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Old 07-25-2012, 07:13 PM   #5 (permalink)
aerohead
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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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