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Topic: Trailer Wind Resistance

Posted By: 86944t on 03/12/08 04:24am

I am towing a 28' Trail Cruiser with a 2005 Jeep Grand Cherokee with an equalizer hitch. The light weight trailer is between 5000 and 5500 pounds loaded and the tow capacity of the Jeep is 6500 pounds. It tows fine up to about 60 mph. After that the transmission has to downshift to 3rd gear if I am going the 70 mph speed limit on the interstate. I am thinking this must be a wind resistance issue with a large trailer on a small tow vehicle.

The Jeep has the full towing package, and doesn't seem to be suffering any overheating while running in 3rd gear for extended periods of time.

My options appear to be:
1) Keep driving 70 and running in 3rd gear
2) Slow down to 60
3) Buy a new tow vehicle with a higher towing capacity.

All of my towing is in Florida, so hills aren't a problem.

I would appreciate the input of other forumn members on this topic.

Rick


Posted By: Bumpyroad on 03/12/08 04:58am

option 2 makes the most sense to me.
slow down and smell the roses.
bumpy






Posted By: SolidAxleDurango on 03/12/08 05:18am

#3

Wind resistance is wind resistance. Your camper's frontal area is where this comes from. A 17' camper has the same frontal area as your 28' camper.

Even though you have a 6500 "tow rating", you are likely not adhering to your frontal area recommendation - generally listed in your owner's manual.

Your Jeep's simply getting too much of a workout.


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Posted By: goodcruisin on 03/12/08 05:25am

It's the wind resistance. You didn't say what size engine you have in the Jeep, but I'm going to guess it's either a V-6 or small V-8. When I bought by TT, which was also a 28' but slightly heavier, I started out pulling it with a Chevy Silverado with a 4.8L. The 4.8L is a small V-8 that doesn't produce much torque. If I left the shifter in OD (4th) as soon as I hit 62 mph it would downshift to 3rd. In 3rd I could run just about any speed I wanted to. To keep from stressing the transmission I would just leave the shifter in 3rd and run in the 60-65 range. However, after just a couple tows I got tired of having to constantly work the throttle and traded it in for an Excursion with a diesel. With that 7.3L diesel I could leave it in OD, set the cruise on 65 and motor down the road. The short answer is if you don't want or need to trade then just slow down. On a 300 mile trip if you go 55 instead of 65 it only takes 45 minutes longer. You'll use less gas and stress the vehicle less as well.


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Posted By: chadsalt on 03/12/08 05:59am

I would run 65 in 3rd (4 speed auto?). What engine/tranny/rear are we talking about here? How fast is the engine turning at 70 in 3rd? I dont worry about what gear the truck is in, only if the engine is in the correct operating range.






Posted By: fpresto on 03/12/08 06:02am

The amount of wind resistance or drag is very misunderstood especially as it relates to speed. A couple of quotes from Wikipedia "About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Therefore, a vehicle with substantially better aerodynamics will be much more fuel efficient. Additionally, because drag does increase with the square of speed, a somewhat lower speed can significantly improve fuel economy." The key is that drag increases with the square of the speed. How this relates to power "Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power."
All this really means is that we are trying to drag a billboard through the air and it requires a lot of power and fuel to do it. Of all the ideas that people come up with to save fuel we all tend to overlook the easiest. Simply reducing speed a small amount will show a very large increase in MPG and reduce the workload on the engine and transmission.


USN Retired
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Posted By: kknowlton on 03/12/08 06:07am

I agree with the other posters on slowing down.

Check your owner's manual; possibly you should not be towing in overdrive (our Explorer's manual specifically said not to). You'll probably be happier with your performance as well as your MPG if you drive closer to 60 mph.


Posted By: blt2ski on 03/12/08 07:44am

Not sure about Fl, but here in Wa st, truck/tow/vehicles over 10K gross, speed is 60 mph vs car speed/less than 10K gross wt, in some area's at 70 mph.

Yes wind resistance is a factor that many do not account for. 2# of additional FA is equal to 1000 lbs of wt added to your trailer. Also aluminum sided trailers are harder to pull than smooth fiberglass models that weigh 300-500 lbs more! 5w's are easier to pull in general terms than ball mount, due to better aerodynamics. non bedroom slide model 5w's with fiberglass sides are the easiest to tow when looking at equal lb trailers. And more than likely, a Glendale Titanium is the easiest due to its super smooth walls and designed front end of the trailer!

On semis, assuming a typical 18 wheeler is a one for HP needs, a full frontal aero pkg will need 30% less HP vs the typical, and a carhauler will need upwards of 30% more hp to move the same wt as a typ 18 wheeler.

marty


05 Chev CC D/A LS Dooley

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Posted By: aateater on 03/12/08 07:45am

Slow down, use your tow/haul mode.


Me, The Wife, and The Kids

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Posted By: willald on 03/12/08 08:12am

I vote #2 and try to do #3 as well if you can.

Most tires used on trailers, are not supposed to be run any faster than 65 anyway, unless you bump up their air pressure by 10 psi.

By doing 70 consistently, you are probably punishing those trailer tires, and could be asking for a blowout, or shortening their lifespan significantly.

And, as already said, wind resistance is your biggest enemy, and is why your Jeep is struggling at highway speeds. You're trying to drag what amounts to a large, nearly flat billboard in 60 mph wind, it takes a lot of power and fuel to do that. Thats one of the reasons you'll notice those that tow a lot, do so with larger, more powerful trucks.


Will & Angela
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Posted By: 86944t on 03/12/08 09:31am

Thanks for the responses. This is a 4.7L V-8 w/about 225 hp. It has a 5 speed auto transmission, and I tow using tow/haul mode. The approximate engine speeds at 70 mph are: 5th - 2000 rpm, 4th - 2500 rpm, and 3rd - 3000 rpm.

Thanks

Rick


Posted By: skipnchar on 03/12/08 09:48am

86944t wrote:

I am towing a 28' Trail Cruiser with a 2005 Jeep Grand Cherokee with an equalizer hitch. The light weight trailer is between 5000 and 5500 pounds loaded and the tow capacity of the Jeep is 6500 pounds. It tows fine up to about 60 mph. After that the transmission has to downshift to 3rd gear if I am going the 70 mph speed limit on the interstate. I am thinking this must be a wind resistance issue with a large trailer on a small tow vehicle.

The Jeep has the full towing package, and doesn't seem to be suffering any overheating while running in 3rd gear for extended periods of time.

My options appear to be:
1) Keep driving 70 and running in 3rd gear You are exceeding the speed rating of your trailers tires by 5 MPH at 70 MPH and doing so with a marginal tow vehicle. You'd be better off not exceeding ANY of the ratings concerning your set up.
2) Slow down to 60
3) Buy a new tow vehicle with a higher towing capacity.

All of my towing is in Florida, so hills aren't a problem.

I would appreciate the input of other forumn members on this topic.

Rick

Option 2 or 3 are both feasible but as long as you're towing on a relatively "billiard table" flat area you probably have no need to change except to stay within the ratings for the tow vehicle and the trailer. Good luck / skip


2011 F-150 HD Ecoboost 3.5 V6. 2550 payload, 17,100 GCVWR -
2004 F-150 HD (Traded after 80,000 towing miles)
2007 Rockwood 8314SS 34' travel trailer

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Posted By: Fast Mopar on 03/12/08 10:23am

86944t wrote:

The light weight trailer is between 5000 and 5500 pounds loaded and the tow capacity of the Jeep is 6500 pounds.
Rick


According to your own numbers, if your trailer weighs 5200 lb loaded, you are right at the magical "80% of max" internet forum weight police rule that all of the experts love to quote. The reality is that if your trailer weighed 500 lb less, you would be having exactly the same problem. The tow rating of your vehicle is not the issue. The answer is to drive 60 mph. As was stated earlier, your tires are probably ST tires rated for 65 mph max anyway.


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preserve the Second Amendment


Posted By: Scott_C on 03/12/08 11:49am

Others are stating things well... Your frontal area is a fixed dimension, but the effective frontal area change with coupled vehicle (read on) and drag changes with speed of motion and direction and magnitude of winds. A larger tow vehicle will only give you more power to "fight" drag unless it's aerodynamic profile better "couples" with the TT. What I mean in a simple explanation is that your little Jeep is probably more narrow and definitely shorter than the TT. With a full-size SUV, van or truck with a cap, the wider body will make a smoother transition of the streamlines around the sides (yeah, there is side drag too) and the increased height will flatten the streamlines over as well. If you can minimize the under currents as well, it all helps.

It's a world of difference towing mine with my 1/2-ton, open bed pickup with a small block V-8 vs. my 3/4-ton and a big block vs. my 3/4-ton when I have the wedge cap (cab high at front and about 16" taller at the back) on the bed.

I would agree with slowing down, savoring the time and easing fuel mileage.

Also, if anyone saw the national news on last Saturday (I think) and can post a link, there was a nasty accident on a major interstate involving a mid-size SUV (Durango) towing a 26 or 28-foot Pegasus that a speed trap clocked at 73 mph before not being able to stop when a semi had a blow out. The news made a big deal about the verified speed and the combination not being adequately able to stop... Yeah, I am one of the many who support "not being in a rush to vacation".


2008 Shamrock 21SS



Posted By: JAXFL on 03/12/08 02:10pm

My only question is... Why do you have to drive 70? Just get in the right lane, relax, lesson to th music or talk to the passangers, drive 60 and over a 200 mile trip... get there about 30 minites later. No big deal.


Happy Trails
JAXFL
2008 3100LTD Sun Seeker
2008 Chevy Colorado Z71 4x4 Auto Toad


Posted By: AlbertF on 03/12/08 01:01pm

Here's my experience:

On a level road, a flat sided 5x10' enclosed cargo trailer (6' high box) with a modest load (total weight about 1500 lbs) tows about as hard as my 5500 lb Airstream at any speed over 60 mph. In fact, it may tow harder beyond 65 mph. (It also doesn't handle any better; I prefer the Airstream with the weight distributing hitch.)

Weight is basically irrelevant on a level road at highway speeds.


Posted By: Pete D on 03/12/08 03:28pm

Quote:

Most tires used on trailers, are not supposed to be run any faster than 65 anyway, unless you bump up their air pressure by 10 psi.

By doing 70 consistently, you are probably punishing those trailer tires, and could be asking for a blowout, or shortening their lifespan significantly.


Not sure about the 10 psi on ST tires, but the published limit for Goodyear is generally 65 mph.

Goodyear Tire Load and Inflation PDF


1998 Ranger 4.0 4x4
1991 Scamp 13'


Posted By: LarryJM on 03/12/08 04:13pm

Pete D wrote:

Quote:

Most tires used on trailers, are not supposed to be run any faster than 65 anyway, unless you bump up their air pressure by 10 psi.

By doing 70 consistently, you are probably punishing those trailer tires, and could be asking for a blowout, or shortening their lifespan significantly.


Not sure about the 10 psi on ST tires, but the published limit for Goodyear is generally 65 mph.

Goodyear Tire Load and Inflation PDF


Yes, but 60 should be the max safe speed IMHO for the ST tires and these folks running around at 70 to 75 probably at max or close to load and might not be inflated properly and then complaining of tire failures really make me wonder.

Larry


2001 standard box 7.3L E-350 PSD Van with 4.10 rear and 2007 Holiday Rambler Aluma-Lite 8306S Been RV'ing since 1974.
RAINKAP INSTALL////ETERNABOND INSTALL



Posted By: sayby1campers on 03/12/08 04:16pm

My question is what's the hurry. Out of curiosity; I looked up speed limits while towing. Most states keep it between 55-66 mph. To me driving the speed limit isn't as important as your ability to stop and perform emergency maneuvers.

Just some food for thought. I personally keep things @ 60 mph. Gas mileage is better than expected and no real difference in drive time.


DW, DD(10y/o) and CoCo our Sharpei(wrinkle dog)


Posted By: LarryJM on 03/12/08 04:21pm

Until someone sets me straight I hear a lot about frontal area, etc., but from my understanding it's the rear drag and under vehicle turbulence where the problem is as to the amount of wind resistance you have. Wind tends to follow the flow of whatever it hits unless it's a flat surface and most vehicles have some aerodynamic styling in the front for smooth flow, it's when it exits the vehicle where the problem and drag occurs. It's actually the "sucking" and not the "blowing" that is the root cause of wind resistance.

Larry


Posted By: nny12972 on 03/12/08 06:43pm

Way back in the 80s, when I first looked into doing my wind deflector for my TV, I found zillions of private and FED studies with many routine findings specifically relevant to both OTR semis and passenger vehicles with recreational trailers----even a few from the 60s which dealt with PU tailgates and PU caps....you could Google 'til the cows come home, but generically those findings routinely included....(and all of the findings I read emphasized that results vary with vehicle shape and spacing between TV & T, as well as the road clearance of the vehicles...)

Frontal wind resistance was the major culprit up to about 55 mph to 65 mph, again, depending on shape and size (soooo, smoothing the frontal shape should help....it should also be noted that both "wings" and solid shapes both performed well in many studies, below 65mph----so, some OTR tractors have wings, and some have full-body fairings----there's are very real $$$ and convenience features which enter into consideration here, too!.)

KEEP IN MIND THAT A HEAD-WIND CAN TAKE YOUR THEORETICAL LAND -SPEED OVER THAT 65mph THRESHOLD PRETTY QUICKLY, and blow a hole in your wallet while it blows your performance! (OPs main concern)

With many vehicle combinations , at speeds above 55mph to 65mph, rear-drag/turbulence (if I recall correctly, increasing an OTR combo speed from 65mph to 75mph, required approximately 30% more HP to maintain that speed.) (soooo, generically, if you wanna' save fuel & perhaps your drive-train, back off!) Between 55 and 65 mph is also where many studies began to find that "drag" could be significantly reduced with a tapered rear end configuration. The OTR folks wouldn't go that route due to max length restrictions, and very few of us T folks are gonna' build a "whale-tail" for our Ts!

I've found that my 18' TH (46sq.ft frontal & smooth belly) and my 24' TT (64sq.ft. frontal and bare 4" cross-members) are like nite and day towing......approximately 4 mpg(+/-15%) difference when hauled with nearly identical T weights. Without my "wing" in use(or canoe on the TV), I lose approximately 2 to 4 mpg respectively.
J


Posted By: LAdams on 03/12/08 07:02pm

The average trailer is about 8' wide and 8' high or roughly 64 square feet of area... That is roughly the equivilent size of 2- 4' X 8' sheets of plywood held broadside to the wind at highway speed...

If you really thing it's the rear drag and under vehicle turbulence that are causing the drag, I suggest you stand in the back of the pick up holding a 4' X 8' sheet of plywood and have your driver accelerate to about 65 MPH - then nail 2 of them together to form a 8' X 8' sheet and try it again... If you survive this little experiment - I think you'll realize where the majority of drag is coming from rather than your aforementioned sources...

Those of us who have many years experience in towing do not see the need to "set you straight" as we know by experience with many different tow vehicles and many different trailer configuration that the prime drag factor in towing is the frontal area of the trailer which we have said many, many times on this forum...

You can talk all the rear drag and undervehicle turbulence you want and expound on the airflow, the sucking, whatever you think is the major cause of drag but the simple facts are these - - -

It is the frontal area of the trailer that is making most all the drag and drag is a square function of speed... Slow down and mileage goes up - PERIOD!!! Yes, weight enters into the equation but if there were no wind drag at all, it would take relatively little HP to keep the load rolling once the initial force of breaking the load loose was overcome...

To the original poster - I suggest you slow down to about 60 MPH - this will increase your mileage, reduce wear and tear on the tow vehicle, and return your TT tires to the speed they were designed to safely tow at...

Les


2000 Ford F-250SD, XLT, 4X4 Off Road, SuperCab
w/ 6.8L (415 C.I.) V-10/3:73LS/4R100
Banks Power Pack w/Trans Command & OttoMind
Sold Trailer - not RV'ing at this point in time



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Posted By: mythree on 03/12/08 07:25pm

I tow a pup and I also have had a travel trailer albiet it was small at 18ft, but i can promise you it is much easier towing the pup


1 newborn ds
2 dd
1ds
1 beautiful dw


Posted By: K3WE on 03/12/08 08:34pm

LarryJM wrote:

Until someone sets me straight I hear a lot about frontal area, etc., but from my understanding it's the rear drag and under vehicle turbulence where the problem is as to the amount of wind resistance you have. Wind tends to follow the flow of whatever it hits unless it's a flat surface and most vehicles have some aerodynamic styling in the front for smooth flow, it's when it exits the vehicle where the problem and drag occurs. It's actually the "sucking" and not the "blowing" that is the root cause of wind resistance.

Larry


I think that technically you are right, that the suction off the flat back side is probably pulling more than the shoving up front- especially since the TV is parting the air to some extent and since the front of trailers often have at least a limited slant/curve/something.

Of course we might need to "get a life" if we go too in depth on discussing the nuances of camper aerodynamics...They're mediocre at best!


Posted By: riverdog on 03/12/08 09:20pm

K3WE wrote:

LarryJM wrote:

Until someone sets me straight I hear a lot about frontal area, etc., but from my understanding it's the rear drag and under vehicle turbulence where the problem is as to the amount of wind resistance you have. Wind tends to follow the flow of whatever it hits unless it's a flat surface and most vehicles have some aerodynamic styling in the front for smooth flow, it's when it exits the vehicle where the problem and drag occurs. It's actually the "sucking" and not the "blowing" that is the root cause of wind resistance.

Larry


I think that technically you are right, that the suction off the flat back side is probably pulling more than the shoving up front- especially since the TV is parting the air to some extent and since the front of trailers often have at least a limited slant/curve/something.

Of course we might need to "get a life" if we go too in depth on discussing the nuances of camper aerodynamics...They're mediocre at best!



Go back a few posts and read from "LAdams". Slow Down...


Posted By: blt2ski on 03/12/08 10:05pm

Actually folks, the rear suction of the trailer can and will make up some difference. In some studies on 18 wheelers, going from duals to super singles, along with a change in the rear of the trailers can each net up to a 5% savings in MPG or 10% total! Does Frontal area also make up some of the problem, yes it does, but, the rear of the trailer is part of the equation to!

Marty


Posted By: Bumpyroad on 03/13/08 04:25am

blt2ski wrote:

Actually folks, the rear suction of the trailer can and will make up some difference. In some studies on 18 wheelers, going from duals to super singles, along with a change in the rear of the trailers can each net up to a 5% savings in MPG or 10% total! Does Frontal area also make up some of the problem, yes it does, but, the rear of the trailer is part of the equation to!

Marty


a while back in another thread I made the rash statement that I remembered from good old school days, about 100 years ago, hearing that the streamlined rear contributed about as much as a streamlined front, with respect to racing cars, airplanes, etc. somebody pointed out that airplanes have pointed noses, more or less. I observed that some of the faster ones also have pointed rear ends.
glad to see this post.
bumpy


Posted By: mkirsch on 03/13/08 07:03am

Color me a little confused here. How are you running below 60MPH in O/D pulling that heavy of a trailer? Most small block V-6 and V-8 engines don't start producing appreciable horsepower until they're over 2000RPMs. At 60MPH, the typical small block V-8 in a vehicle configured more as a "people carrier" than a tow vehicle is only turning about 1700-1800RPM.

With my 3500lb low-profile enclosed utility trailer and 4.8L Chevy I can't hold speed on the flat in O/D at anything below 65MPH. That's where the engine crosses the 2000RPM threshold and really starts to get into its torque curve. At 70MPH, the engine's in it's "sweet spot" and pretty much unstoppable on the flat in O/D.


2002 Chevy 3500 DRW/8.1/Allison & 2000 Palomino B1500 popup TC

-Yes, I haul a popup with a dually. No, I don't think I need a dually to haul a popup.


Posted By: mkirsch on 03/13/08 07:07am

I just had another thought:

With that 5-speed tranny, which gear is "direct?" Does it have a double-overdrive?

If you're able to pull the trailer in the direct gear, that is the one that puts the transmission into a 1:1 ratio, I would not worry about it.


Posted By: nny12972 on 03/13/08 07:48am

My 5-speed manual is 4th-1.00, &5th-0.75......couple that to my 3.08 LS rear-end, and my 5.7 cruises at +/- 65 mph at approximately 1675rpm....can do that all day towing 4.5K# without major wind or hills....occasionally get into 4th w/TH in the hills...occasionally 3rd with the 6.2K# TT.....except for short runs in the mountains or in stiff head winds, my routine towing is 15(TT) to 18(TH) mpg.
J


Posted By: Garfie|d on 03/13/08 08:10am

Bumpyroad wrote:

blt2ski wrote:

Actually folks, the rear suction of the trailer can and will make up some difference. In some studies on 18 wheelers, going from duals to super singles, along with a change in the rear of the trailers can each net up to a 5% savings in MPG or 10% total! Does Frontal area also make up some of the problem, yes it does, but, the rear of the trailer is part of the equation to!

Marty


a while back in another thread I made the rash statement that I remembered from good old school days, about 100 years ago, hearing that the streamlined rear contributed about as much as a streamlined front, with respect to racing cars, airplanes, etc. somebody pointed out that airplanes have pointed noses, more or less. I observed that some of the faster ones also have pointed rear ends.
glad to see this post.
bumpy


Hmmm....you mean like an Airstream?


2001 Airstream Safari 25SS tugged by 2011 Chevy Traverse 3.6L AWD
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Posted By: Bumpyroad on 03/13/08 01:03pm

yep, airstream has the right idea.
bumpy


Posted By: blt2ski on 03/13/08 02:33pm

Airstream, Glendale Titaniums, all have the right idea! There is still more manufactures can do to increase mpgs, as long as we the consumer ask for, buy and get items that will do this. Otherwise, they will design things that use more fuel by being less aerodynamic etc.

Marty


Posted By: LarryJM on 03/13/08 05:00pm

blt2ski wrote:

Airstream, Glendale Titaniums, all have the right idea! There is still more manufactures can do to increase mpgs, as long as we the consumer ask for, buy and get items that will do this. Otherwise, they will design things that use more fuel by being less aerodynamic etc.

Marty


I still stick by my assertion (which might be wrong once someone gives me an authorative cite/link) that it is mainly the sucking/turbulence problem and not the blowing as being the problem. The Airstream front tries to redirect the "Turbulent" air flow created by the back of the TV and the somewhat rounded end does the same for the rear of the trailer. All vehicles as I said in my first post in this thread try and address the front aerodynamics, but after than is where the biggest problem occurs. This is the same issue of the pickup tailgate up or down issue where the aerodynamic bubble of the wind "bubble" in the bed in front of the tailgate actually helps the aerodynamics for wind resistance than simply removing the tailgate completely.

I frequent a diesel truck forum and often see complaints of folks loosing 2 to 4 mpg after either lifting their truck or putting on wider/larger tires and that is because of the increased ground clearance and turbulence.

LAdams example of the plywood is only applicable if trying to hold that up at a right angle to the wind, slant that and the resistance is much less and even the hand doesn't say whether it's the direct blowing on the front of the hand or the sucking from the back of the hand where the major resistance is.

As has been mentioned a pointy front and points back is the best like in blimps and submarines and even formula one cars, but the pointy back except in teardrops is not very efficient use of floor/frame space in a trailer.

I still say it's the sucking and not blowing that is the major problem.

Larry

* This post was edited 03/13/08 05:07pm by LarryJM *


Posted By: Ron Gratz on 03/13/08 06:23pm

LarryJM wrote:

---I still say it's the sucking and not blowing that is the major problem.

I think it is likely that the "drag" force on a truck/trailer combination will have about equal contributions from the increased pressure acting on the cab and from the decreased pressure acting on the rear of the trailer.

I can't cite any references for a truck and travel trailer combo. However, there has been much published about drag forces on tractor and semi-trailer rigs.

External Flow Analysis of a Truck for Drag Reduction shows some pressure information in Figures 6a and 6b. If figure 6a, the high pressure region acting on the front of the tractor is shown by the red color. The low pressure acting on the rear is shown by the blue. Figure 6b shows results for a modified tractor/trailer where the magnitude of high pressure on the front and the magintude of the low pressure on the rear are both reduced giving a reduction in drag force.

This paper gives some estimates of the relative effects of drag forces acting on various parts of a tractor/trailer rig. Figure 4 on page 10 shows pressure effects for rigs with and without cab fairings. Without fairings, the magnitude of the drag force acting on the cab, the front of the trailer, and the rear of the trailer are all shown to be equal (CD = 0.2 for each).

I would guess that for trucks and travel trailers, the "blowing" and "sucking" effects would be about equal unless the trailer has a streamlined front and/or rear.

Ron


Posted By: LAdams on 03/13/08 06:26pm

And most trailers have virtually a flat front end, perhaps just a little angle to the top and bottom from about the 3' level in an attempt to streamline them a bit...

OK - the airstreams don't and hey - guess what - they tow better and get better mileage... How about Trail Manors??? You know the ones that collapse like HiLO's that both tout up to 77% increase in fuel efficiency because of lo profile and decrreased wind drag and perhaps the new VR series of V-shaped TT's from Forest River too ...

Seems like your really hung up on this sucking/turbulence issue... OK, I'll give - it will make a minute to small difference in drag but the majority of wind drag is still generated by the high profile of the TT front end - why is this so difficult for you to understand

Another example is the very small contribution to suck/turbulence are those devices called air tabs (http://www.airtab.com/ The air tab website that customers report a 2-4% fuel savings annualy... If your getting 10MPG towing your TT that means with the air tabs you would pick up .2 to .4 MPG - not exactly a huge gain and not anywhere near the 77% reported by low profile trailer manufacturers like Trail Manor or HiLo...



And that completes my posting to this thread...



Les


Posted By: loveshack1 on 03/14/08 05:16am

You're pulling approximately 72 square feet of billboard. It doesn't matter how aerodynamic the rig "looks", there is still resistance.

As for the options, either #2 or just go out and buy a Dodge 3500 dually with CTD and 6-speed tranny. That'll get the job done!


YOU CAN'T SPELL TRAVEL WITHOUT RV



Posted By: PrivatePilot on 03/14/08 07:46am

Bumpyroad wrote:

yep, airstream has the right idea.
bumpy


So does (Er, did) Award - they're low slung and aerodynamic at both the front and rear, making a huge difference in mileage.


30' Keystone Cougar 5'er, Triple Bunkhouse, SuperSlide.
Chevy 3500 1 Ton long box crew cab dually
6.5 Turbo Diesel, 4.11 Rears, LSD, Fresh rebuild spring 2012.
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Our 2008 western adventure - to the coast and back!

Mark



Posted By: Pete D on 03/14/08 01:34pm

A friend of mine switched from an 18' 'box' TT to a 25' Avion and reported the Avion was a lot easier to tow despite the additional weight.

BTW, I did a 'spring-over' on a Jayco 16' and put on larger wheels/tires to get better ground clearance (repairing dump pipes once was enough for me). I didn't plan it that way, but lifting the trailer brought the bottom of it level with the bottom of my pickup truck (with canopy and kayak).

The result was an easier tow, air-resistance wise,esp in head winds. My guess is that the increased under-side air flow (less air-dam) more than offset the upper increase in area (Upper forward edge of Jayco was slanted back).


Posted By: LarryJM on 03/14/08 03:21pm

Ron Gratz wrote:

LarryJM wrote:

---I still say it's the sucking and not blowing that is the major problem.

I think it is likely that the "drag" force on a truck/trailer combination will have about equal contributions from the increased pressure acting on the cab and from the decreased pressure acting on the rear of the trailer.

I can't cite any references for a truck and travel trailer combo. However, there has been much published about drag forces on tractor and semi-trailer rigs.

External Flow Analysis of a Truck for Drag Reduction shows some pressure information in Figures 6a and 6b. If figure 6a, the high pressure region acting on the front of the tractor is shown by the red color. The low pressure acting on the rear is shown by the blue. Figure 6b shows results for a modified tractor/trailer where the magnitude of high pressure on the front and the magintude of the low pressure on the rear are both reduced giving a reduction in drag force.

This paper gives some estimates of the relative effects of drag forces acting on various parts of a tractor/trailer rig. Figure 4 on page 10 shows pressure effects for rigs with and without cab fairings. Without fairings, the magnitude of the drag force acting on the cab, the front of the trailer, and the rear of the trailer are all shown to be equal (CD = 0.2 for each).

I would guess that for trucks and travel trailers, the "blowing" and "sucking" effects would be about equal unless the trailer has a streamlined front and/or rear.

Ron


Ron,

Thanks for those especially the second and I've looked some more and a lot of the wind resistance on the front of the trailer from my read is a turbulence problem created by the back of the tractor and remember my assertion was the sucking and turbulence are more of a factor than just the blowing and aerodynamic flow of say the frontal area of the tractor/TV.

I did find the following two references that seem to dovetail nicely in general terms with your references.

Quote:

The difference between the testing methods is critical, according to Bob Weber, International’s chief engineer of heavy vehicles. “Since their test does not include the aft section of the trailer, it does not allow a wake to form behind the vehicle as it does on the road,” said Weber.

Both the aft end of the trailer and the size and shape of the wake behind the vehicle “have a profound influence on the overall aerodynamic drag of the tractor-trailer combination,” said Weber. “A vehicle’s wake changes shape and becomes more pronounced as cross winds (and subsequent yaw angles) increase.


From From HERE

Quote:

Aerodynamicists refer to drag as positive or negative air pressure. One can visualize positive air pressure easily enough: put your hand out in the wind at even 50 km/h and you can feel the resistance or drag. You know intuitively that pushing through the air requires effort. That's why in recent years we've taken to putting round-nose tractors out in front of our brick-shaped trailers. But that takes care of only one end of the problem.

A less obvious form of drag is the vacuum that occurs behind the trailer caused by the air rushing back into the space it was pushed out of. Because air can't reoccupy that space instantly, a vacuum or low-pressure area is created until it does -- along with a ton of turbulence. That area of rough, irregular airflow and the partial vacuum behind the trailer is pulling the truck backwards.

Jason Leushen of the National Research Council in Ottawa (NRC) estimates that 130 hp is consumed in the effort. He and his team test truck aerodynamics in a full scale wind tunnel. All day long, you have a 130 hp engine pulling backwards, all day, hour by hour while traveling at 100 km/h.

"Your engine works hard enough to keep you moving forward without having to overcome a 130-hp pull from behind," Leushen says.

In real life there is wind to contend with too. Wind can hit the truck from any angle. If you are driving in a crosswind, the air is passing at an angle to the truck. In fact, in this condition some of the air is passing under and over the trailer and exiting along the side of the trailer. The low-pressure problem occurs along the whole length of the trailer, in effect pulling the trailer to the side. It also creates drag because of the truck's forward speed.

Bob Englar, a research engineer at Georgia Tech says there are three main areas of drag other then the front of the truck -- the tractor/trailer gap, the underside of trailer which includes the trailer suspension and the cross members of the floor, and the back, or as researchers call it, the "base" of the trailer.

"Various wind tunnel tests have come up with coefficient-of-drag (Cd) numbers for these three areas," Englar says. "The important number to look at here is the back of the trailer, at .25, it represents one-third of the total drag. That's a big chunk." (See sidebar).


Here is the sidebar info:



From HERE

I will also note that many, many of the newer trailers and even 5'ers have a more aerodynamic shape on their fronts than in the past. They now even have the V-nosed TT which should help deflect the turbulent air at the trailer front.

Larry


Posted By: WE3CMP on 03/14/08 08:23pm

fpresto wrote:

The amount of wind resistance or drag is very misunderstood especially as it relates to speed. A couple of quotes from Wikipedia "About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Therefore, a vehicle with substantially better aerodynamics will be much more fuel efficient. Additionally, because drag does increase with the square of speed, a somewhat lower speed can significantly improve fuel economy." The key is that drag increases with the square of the speed. How this relates to power "Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power."
All this really means is that we are trying to drag a billboard through the air and it requires a lot of power and fuel to do it. Of all the ideas that people come up with to save fuel we all tend to overlook the easiest. Simply reducing speed a small amount will show a very large increase in MPG and reduce the workload on the engine and transmission.




I was thinking the same thing!!! HAHA No seriously thanks for the great explanation!!!


Posted By: PSDExcursion on 03/16/08 07:07pm

I draft the big 102" wide 13ft 6" high 18-wheelers on Interstate Highways to pick up a few mpg.


2002 Chevy Express 3500 8.1 155" WB passenger van
41 Ft 2003 Thor Citation 41-ZBSR TT w/ Hensley Arrow


Posted By: PrivatePilot on 03/16/08 07:16pm

PSDExcursion wrote:

I draft the big 102" wide 13ft 6" high 18-wheelers on Interstate Highways to pick up a few mpg.


Please get a CB if you intend on doing that and let us guys know what your plans are.

I don't have any trouble when RV'ers do it to me, but I sure do like knowing ahead of time that I can get ahold of them on the radio should I need to, for example, if traffic is about to suddenly scream to a halt in front of me.

Lots of times I struck up long conversations on the radio over hundreds of miles in situations like that, so it can work well, and the MPG gain is staggering if you stay close enough.


Posted By: Airstreamer67 on 03/17/08 06:22am

Mark, I don't know what the laws are in Canada, but in the US, you would be taking a big risk by doing that for a drafter. Should anything go wrong and someone winds up injured or killed, in the US you would be subjected to severe civil lawsuits from the families and companies involved, and the local district attorney could consider criminal prosecution if the acts were considered negligent, even if the accident were not initially your fault.


Posted By: nny12972 on 03/17/08 07:17am

While the torpedo-shaped Ts obviously CAN do better aerodynamically, the shape & style does not fit most of our wants & needs----and for some---the wallet!

Bottom line on the T shape thread.....MOST of us can't do ANYTHING about the shape of the rear of our Ts, but, just as millions of fleet operators have done, we can (and many of us have made "improvements" that work for us) do something about the TV and front of the Ts....so, even though that sucking sound is still gonna' be there, as it is to some extent with EVERY vehicle---realizing that drag becomes significant at about 55-65mph for most rigs, keeping any combo speed down will probably improve mileage noticeably....
J


Posted By: PrivatePilot on 03/17/08 07:27am

Airstreamer67 wrote:

Mark, I don't know what the laws are in Canada, but in the US, you would be taking a big risk by doing that for a drafter.


Generally, the laws suggest that if anyone runs into the back of anyone else, the fault lies with the person at the back. Only if there was clear negligence of the person in the front of the line does that change.

For example:

- Someone tailgates (or drafts, or whatever you want to call it) someone else, traffic suddenly slows down and the first vehicle slows down suddenly and unexpectedly. If the tailgater runs into the back of the vehicle he was draftingm, the fault lies with tailgater.

- Person being tailgated (or again, "drafted") sees the person following him, gets upset about said fact and mashes on the brakes to "teach a lesson" to the tailgater. The fault will probably lie somewhere in between - the tailgater is still likely going to get charged with following too close, but (so long as there's a witness to prove the story) the person to whom he ran into is also probably going to get charged with something as well, probably a much more serious charge like careless or dangerous driving.

Generally, so long as I'm driving safely it's not my responsibility to worry about who is behind me, nor ensure that my driving (or stopping) in the normal course of operating my vehicle ensures that someone won't run into the back of me.


Posted By: loveshack1 on 03/17/08 08:41am

Tailgating or drafting seems fine for the racing circuits, but when it comes to high-speed interstate travel, it sucks! Pun kind of intended.

The last trip we took with someone else, they "tailgated" for about 800 miles. Now, I'm a nice guy, and even tried subtle ways to get the point across not to do it. But we ended up suffering, because they were staying just far enough away to get the full effect of the draft, while I got 6mpg's for that distance.


Posted By: Bumpyroad on 03/17/08 10:51am

"- Person being tailgated (or again, "drafted") sees the person following him, gets upset about said fact and mashes on the brakes to "teach a lesson" to the tailgater. The fault will probably lie somewhere in between - the tailgater is still likely going to get charged with following too close, but (so long as there's a witness to prove the story) the person to whom he ran into is also probably going to get charged with something as well, probably a much more serious charge like careless or dangerous driving."

they won't be able to charge me. I was panic stopping because that dog ran across the road in front of me.
of course the person tailgating me didn't see the dog, they were too close and any bystanders didn't have the same view that I did.
bumpy


Posted By: BarneyS on 03/17/08 02:01pm

Interesting discussion about tailgating BUT it is off topic for this thread. Please, lets stop the hyjack and get back to the OP's question about wind resistance. Thanks.
Barney


2004 Sunnybrook Titan 30FKS TT
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Posted By: PrivatePilot on 03/17/08 01:38pm

Bumpyroad wrote:

they won't be able to charge me. I was panic stopping because that dog ran across the road in front of me.
of course the person tailgating me didn't see the dog, they were too close and any bystanders didn't have the same view that I did.
bumpy


Been tried in court, and lost. Typically the offender of the agressive braking that caused the rear-ender to begin with exhibits signs of the aggression before the accident occurs.

I speak from experience on the matter - I worked with at least one person who did exactly this to teach a tailgater a "lesson", causing an accident, and he did get charged with careless despite trying to use every excuse in the book.

It was pretty serious wreck and the police rounded up plenty of witnesses to the effect that there was no factors that caused it. It's worth adding that the driver in question had a driving record to begin with that made his claims questionable at best.


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