, I say.
Here! Here! Way back when I was in school that was actually the plan in the US... to go metric anyway. In school everything was MKS (or SI) units... I didn't know how good I had it! There is nothing more tedious and boring than plugging in all the conversion factors so you can actually calculate something. It's really easy to make a mistake too.

I remember giving my first boss a hard time for presenting some data with energy in "watt - minutes"... I'm like "why don't you just put it in Joules?... that is an actual energy unit". He never got it...

M/s would be nice... unfortunately nobody measures speed in those units, so conversions would need to be made anyway...
 

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rruff said:
Here! Here! Way back when I was in school that was actually the plan in the US... to go metric anyway. In school everything was MKS (or SI) units... I didn't know how good I had it! There is nothing more tedious and boring than plugging in all the conversion factors so you can actually calculate something. It's really easy to make a mistake too.

I remember giving my first boss a hard time for presenting some data with energy in "watt - minutes"... I'm like "why don't you just put it in Joules?... that is an actual energy unit". He never got it...

M/s would be nice... unfortunately nobody measures speed in those units, so conversions would need to be made anyway...
So you want to know how it is that we got stuck with such backward units (I'm sorry there is no reasonable justification for having a pound mass and a pound force. Geez. English units are bad enough, but do we have to have a screwed up Engineering units system, too?)

According to Physics Today......

At the end of the 18th century, the French Academy of Sciences began work on the Metric system, a system designed to be a universal standard of measurement. Two of the units, length and time, were to be based in part on the circumference of Earth. Time would be based on how long it took the Earth to complete one full revolution, and length would be based on the circumference. Both of these standards were to be measured at or along a specific line of longitude.

It happened that the line of longitude chosen went from Barcelona to Dunkirk, passing through Paris. It so happened that Thomas Jefferson was peeved that the French Academy commission didn't follow his suggestion to measure the period of a pendulum at the 38th parallel (thus deriving the radius and circumference of the Earth) which just so happened to pass very near his home in Monticello. He was also uber unchuffed that the observatory making measurements for this new measurement system was in France and near this longitude datum. The US pretty much immediately lost interest in the French Academy's work, and we were thusly doomed to stupid units (pounds, pound-mass, inches, feet, acres, gallons, cups......) and stupid unit conversions (12's, 3's, 5280's.....).

Go figure. Apparently there is historical precedent for petty decisions like renaming French fries "Freedom Fries." Yup. Because someone or somebodies got pissy, we're stuck w/ a lousy units system and everything that comes with using units that conflict with everyone elses.....like flying probes into planets, stores not stocking a full spread of metric tools, Harley Davidson......:rolleyes:
 

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alienator said:
.....like flying probes into planets, stores not stocking a full spread of metric tools, Harley Davidson......:rolleyes:
Last time I visited the homestead my Dad confessed that his brand new John Deere planter (made at least partially in China I guess) had a random *mixture* of metric and SAE bolts and nuts on it. Can you imagine how much fun that would be to work on?... Like "oops, I guess that was 1/2 inch and not a 13mm"... as you round off the edges of the bolt head. I couldn't believe he didn't send the POS back...
 
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