The drive from South Bend to Louisville is very boring. It's almost entirely highways (although annoyingly punctuated -- this seems to be my Word of the Week, by the way -- with stoplights here and there, particularly in Kokomo).
While I was driving down I-64 between Indianapolis and Louisville yesterday, for no apparent reason, I started noticing the aerodynamic tops of tractor trailer cabs. I am neither a mechanical nor aerospace engineer, so I can't say exactly why this struck me.
I started thinking about when these kinds of things were first introduced. I believe it was sometime during my grad school years; I have a vague recollection of touring a trucking company with my Boy Sprout troop and having the tour guy explain this "great new invention" that saved them $0.001/mile in gas. And that in just a few months, this invention (and all its associated installation costs) would pay for itself.
So I started noticing the different kind of aerodynamic features on the tops of tractor trailer cabs today. Let me state the conditions and assumptions for my study:
- I only examined tractor trailers that are of the 18-wheel or greater variety
- Only trucks with one or more full-sized trailers were examined; no flatbeds, cylindrical liquid-carrying trucks, or other kinds of "big rig" trucks were examined.
- The time of this experiment ranged between 7:30-9:15pm, EDT, on Tuesday, 1 July, 2000. The location was the approximately 110 miles on I-65 between Indianapolis, IN, and Louisville, KY.
- Full-height aerodynamic bubble.
A full-height aerodynamic bubble is a fixture on the top of the cab that extends vertically all the way to the top of the trailer that is being towed by the cab. It looks something like this:
/----> Wind / ____________________________________________ / / | / / | Tractor ----/ / Bubble | Trailer / | / | ==================| | | | | Cab | |I should clarify that there are many types of these full-height bubbles. For one thing, their lengths are highly varied. Some start immediately above the windshield. Others start anywhere to about halfway back along the cab. I won't even pretend to understand why this is, but if I had to make a guess, I'd say that it was somehow related to the angle of the windshield and/or hood (many of which -- contrary to what is shown in the simple pictures in this presentation, are sloped, not completely vertical). Additionally, the widths of these full-height bubbles can vary greatly. Some are have a distinctly rounded point and sharply fall away on the left and right side. Others have a much shallower gradient. Regardless, the one thing that all these fixtures share is that they extend all the way to the top of the trailer that they are towing (plus or minus a few inches). This bubble is typically part of the cab itself -- it seems obvious that the cabs were specifically designed with an overhead bubble in mind. As such, the bubble typically extended behind the cab right to the horizontal beginning of the trailer, providing a seamless channel for the wind to travel along until it hits the trailer surface. One conspicuous exception to this is UPS trucks; their cabs are the classic square/boxy type and have a bubble attached in a seemingly ad-hoc fashion. This leads the observer to believe that UPS "upgraded" their fleet for more aerodynamicicty in a cost saving paradigm shift (it should be noted that Louisville is UPS's national hub; hence I saw a lot of UPS trucks). Most bubbles had some kind of logo emblazoned on them (typically in the front), although some were just a plain color. Of the logos, the vast majority were corporate logos; a few personal logos were sighted (perhaps from independent truckers), but the vast majority were advertising in nature. Some even had lighted logos. - No aerodynamic fixture.
It was surprising to see many trucks still out there with no aerodynamic fixture at all. And the vast majority of these were the HR >
________________________________ Wind | | Tractor ====================>| Trailer | | ================= | | | | | Cab | | The wind comes in and hits both the square front of the cab and front of the trailer. Since the air is not directed, it must effectively 90 degree change in direction. This seems to be obviously inefficient (indeed, while there were a good number of trucks without fixtures, they were vastly outnumbered by the numbers of trucks that had some kind of fixture). - Half-height aerodynamic fixture. The half-height fixtures (not quite rounded enough to be termed a "bubble", like the full-heights) -- and again, we're not talking about lengths or widths here -- typically only spanned half the distance from the top of the cab to the top of the trailer:
Wind ________________________________ | /===============>| Tractor / _______________ | Trailer --/ / | | / | | ================= | | | | | Cab | |The picture is somewhat misleading here -- there were two main aerodynamicicity se awareness of aerodynamicictytypes of half-heights:- Tall cabs. Some cabs were just taller than average -- they had the same classic box/square shape, but had an obvious half-height bubble-like extension on the roof in an attempt to channel at least some airflow.
- Add-ons. Other cabs had half-height bubbles attached to the tops of their roofs (see picture, above). Some of these half-height bubbles had a fin located at their top rear that looked like an attempt to provide some measure of airflow channeling rather than having the wind have to make a sudden 90 degree shift. Something like:
/=====> Wind / _______________________________ / | /============/ | | Tractor / _____________/| | Trailer --/ / | | / | | ================= | | | | | Cab | |
- Wind redirecting slab. The last kind of fixture is typically the simplest -- a small fixture on top of the cab that attempts to provide minimal airflow channeling, but provides some kind of sloped edge rather than force a 90 degree wind turn. There are two distinct types:
- Simple. The simple slabs were typically straight sheets (although a very small number of curved sheets were observed) with slightly tapered edges mounted at an angle on the top of the cab:
/================> Wind / ________________________________ / / | / / | Tractor ========/ / | Trailer /\ | / \ | ================= | | | | | Cab | |Of course, as with the full-height bubbles, the exact horizontal placement and width of the slab varied. Note the strut support on the back edge. Some struts were designed such that the slab could be lowered for when the cab was driving without a trailer, which seemed like a good idea:Wind ========================================> Retracted slab _______ ================= (No trailer) | | | Cab | - Airfoil. Other slabs were clearly influenced by airplane wing design. They tended to look like inverted mini-wings on the top of the cab, something like:
/================> / ________________________________ Wind / | / | Tractor / | | Trailer ____________/ / | _) | ================= | | | | | Cab | |Although the ASCII art really doesn't do it justice. The mini-wings typically did not span the entire width of the cab -- they were usually between 1/2 and 3/4 of the width. I can't really speculate why an airfoil design would be helpful in such a case here. i.e., I'm not sure what the "win" is in using an airfoil over the simple slab -- the airfoil must be more expensive to design and build; it must have better wind directing capabilities somehow.
- Half-height aerodynamic fixture. The half-height fixtures (not quite rounded enough to be termed a "bubble", like the full-heights) -- and again, we're not talking about lengths or widths here -- typically only spanned half the distance from the top of the cab to the top of the trailer: