Seems like many flatbed trucks I see are actually curved when empty so that the center is higher than the ends. Is that to carry heavier loads?
Also lately I have noticed a big white metal strip hanging below the trailer - what is that for? Maybe to increase gas mileage?
Yes to #1. It flattens out with a heavy load. W/regard to #2, I believe it’s to control water/spray off the wheels.
Yes to both. The skirt is to cut down on wind resistance thus saving fuel. There are also fold out panels on the back to cut down on drag.
Why are the skirts fairly new? Nobody thought of the idea until recently?
Gas was cheap.
I believe it is only aluminum flatbeds that are curved (and always have a sign - DO NOT PUSH) as pushing will bow it upward.
Well, not to split hairs, but the class 8 tractor trailer rigs use diesel fuel not gas.
But you did hit the nail on the head. Fuel costs are driving an aerodynamic revolution in the trucking industry. The frontal shapes of the tractor trucks, cab mounted wings, trailer skirts and rear skirts all decrease the drag coefficient of what is basically a brick. These tractor trailer rigs put on a LOT of miles in a years time and even a small gain in miles per gallon can pay back huge dividends. Tractor trucks are now developed using a wind tunnel where the engineers scrape every ounce of wind resistance out of the design they can.
Diesel fuel is close to or over $4.00 per gallon in most parts of the country. An over the road rig usually travels 600 to 700 miles a day. If you can raise your mpg from 6 mpg to 8 or 9 you can see a huge difference in a years time.
Example:
700 miles at 6 mpg = 116 gallons of fuel. 116 gallons x $4.00 per gallon = $464 per day in fuel.
700 miles at 8 mpg = 87 gallons of fuel. 87 gallons x $4.00 per gallon = $348 per day in fuel
That’s a $116 per day in fuel savings. $116 per day x 317 days per year on the road = $36,772 savings! That’s what kind of difference 2 mpg makes to these rigs.
Here’s an example of the semi trailer skirts and the wind tunnel testing that’s done;
http://www.freightwing.com/aeroflex-classic.php
The trailer skirts are one of several technologies to reduce aerodynamic drag:
http://www.epa.gov/smartway/technology/aerodynamics.htm
According to that site, they can reduce fuel consumption by over 4 percent.
Just a quick hijack: Which is more fuel efficient, flat nose (Freightliner) or Peterbuilt (long nose)?
Cabovers have greater aerodynamic drag, which is why you rarely see them anymore on long-haul trucks.
As mentioned, the trailer fairings/skirts increase fuel economy. And 4% may not sound like much but it’s huge in the realm of big rigs, especially when a fleet can multiply that savings across 1000 or more trucks.
It’s only a matter of time before you start seeing huge fairings hanging off the back of cabs to try and eliminate the gap between the trailer and truck which is another big source of drag.
The bowing (higher in the center) without a load is also used in building construction, where it is knows as “cambering” a steel beam. Basically, the beam is bowed in the way you describe with a goal of it ending up flat under the expected load.
Here is a link to a structural engineering article on how beams are cambered. A google search for “steel beam cambering” will reveal a lot of info.