In simple terms. I looked but couldn’t find anything. Coefficient drag?
Cour d’Alene (sp?), as in Ironman CdA
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In simple terms. I looked but couldn’t find anything. Coefficient drag?
A erodynamic
C oefficient of
D rag
The combination of drag coefficient and area is CdA, a multiplication of the Cd value by the area.
Scott
the only way i have used CdA is in talking about the Ironman Coeur D’alene.
thats how I know it…Cd (coefficient of drag) * A (surface area) = CdA
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As others have said, CdA is the multiplication of Cd (drag coefficient) and A (area). Cd is a value associated with a shape, basically it is how slippery it is, how easily air moves over and around it. For instance a flat square would have a high Cd because it is difficult for air to move around it (think putting your hand out the car window at speed with the palm facing forward), but an arrowhead would have a lower Cd because the shape lets air move around it more easily. Once you know how slippery a shape is, you can calculate how much drag it will create at a given speed by multiplying the Cd by the area, thus CdA.
So pretend you have 2 cyclists:
Cyclist 1 is wearing a sleek aero helmet (low Cd “theoretically”) and has a flat back (let’s pretend for this argument that flat back is always faster/lower Cd), etc, let’s pretend his Cd value is .1
Cyclist 2 is using a regular helmet (higher Cd) and sitting more upright let’s pretend they have Cd of .2 because it’s hard for air to move around them
If both cyclist have the same area, then cyclist 1 would have half the CdA that cylist 2 would. If the area was 10 square meters (an absurd amount obviously) then cyclist 1 would have a CdA of 1 (10* 0.1) and cyclist 2 would have a CdA of 2 (10 *0.2). But if cyclist 2 was half the size of cyclist 1 in area, they would have the same CdA.
Once you know the CdA of something or someone (including the bike/wheels/everything) you can then figure out how much drag is created theoretically when wind moves over them or they move through the air. Real calculations are complicated by a lot of things, but that’s basically it.
Does that make more sense?
Cadmium Aluminite. New(ish) frame material. Half as dense as your average carbon weave and just as strong.
Turns into popcorn when it gets over 150 degrees F, though.
Also, has a natural odor similar to used bandaids.
Cadmium Aluminite. New(ish) frame material. Half as dense as your average carbon weave and just as strong.
And I assume that the cadmium element will make it highly dangerous?
I’m just learning about this myself, but from what I can tell the usefulness of this number is that it describes changes to the two main things that are involved in ‘aero-ness’: how small something is, but also how ‘slippery’. So part of the reason an arrowhead is better than a hand is because it has a much smaller area, and is also the predominant reason why getting lower tends to be better. Then there are things like wrinkled jerseys, flat shapes, etc. that are bad because of not being slippery, and this pertains to the Cd part of the number. Also, consider the careful shaping of aero frames. By combining the two variables, you get a meaningful expression of the resistance being provided in different situations, since one could go up and another down.
I’m sure if this isn’t exactly right, someone will correct me…
The fundamental equation for aerodynamics is:
D = cD * 1/2 * rho * V^2 * A
Where:
D = total drag (in grams*)
cD = coefficient of drag (no units)
rho = air density (kg/m^3)
V = speed (m/s)
A = frontal area (m^2)
From that you can work out the drag on most basic shapes.
*edit - well, grams if multiplied by 1000/9.81!
Are you objecting to something I said? (There probably was something sloppy in it)
Right, but most triathletes I know don’t have control over the air density and are trying to optimize speed, so work on reducing total drag by decreasing cD and A. No?
An approximation because the mathematics are complicated… like 1/2mv^2 … see CFD
No sorry, the reply button above your message was just the closest one!
Not any more dangerous than riding a bike made mostly out of 5-year-old laptop batteries.
From memory, the EU has banned it (they were being very difficult when I was last involved in this sort of thing 2 years ago) which kind of limits the market 
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Er…um…I was …heh…you see…It was just a test. That’s it. A test!
Oh. That’s simple.
There’s a block in some French lab somewhere that weighs exactly a kilo. As the weight of that block changes, the definition of a kilo changes.
I never claimed that it was sensible eh?!?
http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_International_Prototype_Kilogram
“After the International Prototype Kilogram had been found to vary in mass over time”
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An approximation because the mathematics are complicated… like 1/2mv^2 … see CFD
Not really. CFD is used to determine Cd, but it should be relatively stable in the sets of Reynolds and Mach numbers that a bike operates, and the 1/2 CdArho*v^2 formula holds up pretty well. If someone were testing an object in a wind tunnel, and it was going to operate over a wide range of Reynolds and Mach numbers, Cd would be tested and calculated for each of these regimes. Scaling effects (speed, density, and linear size determine Reynolds number, and speed vs. speed of sound in the medium determine Mach Number) are always a challenge when trying to translate results at one speed or area to another. This is particularly apparent when trying to use a wind tunnel to calculate Cd and Cl for an object of a different size than the test model - for example, wind tunnel testers who use model airplanes to represent their full-size counterparts must make many adjustments to their results to account for scaling effects. In fact, aerodynamicists sometimes use pressurized wind tunnels or cryogenic wind tunnels to allow better scaling of Reynolds vs. Mach number (since Reynolds is affected by density, but Mach isn’t). The nice thing about testing bikes and riders in a wind tunnel is that you test a full-sized model at speeds which are representative of how fast the object wil be going.
Even CFD is only an approximation (turbulence models in particular are a source of error), and generally has to be validated by wind tunnel and full-scale testing. Ultimately, it is just another way of calculating
1/2mv^2 is complicated?
Cd is a dimensionless number, and accommodates the otherwise unquantifiable aerodynamic traits (“shape”) of an object.