This page is dedicated to conversations about aerodynamics
There are several sources available concerning fuel savings and aerodynamics and you can spend a lot of time trying to locate them. We offer you some good links here. One of my favorites is this one; Cummins MPG Guide. In it is a wealth of information for saving fuel dollars. Everything from aerodynamics to weather conditions and their effects on your fuel mileage is covered here along with some good ‘rules of thumb’. If you have not done so already I highly recommend that you download this document and use it to pick up the fuel saving ‘low hanging fruit’ first, before you dive into upgrading your aerodynamics.
Now that you have studied the Cummins paper, let’s talk aerodynamics as it relates to trucking. The aerodynamic laws of physics are fairly straight forward with the most important part being the fact that as your speed increases above 50 mph the aerodynamics of your truck become increasingly more important. A good mathematical explanation can be found here.
Basically, as your speed increases the aerodynamic drag forces really become THE major player for sucking up fuel dollars. One simple solution is to never drive faster then 55 mph, but as simple solutions sometimes do, it’s just not that practical in real life trucking. I mean, we not out there trucking for the fun of it (although I honestly have done that) but we make money in trucking moving freight and the more we move the more income we make. So right off you are not going to impress me by telling me to slow down. We are already regulated to drive only a certain number of hours in a week so if we want to make as much money as we can using our limited time resource then we need to drive as fast as possible. But, there is a slight problem with the going faster thing. According to the Cummins paper if we drive at 70 mph we'll be losing 0.1 mpg for every mph above 55. Doing the math I get that we will only get 5.0 mpg at that speed instead of 6.5 mpg at 55 mph. Figuring fuel at $4/gal and a 10,000 mile month it will cost us an extra $1,850 more in fuel driving at mostly 70 mph unless we can improve the aerodynamics somehow. Lets investigate.
Here’s a mental picture that I have developed for understanding the aero-problem of trucking, in this case your basic tractor and 53’ box trailer. There are 2 scenarios here.
One is in still, calm air and/or driving into a head wind. The other is driving into a wind blowing from the side, say between 10 to 45 degrees off my ‘bow’ if you will.
In the first condition your truck is heading down the road at speed with all of the front of the tractor hitting the air as you move in a forward direction. You get frontal air drag as the tractor is pushing through the air. As that air flows around your tractor it wants to come back in behind the cab as it moves to the rear. This creates a vacuum or low pressure area that is now pulling the tractor back. Then, all of a sudden there’s the front of trailer and the air smashes and pushes on that whole area - unless you have a newer high-top tractor. (That is why this air gap between the back of the tractor and the front of the trailer is so important).
Down near the ground the air reaches the back of the drive tires and then it creates another negative pressure area along with a lot of un-organized airflow behind the drive tires and under the trailer all the way back towards the trailer tires. The airflow above this mess is flowing along the sides and top of the trailer until they get to the back doors and then they create the well known vacuum pocket there as the airflow is ripped off and away from the trailer sides and top. Think about the downstream side of a flat plat placed in a stream flow of water.
Now back to the swirling, rolling mess under the trailer. This mess of air moves along under the trailer until it hits the trailer axles, wheels, brake cans and etc. and would look like a wave crashing onto rocks if you could see it. Well actually, you have seen this unorganized airflow whenever you have passed a plain trailer in the rain. The turbulence can be so great that when raining you can hardly see the front of the truck and the lower you are sitting in your vehicle the worse it is. This effect is actually very frightening to a lot of the motoring public that shares the roads with our trucks, even the guys but of course you will not hear much from them. This splash and spray issue is a huge concern with car drivers but it has been ignored since everyone believes there is no solution. And to be honest, there wasn't a good solution until the AirWedge came along. We can talk about that later.
So there you have it, a description of the airflow around a 'normal', naked truck in calm or head wind conditions. So what are the options that a trucker can do to improve this situation? First thing that comes to mind is to make the cab's front end more aero-smooth. The truck industry has and is doing that. So what about the trailer? We could make them more rounded and all that but let's face it, we are moving freight and rounded trailers will not work well when trying to fill it with square packages, talk about your square peg into a round hole....
OK, so the trailer shape is not going to change any time soon, now what? How about attaching something along the sides of the trailer along the lower edge and hanging down close to ground to keep the nasty ole swirling, rolling air out all together? OK, let's see what happens to the air flow with that.
The air flow along the drive tires can not get in behind the tires now, so far so good. The air is now flowing along until it gets to the trailer tires but in calm air the air just keeps moving along towards the rear of the trailer where it joins its buddies from above and adds to the rear turbulent vacuum area. No crashing and splattering onto the trailer axles, tires and such. Let's put it on a track and see what happens. Well I'll be dipped! The track test shows around a 4% improvement with some skirts showing even better the deeper they are towards the ground. Hey, this is great stuff. We can make 'em out some cheap ass plastic crap or maybe even some of that stuff they're using for trailer side walls that we can get for cheap and charge a real good price for - let's do it. Let's call them 'skirts'.
So they get that design all EPA, CARB verified on a track under testing procedures that include an oval or circular track of at least a whole 1.5 miles long and we'll test it only if the winds are below 12 mph with gusts up to 15 mph. Sounds like my normal driving conditions alright-NOT. But who am I to question the EPA's wisdom? Besides it gives all us something to compare against right? (this is what I used to say. But, if the conditions are not close to real life trucking, are these tests going to mean anything to a trucker in the end except one product does better then another on a track?) Has anyone ever thought that if the wind is fairly steady that when you are on one side of the track you are running into the wind and then on the other side you are rolling along with the wind? Isn't that like no wind at all? With the negative part running into the wind cancelled out by the positive part when running with the wind?
So now we get California passing laws that if we want to drive trucks in their state we will have some kind of aerodynamic treatment that passes their test mentioned above. So they did and we bought and installed and then we found out the rest of the story-how they work in real life trucking operations. Here is a list of what we learned;
1. We only see about one half of the advertised test improvement numbers. Sometimes we are doing worse then we did with our 'naked' trailers.
2. They are very hard to keep in place especially after strikes with deep docks. Lots of times they just look like crap all bent up or flap'n in the breeze. Extreme temperatures cause the surfaces to look like ocean waves.
3. Our trailer tires are blowing out with increased frequency (something about trapped heat from the motor, trans, rear ends and tires) and if we have a spare tire in the rack behind the skirts we can not get to it easily. It is safe though because most crooks are too lazy to try and get them out from under there.
4. We can't see under the trailer during our pre-flight walk around and more than once a failure occurred in there and we never seen it coming because it is just too hard to check now.
5. Our trailers do the Watusi back there in any kind of side wind and now we are more tired then usual trying to stay ahead of it going down the road.
6. Our maintenance costs are up with skirt repairs and my mechanics are always grousing about them. It's hard enough to keep moral up as it is.
7. They sure looked good in the University Labs and on the tracks though.
8. At around $1,500 installed they are inexpensive enough and they get us into California.
9. Did I mention they look like crap?
Lets continue this discussion as it relates to the next great University design; the 'Boat Tails'. For now let's just agree that they too test out well on the track and they actually are provided good numbers in the real world. And apparently skirts and tails together can generate good fuel numbers in the real world. I know of one outfit claiming + 8% using both tails and skirts. Just a few details should be mentioned here about the tails;
1. Driver input is required to open and close the tail assembly.
2. If auto-deploy tails are used then the cost goes up along with more stuff to fail.
3. If the tail fails due to damage or otherwise it is generally un-fixable by the driver. Increased roadside expense or loss of your aero benefits if one or both of the tails can not be opened.
4. Ice, snow and road grit play havoc with the tail's deploying system. Not a problem if you never drive in snow or gritty areas.
5. Tails are more expensive then skirts at around $3,500 installed or so I've heard. I've heard skirts and tails run close to $5,000 together. That's going to take a while to pay back.
Now for the importent part.
Before we go too far here we need to discuss the other wind condition - the side-wind. In side-wind conditions we have the same effects as above except the negative (vacuum) areas increase along the whole down-wind side of the truck and trailer plus we have increased positive pressures from the airflow hitting more of the unprotected trailer front, sides and the unprotected front of the trailer bogie area. I estimate a side-wind condition occurs about 85% of our driving time. Go here to see a wind map of the day and look at your route location. Lots of time the wind is seen to be in the 20 to 30 mph range and not blowing along your route but rather right at you in a side-wind condition.
Our friends up north in Canada have published a very informative paper by the National Research Council Canada in May 2012. You can see it here. I have highlighted in yellow, statements inside this copy as it pertains to this next discussion.
First item to keep in mind is that aerodynamics is 50% of our motor's power requirement at 50 mph. It is 70% at 70 mph. So if we want to go faster to pick up that extra half load by the end of the driving week without losing money to drag then we need to do something about improving our aerodynamics!
According to the National Research Canadian Report:
"The wind-averaged drag is the most important metric for evaluating vehicle drag because some drag-reduction technologies improve aerodynamic performance only for a subset of wind angles (typically centered on zero yaw)" - One technology that meets this statement is the skirt discussed above. It works well in zero yaw or straight-on calm wind conditions but when hit with side-winds it captures air in that area at the rear of the skirt and before the trailer tires. Just like a parachute. But as the report states, "Only zero-yaw drag-coefficient results are required for reporting".
Also from the report:
"The primary recommendation of the NAS report in regards to aerodynamic drag reduction is a
requirement for such technologies to be evaluated on a wind-averaged drag basis to take into
account the effect of typical terrestrial winds." - These guys are NOT making a good case for our EPA, C.A.R.B. test protocols at all. It's starting to look like winds are important when dealing with truck aerodynamics.
Another quote from NRC:
"Under such cross-wind conditions, the aerodynamic characteristics of a ground vehicle, especially tractor-trailer combinations, behave differently than when the ambient winds are either static or when
they are in line with the vehicle motion".
"For moderate cross-winds the yaw angle can exceed 10º, and under these conditions the drag of a tractor trailer can exceed 50% of its zero-yaw drag load. Therefore, the wind-averaged drag coefficient is the most critical aerodynamic performance indicator for long heavy vehicles such as trucks or buses because it represents an average of the typical wind conditions under which these
vehicles will operate."
And then as if side winds were not important, apparently turbulence also plays a large part in this game:
"A drag reduction device may show negligible sensitivity to turbulence when winds are aligned with the truck motion, but can show different levels of drag reduction in cross-flows depending on the
turbulence environment. The characteristics of the turbulence environment are important for adequately assessing the performance of drag-reduction devices for road conditions."
So there you have it. Straight from the Ph.Ds, just what we have known for years here at Airman Inc. Geez it's nice to finally get some validation! The most important part of truck and trailer aerodynamics has to do with how we deal with side-winds and turbulence-not how they work going around a track in no wind conditions.
If you are still with me up to this point I commend you. I'm not sure how many will stay with this discussion up to here, but now for the rest of the story........
Remember how we talked about the roiling, unorganized flow that developed under the trailer and behind the rear-most driver tires? Well, we studied this area using real trucks on real roads using smoke and cameras. What we found is that in straight-on wind conditions, not only is that whole area just a swirling mess, it also moves along with the trailer with some wind components actually moving faster forward then the trailer - at times. Totally unexpected and we believe totally unknown to many in the industry. What this means is that whatever we might place in this area under the trailer will not be working as one might expect because everyone is thinking that the air under the trailer is moving just as fast as the air along side the trailer - WRONG. Good for us to know and it explained a lot about the test results we were getting.
What about those test results you speak of? you might be asking. Here it is. Our AirWedge units are designed to modify the airflow just before the trailer bogies and we have great success with that. The interesting part is that during our over-the-road testing campaigns we always got great improvement numbers when we were running at 70 mph into a side wind between 20 to 40 degrees off beam direction. Also our EPA track tests came up short. So what was going on? We verified our ideas with the smoke tests. When running with an AirWedge it must be presented into an airflow in order for it to work. If it is aerodynamically hidden behind the tractor (as during a test on the track) then there is not enough airflow to generate large positive results. When running into a side-wind then the AirWedge has the airflow required to do it's job and reduce the trailer's air drag.
About now I figure you are thinking "OK, that sounds reasonable but side-winds only happen part of the time." Please do me a favor, during your run next week please keep track of the amount of time you run into some kind of side wind and then let us know because I think we will be surprised as to how often that does occur. We have not located a study on how much time we spend driving in side-winds, it just seems like most of the time to me.
In order to make the AirWedge work in all wind conditions we have designed forward mounted 'winglets' that attach near the outside edge of the trailer just behind the landing gear. These winglets remove the turbulence behind the drive tires and feeds the AirWedge when your truck is moving through straight-on airflow while not changing its side-wind performance. These two devices work together delivering the best aerodynamic solution out there.
Now we are getting reports of fuel mileage increases over 10% above baseline. How would you like to see your rig pull in a 7.2 mpg while driving at 70 mph in a side-wind?
The rest of the story is livability with your aerodynamics. We have that covered also but on another page later on.................
Give us a call sometime to discuss questions you might have............
Bob Henderson 503-990-2069
John Buck 517-404-2407
Reg Durham 480-595-9805