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.
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I've gleaned the following from my train-wreck of a library.As always,the material is dated,however,in light of pickups and SUVs,the material may help with respect to cost-benefit scenarios on potential mods.
There is a general assumption that bellypans are a must for low drag, however,from at least one of the following citations,unless the work is carefully integrated into the underbody,only mediocre returns may be realized. So here we go :
1950s,wind tunnel studies at University of Wichita show a potential for 6.3 % drag reduction with a full bellypan for passenger cars of the day.
Wind tunnel studies at the University of Michigan demonstrate a potential drag reduction of 5.8 % with employment of a full bellypan.
Walter Korff,of Lockheed Aircraft ( designer of Cd 0.115 Goldenrod,land speed record car,409-mph ) suggests that a 15.38 % saving can be had with a full bellypan.
NASA,at Dryden Research Center,Edward's Air Force Base,California ( I was there but didn't get to see any of it ) achieved a 15 % drag reduction on a Ford Econoline van,with a carefully executed and highly refined full bellypan.
April,1975, CAR and DRIVER Magazine attempts ecommodding on Dodge van with crude bellypan,and meet with great frustration as their labor of love nets them only a 4 % drag reduction.
Late 70s, Ford Probe-III concept car becomes Ford Sierra/Merkur XR4Ti,and deteriorates from Cd 0.22,to Cd o.32.The movable front valance and full bellypan are deleted from the production cars.Underside losses do to deletion are estimated at Cd 0.086,and 26.87 % increase in drag.
The late 1970s Audi 100 scores a 15 % drag reduction with a full bellypan.
1980,a full,but less than ideal bellypan gives my 1970 VW bus a 8.726 % reduction in drag and extra 1.038 mpg.
1986 Subaru XT nips 3.1 % drag with front pan,lowered and flush trunk floor,and lowered and flush with bottom of car bumper fascia.
1986 FIAT wind tunnel research reports that a full bellypan is good for 0.07 off the drag coefficient, 0.08 if combined with the 2-1/2 degree upswept diffuser.
The full bellypan,combined with rocker-panel extensions on the T-100 helped push the mpg from 30,to 32 mpg, a 6.66 % improvement in mpg and 11 % drag reduction.
As you can see,there are no hard and fast rules that apply to under-bodies,and you'll have to consider your own project on a case-specific basis. I've got more info on belly-pans, however, their performance is embedded within that of "kits" that were done on vehicles,and I'll have those in future " case-studies".
In 1962,HOT ROD Magazine exclaimed that "...all cars with low Cd's will be underpanned." So far they've been right.