Quote:
Originally Posted by Clev
I'm reading that fascinating thread that slowmover posted, and somebody posted an interesting data point:
Interesting, and goes against everything truckers have said in rallying for the repeal of lower speed limits for trucks.
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Don't be too quick on the gun. The world of four wheelers and big trucks only overlap in imagination. It is difficult to maintain a 50-mph headway in a big truck . . a 10" stop in a car can have, at equivalent, 30" or more in a heavily-laden big truck. Then, I have plenty of times not been able to re-gain 65-mph for more than 30" since departure as the truck is so heavy, the truck straining; hills, traffic, etc. At the end of a workday one's average speed is not very high. 70 mph allows one to keep speeds, overall, at a decent clip. One can trip plan for a 60 mph day in a car, but a big truck is 50 mph (industry average); this is all hours on the road.
As to streamlining pickup truck and travel trailer, the "rule" is 24-30" for a gap that can be faired. On my rig, the distance from TV (tow vehicle) tailgate to TT (travel trailer) front was 60". Among the above quotes was this:
At distances over 24" inches between the trailer and the tractor, drag goes up considerable. By 48" you have the drag of TWO FULL FRONTAL AREAS. Might as well have the trailer beside the truck!
I think it's important to note two things about trailers: the one used for pleasure, and the one used for business. The first may only be pulled, say, 5,000-miles/year. The second is the one on which to pay special attention (going for length to cut height; reduced width, etc) as it may travel 10X as far. In which case going to FEATHERLITE to custom-spec a trailer would be, IMO, worthwhile.
This is the way I wrote it in an email to another guy:
Were I in a position to spec a cargo trailer I would do my best to keep height and width to a minimum, about 6'H and 6'W for purposes of fuel economy. It wouldn't be higher than my truck and certainly not wider. I would rather have an extra ten feet in length (well, five feet plus) than another one foot in heighth or width. An 8'H x 8'W x 20'L trailer could be 6'H x 7'W x 30'L if cubic capacity had to be identical. (Which I doubt). Here are the words of an expert:
"I would rather tow a trailer that is 6 feet longer and 2000 lbs. heavier than one-foot taller."
The difference between the two trailers is 64 sq/ft frontal area versus 42 sq/ft frontal area; a reduction of one-third.
All corners and edges should be greatly radiused. My nine-and-half-foot tall 34' travel trailer had such, and wind just rolled off of it, instead of piling up against -- and pushing against -- the trailer wall edges and corners. I haven't found the source, but have seen quoted numerous times that a square box versus a rounded/aero trailer had road horsepower differences for the same speed of 50HP and 30HP respectively. AIRSTREAM maintains that their trailers take 20% less fuel to tow, all other things the same.
The best travel trailer is an aerodynamic, all-aluminum, independently-suspended one. AIRSTREAM is the only survivor of that era. I averaged 13 on the backroads with my SILVER STREAK, and 15 mpg Interstate at 63 mph/ 1,850-rpm. Had I reduced to my current solo speed of 58-9 mph, I would (I believe) have broken 16-mpg (34' trailer). I am not alone at this. The "best" mpg appears to be, currently, with European turbodiesel SUV's pulling AIRSTREAM trailers at 16-mpg and better.
V8 diesels do poorly compared to 6's, but both benefit from many, many details related to hitch rigging, alignment, tires, etc, etc.