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Old 07-25-2012, 12:48 PM   #1 (permalink)
GRU
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Drag coefficient

I was compairing different car's and their Cd. From reading about boat tails and the streamlining template on this forum i expected that most cars that have a clean body with a small angle taper at the back will have a better Cd than cars that are hatchbacks.

The following cars/truck have a suprisingly low Cd.

2008 Chevrolet Tahoe Hybrid Cd. .34
2010 Ford Escape Cd. .29
2005 Honda Odyssey Cd. .30

How can such a huge vehicle that has the opposite of a boat tail be better than most small, fuel efficient cars with a Cd. of about .35? If they can lower the drag on these big SUV's without a boattail, then what's making them better than sports cars from the 90's?

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Old 07-25-2012, 01:02 PM   #2 (permalink)
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...lookup the Cd for the GM VOLT, it's a notchback with a sub-0.30 Cd value.
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Old 07-25-2012, 02:10 PM   #3 (permalink)
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Cd is just the coefficient of drag. To calculate the loss, you have to factor in the frontal area, so a Tahoe may have a fairly low Cd, but it's still as big a barn door and costs more fuel to push it through the air.
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Old 07-25-2012, 04:48 PM   #4 (permalink)
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The trailing vortices from the greenhouses of 3-box designs must take more energy than the random wake behind a squareback. The squareback and the tapering in towards the rear are Kamm's secret to success.
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Old 07-25-2012, 07:13 PM   #5 (permalink)
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How can

Quote:
Originally Posted by GRU View Post
I was compairing different car's and their Cd. From reading about boat tails and the streamlining template on this forum i expected that most cars that have a clean body with a small angle taper at the back will have a better Cd than cars that are hatchbacks.

The following cars/truck have a suprisingly low Cd.

2008 Chevrolet Tahoe Hybrid Cd. .34
2010 Ford Escape Cd. .29
2005 Honda Odyssey Cd. .30

How can such a huge vehicle that has the opposite of a boat tail be better than most small, fuel efficient cars with a Cd. of about .35? If they can lower the drag on these big SUV's without a boattail, then what's making them better than sports cars from the 90's?
*A vehicle with absolutely zero roof camber and zero body sides camber would have a drag minimum of about Cd 0.5.
*By having a small amount of either it's a no-brainer to achieve Cds on the order of 0.29.
*The 2013 Ford Focus hatchback with only 26% of aft-body has Cd 0.29.
*The 2012 Prius V has 25% aft-body for Cd 0.29.
*The Prius II @ 32% is Cd 0.26
*Insight gen-II 33% is Cd 0.28
*Mercedes-Benz Boxfish @ 32.5% = Cd 0.19
*Renault Vesta II @ 28.5% = Cd 0.19
*Chevy Citation IV @ 47.7% = Cd 0.18
*M-B C-111 III @ 100% = Cd 0.178
*AeroCivic @ 50% = Cd 0.17
*1993 GM EV1 LSR (USAC) @ 55% = Cd 0.156
*Shamu (UC Davis ) @ 62% = 0.15
*GM Aero 2002 @ 53.7% = Cd 0.14
*Ford Probe V @ 54.6% = Cd 0.137
*1921,Jaray 'pumpkin seed' @ 73.5% = Cd 0.13
*1987 GM Sunraycer @ 89% = Cd 0.12
You can see that the elongation to the aft-body translates into lower drag as Hucho suggests.
If you compare the Prius V and 2013 Focus to the 'Template' you notice a dead match and the Cds are within 3.5% of predicted values.
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Old 07-25-2012, 10:21 PM   #6 (permalink)
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I see but i guess my question is, if 2 very different (frontal area) cars with the same Cd were going down the highway at 60mph wouild it take the same amount of energy to push them at that speed (assuming that weight and rolling resistance was the same)?
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Old 07-25-2012, 11:23 PM   #7 (permalink)
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No, because it's the product of frontal area (A) times Cd that counts, so if the frontal areas are different then their aero drag forces will likewise be different.

The smaller the ACd product is, the lower the resultant aero drag force is.
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Old 07-26-2012, 12:06 AM   #8 (permalink)
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Quote:
Originally Posted by GRU View Post
I see but i guess my question is, if 2 very different (frontal area) cars with the same Cd were going down the highway at 60mph wouild it take the same amount of energy to push them at that speed (assuming that weight and rolling resistance was the same)?
Don't confuse Cd with CdA; If CdAs are the same then the aero load is the same.
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Old 07-26-2012, 09:12 AM   #9 (permalink)
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Old 07-26-2012, 09:35 AM   #10 (permalink)
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So, in all these threads looking to gain insight on the most aero-efficient cars out there, we should perhaps be listing the CdA, and not the just the Cd?

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