[aerohead]

A few general notes before we jump in,as there still appears to be some confusion over terminology.---------------------------------------------------- "Frontal Area" has nothing to do with the front of anything.Frontal area of your rig will be determined by the maximum cross-sectional area of the tractor ( if it happens to be larger than the trailer you pull,or will be determined by the maximum cross-sectional area of the trailer,should the tractor be smaller.Some journalists refer to the smaller AF of a conventional tractor compared to a COE,referring to the "nose" of the conventional.This is a complete misnomer! ---------------------------------------------------------------------------- The second point I'd like everyone to ponder when they're thinking about Class-8 aerodynamics, is that your GVW is 117,620 pounds ( 53,463 kg ).Let me repeat that.Your GVW is 117,620 LBS ( 53.463 metric tons ).How is that? When you push 95 square-feet of frontal area down the road,you displace 95 X 5,280 feet of air for every mile you travel.That's 501,600 cubic feet of air for every mile.At standard temp and atmospheric pressure,that's 501,600 X 0.075 ponds per cubic foot,or,37,620 pounds of air.18.81 tons of air for every mile.If you are running 80,000 LBS gross,and you add on the mass of air you're boxing every mile,you're actually "moving" 117,620 pounds.----------------------------------------------------------------- Okay,so let's look at some mods.Back in the early 1970s,before the first Arab oil embargo,when diesel was selling at $0.18 / gallon,the typical 18-wheeler had a Cd of around 0.85-0.88 and returned about 5.0 mpg ( maybe 4.5 mpg for anyone hauling a cattle- trailer which had Cd 1.0+ ).--------------------------------------------------------------------- From NASA/Freightliner research,it has been determined that a conventional tractor has no inherent advantage over a COE tractor.Both can be effectively streamlined.Presently,the lowest drag rig is a COE design.---------------------------------------------------------------- The careful profiling of the tractor's nose along with an integrated airdam yielded a 4.8% drag reduction.---------------------------------------------------------- The rooftop air deflector ( whether a "add-on" or integrated into a sleeper ) provided a 15.7% drag reduction.------------------------------------------------------------- Cab extenders and gap-sealers to seal tractor/trailer gap returned a 19.7% drag reduction.-------------------------------------------------------- Full-length lower side-skirts provided a 8.7% drag reduction.------------------------------------------------------------- A full-length belly pan gave a 5.9% improvement.--------------------------------------------------------- And finally,a boat tail was good for a 4.8% improvement in drag.------------------------------------------------------------------ With full aero-mods,the drag for the rig can be reduced to a level of about Cd 0.50.Without trailer mods we're pretty much stuck there.----------------------------------------------------------------- With any particular trailer "style",mpg will vary directly with trailer height.Where a 13'-6" trailer gave 5.22 mpg,a 13'-0" would give 5.32 mpg,and a 12'-6" would be 5.4 mpg.------------------------------------- Full-length lower skirts on the trailer are good for about a 19% drag reduction.------------------------------------------------------------- A full trailer belly pan will cut drag by 14.7%.---------------------------------------------------------------- A truncated boat tail of 5-degree angle and 32 inches ( 800mm ) is good for 12.6% reduced drag.------------------------------------------------------------------ The best combination created so far demonstrates a 65.5% drag reduction compared to the typical rig of the early 1970s and a mpg increase of almost 33% just from streamlining.