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Originally Posted by tjts1
They got the angle right at the factory.
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Not quite. The "right" rear roofline would look a lot like a Prius, or the latest Mercedes E Class coupe (if you prefer a trunk), or like the Chevy Volt (if you prefer a "notched" hatchback style like the 318t).
The effective angle of the rear of the 318t (end of roof to tip of decklid) is ~20 degrees. The glass alone is closer to ~23 degrees.
The ideal angle, per the diagram you posted, is ~13 degrees. Not coincidentally, that's much closer to what you'll find on the Prius, E Class coupe or Volt.
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Either way you are increasing the size of the low pressure zone behind the vehicle which is counter productive.
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Tuft testing of the 318t would probably show that there turbulence & flow reversal on the rear deck lid. That means flow separation (low pressure wake) starts at a point on the rear glass, not at the trailing edge of the "deck lid". It may seen counterintuitive, but a properly designed batman thing will permit controlled pressure recovery further aft and lower down, and would result in a smaller overall wake.
Given the angle of its rear glass, the 318t needed either a taller rear deck lid, a longer one, or a big-ass spoiler for better aerodynamics from the factory.
(Chevy Volt - GM image)