Note from Darin (admin): this installment is part of a series posted by Phil (aerohead) about the effectiveness of various aero mods - usually with quotations and citations to source data.
See the aero mods data index here.
First let me apologize,as the following is just a mixed bag of assorted tidbits published over the years.There is not a lot of comparisons which specifically target roof changes.Here goes: 1959 Chevrolet BelAir fastback vs Impala hardtop (same car just different roof),Cd 0.44 vs Cd 0.48 (10 % difference).
Jaguar XKE open convertible vs closed, Cd 0.85 vs Cd 0.55 (a 54-per cent difference).
1961 Vauxhall station wagon vs notch back, Cd 0.36 vs Cd 0.47 (a 30-per cent difference).
1962 Chevrolet Corvair Coupe (taper-roof) vs sedan notch back, Cd 0.40 vs Cd 0.42 (5-per cent ).
1939 Mercedes-Benz 170 vs Kamm/Fachsenfeld K-3 -bodied Mercedes 170, at 55-mph, 18.1 miles per gallon vs 29.4 miles per gallon (62-per-cent ).
From: A Method for Estimating of Drag Coefficients, by R.G.S. White, White allows a range in drag coefficient of up to 0.0665 difference, based of roof design
As mentioned in the quotes, Hucho reports that the fastback with 15-degree angle achieves the lowest drag over other roof designs.
Ford Motor Co. gets a 6-percent drag reduction on the Probe GT production car, with 71-degree, rather than 45-degree backlight.
Dr.Michael Seal, Western Washington Univ., chooses the Abarth/Fiat "double-top" roof design for the Viking 100-mpg series cars. This roof is used on current Toyota Prius.
SAE Paper 860211 reports drag reduction of Cd0.004 when tumble home angle is increased from 25,to 40-degrees on a fast-back car. Notch back cars see a Cd 0.003 improvement for same change. Squareback cars suffer a -Cd 0.005 loss with same change.
Subaru nets a 5.7-per cent drag reduction for XT, when they boat-tail the sides of the backlight 3-inches over the span.
Hucho publishes that a fast-back angle of 23-degrees demonstrates same drag as square-back. He also demonstrates that integrating drip rails into the A-Pillars on the VWs was good for a 7-per cent drag reduction. Simply raising the trunklid could achieve an 8-per-cent improvement.
Texas Tech University Aero Lab publishes: pickup cab wing good for 6-per cent drag reduction,1/2-tonneau gives a 6-per cent drop, full tonneau gives 7-per cent, cab wing and 1/2 tonneau together nets a 17.5-per cent gain, and an "aeroshell" is good for a 20-per cent drag reduction.
-HONDA Civics show a spread in road horsepower for three roof designs: CRX (fast-back), 3-door (hatch-back), and 4-door (notchback). At 50 miles per hour, the fast-back needs 10.5-horsepower,the hatch-back requires 12.0-horsepower, and the notch-back absorbs 13.0-horsepower. While I don't have data for the standard CRX at 70-miles per hour, a comparison for my streamliner shows that at 70-mph,as compared to notch-back, the road horsepower for the two cars is: 19.5-hp,and 27-hp respectively, a 38-per cent difference.
There is additional data published for roof performance, however it is embedded within a mixture of other modifications made to specific vehicles. I will present this data in a follow-up installment entitled "case-studies".