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aerohead 05-24-2008 04:18 PM

mods-data-% change or Cd change( installment#5-roofline data)
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.

End note.


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".

aerohead 07-07-2009 04:59 PM


Originally Posted by zacksewer (Post 113332)
Hi there. Are you still up with these infos here? Sorry to bump this thread but does somebody here know about some more updated thread on this?

Lately,we're enmeshed in controversy over the aerodynamic "template",which was submitted as a tool to simplify aft-body modifications to any vehicle while maintaining a high degree of confidence for attached flow and positive drag reduction.

winkosmosis 09-24-2009 12:11 AM

Very interesting that a tonneau and cab wings are almost as good as an aerocap.

I assume wiings are the triangular sheets that extend the sides of the cab.

aerohead 09-26-2009 02:01 PM


Originally Posted by winkosmosis (Post 129453)
Very interesting that a tonneau and cab wings are almost as good as an aerocap.

I assume wiings are the triangular sheets that extend the sides of the cab.

Yes,they project down from above while simultaneously tuck in from the sides.From the rear they would appear like the letter-C tilted over onto open end.

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