XR v Cossie (Cosworth) Cd

XR4Ti / Sierra / Sierra Cosworth Discussions - Questions, problem resolution, general talk, technical tips and modifications.
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anglin
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Post by anglin » Thu Jan 31, 2008 9:36 am

CV12Steve wrote:Later, the wind tunnel would confirm an excellent translation, so good in fact that the North American car improved slightly on the European car's coefficient of drag, of 0.32."
MOCA... nice reference. I wonder how much of that slight improvement was related to the fuel filler door versus the exposed fuel cap of the XR4i. I also wonder if the intent of the rear window moldings on the late cars was to trip the air flow (same concept used on truck cabs and beds to reduce the effective drag area by controlling flow separation). It would explain the increased turbulence allowing more rain on the rear window as Johnny has observed.

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Post by CV12Steve » Thu Jan 31, 2008 7:16 pm

anglin wrote:I also wonder if the intent of the rear window moldings on the late cars was to trip the air flow (same concept used on truck cabs and beds to reduce the effective drag area by controlling flow separation). It would explain the increased turbulence allowing more rain on the rear window as Johnny has observed.
That same "I think I remember . . ." part of the brain says they were for cross-wind stability related to rear lift.
Stephen

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Mike McCreight
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Post by Mike McCreight » Fri Feb 01, 2008 8:48 am

Not that it makes any real world difference, but the #s went from 0.32 to 0.34 when the tire size increased from 14s to 15s. The taller the car is, the more air it has to move.
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Re: ?

Post by CV12Steve » Fri Feb 01, 2008 4:00 pm

Mike McCreight wrote:Not that it makes any real world difference, but the #s went from 0.32 to 0.34 when the tire size increased from 14s to 15s. The taller the car is, the more air it has to move.
That might have been because of under-car air flow changes, and/or the interaction around the cladding "bulbs" at the wheel openings with the ground plane. The same body shape shouldn't change Cd just from being higher, but the area the Cd is acting on will appear greater.
Stephen

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