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Old 05-31-2008, 02:30 PM   #1 (permalink)
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aero mods-data-% change or Cd change ( installment # 7-Wheels/Tires/Wheelcovers/etc.

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.


I've left out the new pre-tire airdams,as I believe they have been addressed by other members and I don't have any additional info on them.

First,a general note. An ideal body form,say,with a Cd of 0.04, when brought into ground-effect,will see it's drag coefficient degrade to Cd 0.09. The addition of wheels and tires further degrades the drag coefficient to 0.14 ( from VW and Fiat research ). Presently, Cds for a "real" vehicle,in ground-effect,have touched 0.10, as with the HONDA Dream II solar-car.And you may argue as to whether or not the HONDA typifies a real car or not.The example is to illustrate the extremes one must go to,to achieve those kinds of numbers.

So from my rat's nest, I've collected some tidbits chronicling observations or reports of those attempting to deal with the rubber donut beasties over the decades,here we go:

1938, front wheel skirts are added to the Adler-Jaray and a 2.6 % drag reduction is measured.

1954, from wind tunnel research on the GM Firebird I "Dream Car",by enclosing the exposed wheels within fenders,the drag is cut from Cd 0.335,to Cd 0.311 (7.7 % ).

1958, Mercedes-Benz encloses the wheels of it's 2.5-liter Gran Prix Racing car with full body work,and despite larger overall sectional area,drag is reduced 20 % and top speed increased 11 % ( JEEP and HUMMER owners,this applies to you!).

1961, Deutsch / Romani,Panhard C.D. achieves Cd 0.15 ( over D.B.Coupe's Cd 0.18 ),by using fairings behind the wheels.The car hits 127-mph with 60 Bhp.

June 1962, HOT ROD Magazine credits either front or rear wheel skirts with a Cd reduction of approximately 0.02.

1963, Walter Korff ( Lockheed Aircraft ) claims "A smooth,slightly crowned disc out to the sidewall ( MOON EQUIPMENT CO. ),on each side of the wheel, provides a big improvement. Covering the wheels and suspension with an envelope type body is better; smoothly streamlining the envelope is best by far."

1963, Korff estimates open-wheeled Gran Prix car (remember Mercedes?) at Cd 0.75 and 7square- feet frontal area,and CdA of 5.25 square -feet.( this implies that the Mercedes race car,by enclosing the wheels achieved CdA 4.2 square-feet (something also to think about as regards to the 1987 Oldsmobile AEROTECH Land Speed Record Car built on March Indycar chassis).

1963, Korff allows for a 7.6 % drag reduction with use of front wheel skirts in his recipe for a Cd 0.21 car.

1969, R.G.S.White makes no mention of undercar modifications except for the bellypan in his recipe for a Cd 0.24 automoblie.

1982,Pontiac Trans Am Firebird wind-tunnel studies show Larry Shinoda's trick aero wheel-covers trim 0.027 off drag coefficient,compared to a open wheel.

1986, wind tunnel development work for Subaru XT show a drag increase of 5.1 % when tire size is increased from 155 to 185 series radials.Drag is reduced 2.0 % due to "air flaps" steering air over rear tires.Completely flat/sealed wheel-covers show a 2.75 % drag reduction.

1984-1987 HONDA CRX shows jump from 165 to 185 series radials increase drag 9.3 %.

General Note: Korff published drag for opened-wheeled go-kart and motorcycle at Cd0.90.Hoerner also reports open motorcycle at Cd 0.90.Easy Rider's Harley-Davidson streamliner motorcycle and Kenny Lion's #919 BMW-powered streamliner bike (same body as Easy Rider's ) score Cd 0.11 in Texas Tech Aero- Lab wind-tunnel,1991,a bit better than Burt Munro's Indian's open cockpit bike record-holder,1967.

Final thought,the HONDA Dream II at Cd 0.10 utilizes lazy-Susan turntables for the steering wheels.Only the lowest part of the tires are exposed to the air stream and in between suspension travel,the front wheel/suspension members have no discernable "opening". Clever!

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