What if any is the towing effect based on the shape at front of a trailer. ie: horizontal V shape VS curved shape. Tried towing a 20 ft trailer today with the V shape and found wind resistance was very severe when accelerating upgrade and onto highway. One way to describe this is like pulling 2 pcs. of plywood behind your vehicle.
Effect of Aerodynamics at front of trailer
- Thread starter RBL
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Rancher Will
Well-known member
Our personal experience indicates that aerodynamics is a factor with towing an RV. Our present RV, a Montana 3750FL is the largest of the five that we have owned over 50+ years. It is also the heaviest of the last three. Considering my observations the design of the Montana that we now tow makes it the less air resistant than any that we previously owned. The 39 Ft Monty is certainly easier to tow with less air resistence, than the older style 32 ft model (different brand) that we traded in. In fact there is so little air resistance with the Monty that our Dodge Dually Diesel pulls it almost like there is no effort at all. And the fuel MPG is almost the same whether we are pulling or not.
Gary RV_Wizard
Site Team
Wind resistance is a huge factor in towing, both for fuel economy and handling reasons. It seems obvious that aerodynamics should be a major consideration when choosing a rig. The problem is, visually aerodynamic designs rarely have much practical impact. Air flow across a complex surface like a truck and trailer or a motorhome is seldom what it appears and you have to model the particular configuration in a wind tunnel to learn what it actually does at speed. And even if a trailer itself performed well in a wind tunnel test, that test becomes invalid as soon as a truck or SUV is placed in front of it because the air flow is radically different.
For the same reason, add-on devices like air deflectors and "air tabs" seldom produce measurably better results. The technologies involved are sound, but the tuning necessary to make it work in each RV is well beyond what is practical for most owners.
Furthermore, almost anything you do up front is going to be pretty much totally defeated by the numerous protuberances on an RV - air conditioners, awnings, satellite domes/dishes, antennas, skylights, etc
One indication of how difficult it is to improve aerodynamics is the lack of such things on commercial truck & trailer combos. These rigs travel 100k+ miles/year and get terrible mileage, often in the 4-6 mpg range, so anything that improves their performance has a large and immediate payback. So why don't you see torpedo-nosed trailers, air deflectors and such on nearly all larger truck & trailer combos? You will see a few, but by-and-large the payback is too little or too unpredictable to make them generally applicable.
For the same reason, add-on devices like air deflectors and "air tabs" seldom produce measurably better results. The technologies involved are sound, but the tuning necessary to make it work in each RV is well beyond what is practical for most owners.
Furthermore, almost anything you do up front is going to be pretty much totally defeated by the numerous protuberances on an RV - air conditioners, awnings, satellite domes/dishes, antennas, skylights, etc
One indication of how difficult it is to improve aerodynamics is the lack of such things on commercial truck & trailer combos. These rigs travel 100k+ miles/year and get terrible mileage, often in the 4-6 mpg range, so anything that improves their performance has a large and immediate payback. So why don't you see torpedo-nosed trailers, air deflectors and such on nearly all larger truck & trailer combos? You will see a few, but by-and-large the payback is too little or too unpredictable to make them generally applicable.
lavarock1210
Well-known member
- Joined
- Nov 7, 2010
- Posts
- 484
"And the fuel MPG is almost the same whether we are pulling or not." Sorry but if you are pulling a 30+ foot 5th wheel there will be a MPG hit. SO you are saying you only get 10-12 mpg when you are not pulling!!
Ernie n Tara
Well-known member
Gary is spot on in his analysis, however serendipity also plays a part. My previous toad, a 2006 Jeep TJ had no obvious effect on mileage. When I replaced it with a 2012 JK, my mileage immediately dropped by 12% and stayed that way. If anything, the 2012 is slightly more aerodynamic although its also slightly taller. The gods just liked the '06 better.
Ernie
Ernie
kenb1023
Well-known member
When we had the 5er we got on average 13MPG. It was a 34' Jayco Designer at 13000lbs. When I pulled the cargo trailer fully load (16" Box at 7000lbs) we got 13 MPG and when we pulled the boat (21' Tahoe at 4500 lbs) we got 15 MPG. All different shapes, weights and tire sizes but MPG was close. I figure the boat had less drag and weighed less so that was why it got better MPG. The cargo trailer is just a big box on wheels so lots of wind resistance and it was pumper pulled. The 5er was sort of rounded in the front making it more aerodynamic. The MH, well it is just a big brick pushing through the air.
Rancher Will
Well-known member
Some of the above comments, well intentioned I'm sure, are out of date.
Modern technology has resulted in improvements in cost efficiency over the past few years. With our late model Dodge D3500's (not 4wd) I do average better than the common MPG figure that was normal a few years ago. Pulling our Monty nearly 20,000 miles per year over the past two years, keeping careful actual records of miles and fuel purchases, my average MPG has been15 to 18 MPG per trip. The various MPG are more dependent upon where we go (terrain), weather, and type of fuel (bio-Diesel, winter or summer blend, etc.) I do know that fuel type will make a difference of up to about 2 MPG on any trip. Speed is also a factor and I know that I get the best MPG at the most efficient speed of approximately 60-65 MPH with our RV Dodge rigs the way they are designed with the Cummins engines, manual transmissions and axle ratios. I have also noticed a factor, especially on our semis, from tire tread design and I suspect that tire design may have some effect with RV towing although I cannot say for sure.
Pulling our ranch trailers show a different result based upon the trailer for each of our Dodge trucks. The configuration, weight, load, (livestock, hay, machinery, etc.) all have effect on MPG, as does the actual road surface, terrain, weather, etc. None of these trailers are as aerodynamic as the Monty RV although the horse trailers are close and lower height, so they get the best ranch trailer MPG normally.
The days of 3 to 4 MPG with simi-trucks are also a thing of the past for most. The nine semis that I own, with 9 full time drivers, average 7 to 9 MPG, sometimes even more, again depending on trailer and load configuration, and type of fuel, etc.
In addition to ranching I have been in the trucking business since the 1960's. When fuel was cheap, (16-18 cents per gal) we used smaller, higher RPM engines in our semis. As fuel prices increased, trailers were allowed to get largers, and loads heavier, we learned that not only aerodynamics became more important, but also enginering design kept up. Among other things we learned to get the most effient MPG by including enginering advancement to increase the horsepower (torque) from the common 350 to now 650, an up, engines that run at slower RPM. Higher horsepower at slower RPM give the best efficiency. This is why today you commonly see semis with trailer wind fairings on the front, under the trailers, with smooth side panels, truck frontal, side and lower portions designed with low resistance Also all of our trucks have been equiped with APU's and auto chains for years.
I hire my own service mechanics and all of our ranch equipment and trucks equipment is serviced on a regular basis and kept in top condition in our shop. I believe that this also contributes to our efficiency results.
The above refers to diesel fuel equipment. I know the following is off subject but I believe a factor that may not be commonly known.
I live and drive in an area where we have access to both gasoline with and without ethanol. My wife and I drive a Lincoln Towncar, and a Chrysler 300C, sedans. When we fuel with pure gasoline we average about 8 to 9 percent better MPG compared to gas containing 10 percent ethanol with both cars. The fuel tank wagon delivers pure gasoline to our bulk tanks on our ranch from Wyoming and we buy pure gasoline when we regularly need to fuel at stations in Wyoming. When we drive in Colorado we fuel with ethanol gas since it is required at Colorado gas stations. So we regularly get a chance to check the results.
Modern technology has resulted in improvements in cost efficiency over the past few years. With our late model Dodge D3500's (not 4wd) I do average better than the common MPG figure that was normal a few years ago. Pulling our Monty nearly 20,000 miles per year over the past two years, keeping careful actual records of miles and fuel purchases, my average MPG has been15 to 18 MPG per trip. The various MPG are more dependent upon where we go (terrain), weather, and type of fuel (bio-Diesel, winter or summer blend, etc.) I do know that fuel type will make a difference of up to about 2 MPG on any trip. Speed is also a factor and I know that I get the best MPG at the most efficient speed of approximately 60-65 MPH with our RV Dodge rigs the way they are designed with the Cummins engines, manual transmissions and axle ratios. I have also noticed a factor, especially on our semis, from tire tread design and I suspect that tire design may have some effect with RV towing although I cannot say for sure.
Pulling our ranch trailers show a different result based upon the trailer for each of our Dodge trucks. The configuration, weight, load, (livestock, hay, machinery, etc.) all have effect on MPG, as does the actual road surface, terrain, weather, etc. None of these trailers are as aerodynamic as the Monty RV although the horse trailers are close and lower height, so they get the best ranch trailer MPG normally.
The days of 3 to 4 MPG with simi-trucks are also a thing of the past for most. The nine semis that I own, with 9 full time drivers, average 7 to 9 MPG, sometimes even more, again depending on trailer and load configuration, and type of fuel, etc.
In addition to ranching I have been in the trucking business since the 1960's. When fuel was cheap, (16-18 cents per gal) we used smaller, higher RPM engines in our semis. As fuel prices increased, trailers were allowed to get largers, and loads heavier, we learned that not only aerodynamics became more important, but also enginering design kept up. Among other things we learned to get the most effient MPG by including enginering advancement to increase the horsepower (torque) from the common 350 to now 650, an up, engines that run at slower RPM. Higher horsepower at slower RPM give the best efficiency. This is why today you commonly see semis with trailer wind fairings on the front, under the trailers, with smooth side panels, truck frontal, side and lower portions designed with low resistance Also all of our trucks have been equiped with APU's and auto chains for years.
I hire my own service mechanics and all of our ranch equipment and trucks equipment is serviced on a regular basis and kept in top condition in our shop. I believe that this also contributes to our efficiency results.
The above refers to diesel fuel equipment. I know the following is off subject but I believe a factor that may not be commonly known.
I live and drive in an area where we have access to both gasoline with and without ethanol. My wife and I drive a Lincoln Towncar, and a Chrysler 300C, sedans. When we fuel with pure gasoline we average about 8 to 9 percent better MPG compared to gas containing 10 percent ethanol with both cars. The fuel tank wagon delivers pure gasoline to our bulk tanks on our ranch from Wyoming and we buy pure gasoline when we regularly need to fuel at stations in Wyoming. When we drive in Colorado we fuel with ethanol gas since it is required at Colorado gas stations. So we regularly get a chance to check the results.
Gary RV_Wizard
Site Team
Thanks for the insight, Will. And the more recent, real world data as well.
Anyone who says they get better fuel mileage at 60 to 65 mph obviously haven't tried towing slower than that. Or they either don't know how to calculate fuel mileage, or they are trying to convince themselves that it's OK to tow at that speed because they don't want to drive slower than 60 mph! Sure, fuel mileage is better at 60 to 65 than it is at 65 to 70 mph - duh! Several years ago, a company ran fuel mileage tests on a loaded semi trailer rig (diesel). Amazingly, it got the best fuel mileage at 37 mph in a lower (I believe it was third) gear! So I am highly suspect of the people who say their towing rigs bet their best fuel mileage at 60 to 65 mph! BTW, fairly recent tests on cars and trucks (not towing) showed that the vast majority of them get their maximum fuel mileage between 50 and 55 mph. What a surprise. My long-term driving experience shows the same thing when driving solo, and when towing a camper trailer, and with a slide-in cab-over camper on a 1/2 ton truck, and another one on a 3/4 ton truck. I got maximum fuel mileage in the same 50 to 55 mph slot on all of those combinations! BTW, I did try going 65 mph with those rigs, and fuel mileage took a noticeable hit when I sped up to that speed. Remember, air resistance increases at the square of the velocity! Think about safety. You are safer when towing at 60 mph and below than you are above that speed, and you are saving fuel, decreasing tire wear, and decreasing wear and tear on your tow rig. Slow down and enjoy the scenery, and save money and increase your safety in the process!
JoelP
Well-known member
Hi 65Hemi
I see that you are relatively new to the forum, so I am please to see that your are posting. Just to point out that you have posted to a string that is 4 years old. It would be more likely to be read if you initiated a new string so more people would look to see what you are saying. I tell you this because I have done the same thing in the past, not always noticing the age of the string. Please keep your posts coming.
Joel
I see that you are relatively new to the forum, so I am please to see that your are posting. Just to point out that you have posted to a string that is 4 years old. It would be more likely to be read if you initiated a new string so more people would look to see what you are saying. I tell you this because I have done the same thing in the past, not always noticing the age of the string. Please keep your posts coming.
Joel
blw2
Well-known member
To touch on the original question...
shape does have some affect, but it's small compared to other variables
If you look at the equation to calculate drag force
you'll see that the force is based on the square of the speed.... meaning that for a small increase in speed you get a huge increase in drag
for the variable of frontal area it's a 1 to 1 multiplier....
what you described as sheets of plywood is a pretty good analogy of the frontal area part of the drag force equation
double the area doubles the force
now looking at shape
Google drag force coefficient, and you'll find drawings with numbers beside different shapes. 1 would be no change in drag, bigger than one would be worse, but less than one means decrease in drag
objects with a flat front such as a cube approach 1....so no net affect on drag.... some are actually a little better or worse, but close to one.... and subtle changes aren't going to make a big difference
If you can take it all the way to the best shapes, such as a streamlined teardrop.... then you start getting into multipliers like less than 0.1 (90% savings).....but this isn't realistic for RV's
So angling the front face is something, but not much at all when compared to the affect that speed and frontal area have
shape does have some affect, but it's small compared to other variables
If you look at the equation to calculate drag force
you'll see that the force is based on the square of the speed.... meaning that for a small increase in speed you get a huge increase in drag
for the variable of frontal area it's a 1 to 1 multiplier....
what you described as sheets of plywood is a pretty good analogy of the frontal area part of the drag force equation
double the area doubles the force
now looking at shape
Google drag force coefficient, and you'll find drawings with numbers beside different shapes. 1 would be no change in drag, bigger than one would be worse, but less than one means decrease in drag
objects with a flat front such as a cube approach 1....so no net affect on drag.... some are actually a little better or worse, but close to one.... and subtle changes aren't going to make a big difference
If you can take it all the way to the best shapes, such as a streamlined teardrop.... then you start getting into multipliers like less than 0.1 (90% savings).....but this isn't realistic for RV's
So angling the front face is something, but not much at all when compared to the affect that speed and frontal area have
Larry N.
Well-known member
Since this thread has been revived, I thought I'd add a little to Will's data:
My 2007 F-150 Triton V8 is a "flex fuel" vehicle and, when I had a TT, I tried towing using normal gas (10% ethanol) and E-85. With E-85 my mileage went from 14 to around 11 mpg through the same types of roads. Regular was cheaper.
My 2007 F-150 Triton V8 is a "flex fuel" vehicle and, when I had a TT, I tried towing using normal gas (10% ethanol) and E-85. With E-85 my mileage went from 14 to around 11 mpg through the same types of roads. Regular was cheaper.
michaelrider5
Member
Our Evergreen Element SRK 26 is (in car speak) slammed and has a radically sloped nose. We replaced a smaller, lighter trailer that was essentially a box with 4 wheels with it. The gross trailer weight is 6800# on the element and was 4500# on the smaller trailer. We get 2-3 mpg better fuel economy towing the Element (about 13 in our 5.7L Tundra VS 10 towing the box). The best part is how little power it uses to maintain 60-70 mph. We were running on a smooth, dead straight empty road once and just to see how fast our rig would go I eased on the gas. I let off when I passed 90 mph. The trailers tires are rated for 65-70 max and I didn't want to find out what a blow out would be like at that speed.
Lou Schneider
Site Team
- Joined
- Mar 14, 2005
- Posts
- 13,321
RBL said:
I just wanted to point out this comment from the original post is characteristic of an aerodynamic RV.
Wind resistance predominates when you're at cruising speed on a level road. Weight comes into effect when accelerating or going uphill.
If you have high wind resistance, that's what predominates so subjectively you don't feel the extra drag from the weight as much.
An aerodynamic RV uses less power at cruising speeds, so the extra power needed to accelerate or drag its weight uphill is more noticeable. My aerodynamic Hall Chapparal motor home performs in just the same way. Because it takes so little throttle to stay at cruising speed, I feel the weight more when accelerating on an uphill freeway on-ramp.
All of this is of course why there are wind tunnels to test the effects of varied shapes and speeds for "drag", and what "Looks Right" may not "Be Right". And not only is the front of the trailer a factor, so is the shape of the tow vehicle, and the shape of the rear of the trailer. Google "Kamm effect" for info on this.
Read some aerodynamic texts. Drag comes mostly from how the airflow is closed aka the back. How it penetrates the air is much less important. More curved front/sides have more to do with buffeting performance than drag. A raindrop is a good example of a aerodynamic shape. Rounded but more blunt up front and pointed at back.
Back in oil crisis days of the 70's dot sponsored several studies about drag/mpg for trucks. Pointed tails and keeping airflow from underneath were biggies. Trucking companies ignored the pointed tails because of reduced capacity. Just recently saw a couple of semi's with foldable aero tails on trailer.
Back in oil crisis days of the 70's dot sponsored several studies about drag/mpg for trucks. Pointed tails and keeping airflow from underneath were biggies. Trucking companies ignored the pointed tails because of reduced capacity. Just recently saw a couple of semi's with foldable aero tails on trailer.
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