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Old 09-30-2017, 03:59 PM   #307 (permalink)
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Originally Posted by Merlyn2220 View Post
If I'm reading the truck data correctly:

Cd = 0.435 with everything and the joints taped up on the bed cover (aka Tonto)
Cd = 0.458 as above without the tape (+5.3%)
Cd = 0.357 removed the bug deflector (-22.1%)
Cd = 0.338 removed the air dam (-5.3%)
Cd = 0.450 stock (no Tonto, bug deflector, airdam) (+33.1%)

Is that an accurate summary of the truck data? I ask because there's a prevailing sentiment that the front end geometry isn't as important as the rear end geometry. I recall reading a post about Bonneville testing where they said that extensive front-end changes made little to no difference in speed. The bug deflector seems to be a huge hit to Cd performance, negating the advantages of all the other aero details. So clearly the front end matters a lot. I wonder how bad it is with the bug deflector and nothing else...

Maybe the right way to look at front vs rear is, "don't do anything really bad on the front, or you'll lose all the work you've done on the rear."
Layne printed the entire data set for me.So here's a stab.
Looking at the raw data,the changes reflected:
*Taped gaps = delta 3.03%
*Bug deflector delete = delta 14.92%
*Airdam delete = delta 2.79%
*Tonto delete = delta 15.34%
From worst-case,to best-case,there was a 15.35% delta CdA,using 36-sq-ft frontal area and the reported Cds of 0.7815 vs 0.6615.
The Chevrolet C-10 from the same era was reported @ Cd 0.535 if you reverse-engineer from the 1986 body-in-white C-1500.The JEEP Comanche pickup was Cd 0.55.Some messengers will have you just estimate Cd 0.50 for this time period,as these values weren't offered freely in the press.
Gumby's Dodge shares some of the liabilities that my '62 has.Erupting windshield weatherstripping, inset side glass,plus exposed rain gutters,and windshield header overhang conspire to frustrate attached flow around the greenhouse area.
The mid-80s new truck designs addressed these shortcomings.Cd 0.55 goes to Cd 0.45,then to around Cd 0.35 for today's RAM 1500 with tonneau and grille-shutters.The GM Holden Ute of late is Cd 0.309 ( more Ranchero/El Camino than a 'real' pickup).
To your point though,Hucho would probably tell you,that for passenger cars,as of the mid-80s,there was little low-hanging fruit with the forebody.The real action was in the aft-body.
Gumby's Dodge and most pickups would be an exception.
And it's a good idea to look at the drag index as well as the Cd when evaluating changes.
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