this is in responce to isitfast's post. this is gonna be lengthy so play along boys and girls and you might learn something....
1st: the reason everyone is stating mpg and rpm's is that since very very few of us have access to a wind tunnel this is the best way that we can tell what effect our mods, tweaks, tunes, etc etc, have had on our vehicle.
2nd: it is all aerodynamics thats the problem! hold a sheet of plywood out in front of you and try running, can't do it huh? And the faster you try to run the more energy your body expels. This is physics.
3rd: someone mentioned the ribbed siding, this causes turbulence, not a desired effect for aerodynamics. the ladder, roof vents, and airconditioner are'nt doing you any favors either.
4th: and this is the aspect no one has hit on; almost all new vehicles sit higher in the back. why is this you ask? a couple of reasons; 1st is to be able to handle weight without dragging the rear bumper. 2nd and most important is aerodynamics. it's easer to make air flow over the top of the vehicle than under the vehicle. ever look under a truck? not a very smooth surface is it? if these components are exposed to air flow it causes turbulance, which in turn causes drag, which causes a reduction in MPG and an increase in RPM'S.

now i know some of you are saying B.S. to make my point go look at the bottom of a nextel cup car... smooth belly pan, now go look at the bottom of an indy car... smooth belly pan, now go look at the bottom of a formula one car... you guessed it. do i also need to mention that all of these cars, when sitting still all sit higher in the back. these are some of the fastest cars in the world think they know something about aerodynamics?

now is it fast said he put a leveling kit on his truck, this lifted the front end which in turn allows more air to get under the truck and create more turbulance, or more drag. i cant tell from his picture if when hooked to the trailer the back of the truck is lower than the front. if it is now not only is more air getting under the truck, but you are now compressing it, this has to be to some degree worse than turbulance. also in his pic it looks like the trailer is riding nose high... you guessed it. the trailer is allowing more air under it and compressing it. he also said that when he drives in certain directions it feels like there's more drag. this isn't surprising or mysterious, any wind at all is going to create a "ram air" effect UNDER the truck and trailer making a bad situation worse.

the solution, which nobody wants to hear, is lower the front of the truck and take the time to get your trailer set up right.

i did lift the front of my truck, but only an 1 1/4, by modifying a Daystar leveling kit. the back is still 3/4 higher than the front.

i also got turned on to this website that tells how to properly set up your trailer and tow vehicle. troubleshooting tips included.
www.rvtowingtips.com

i just got down setting up my toy hauler the way they describe and all i can say is WOW. 24' TH, weighing 7750 lbs, 65mph, 1700 rpm, in sixth gear. NICE and what a pleasure to drive, there is very very little porpusing and when passed by a semi absolutely no sway.

one other thing anyone that has put different size tires than stock on really needs to call that tire manufactor and have them cross reference what air pressure you should be running. here's a hint it's probably not what's on your door pillar label. you'd be suprised, i was.

next time you see a plane or ocean going ship think... why is the front and back pointy and why are the surfaces that contact air or water,respectively, smooth? AERODYNAMICS

class dismissed

 

I can attest to the mileage changes when altering the stock aerodynamics. I run a wing when towing heavy and it helps to the tune of +0.9 mpg on average. That same wing COSTS me 2.1 mpg when I'm not towing. More food for thought...

Richard

 

I don't know what era of F1 and Indy cars you have been looking at, but I ones I have seen are NOT that smooth, otherwise they would not generate the ground effects required to be competitive. The wings add, but proportinately create much more drag than channeling the air that passes under and through the bodywork and bellypan areas.
Then there is the issue of turbulance raising RPMs...if that is the case, I suggest you may have a clutch or TC problem, unless of course you are thinking it is causing wheelspin.....but I doubt it.
Of course trying to run faster with a sheet of plywood will require more energy...for that matter, trying to run faster with no sheet of plywood will require more energy. That is just common sense. And there was the game show that recently aired where one of the tasks for some females WAS running with a piece of plywood. They actually did quite well until they got directly in the path of the large fan blowing on them, and then only the strongest crossed the finish line. I don't remember you mentioning the fan, and as I stated some of them DID cross the line anyway.

The fact is that I wouldn't have even bothered posting on your thread except that I have learned there are some TRUELY wise and intellegent people on this forum, and as much as I enjoy and appreciate them sharing their knowlege, I found your post condecending and maybe a bit insulting.
Ohh, and for the record......I DIDN'T learn anything.
Mike

 

zamdar1,
Your post was right on the money, very informative. Simple physics is what your explaining and if anyone doesn't understand that then don't bother with them, they will never under stand anyhow.

My previous 5th wheel was nose high and I wanted it to ride level. I put the spring purch and springs under the axle instead of on top like it was when I bought the camper. I noticed immediately 1 - 1 1/2 MPG increase depending on wind conditions.

 

Man not again......the guy had 42" tires 3:73's turned 1800 rpm at 78 mph towing barn!..(a very nice one..)....I'm sure you could increase mpg a teany bit if you had an air tunnel to play with but I know for sure a set of 4:10's would help!......4:75's if you could find em!....... Put that sheet of plywood on a bicycle get er up to 15mph in high gear! stay there for awile. Then put in a gear you can pedal fast enough to hit 15mph. Which one do you want all day? I agree with all of the aerodynamics fixes but to make a big improvement it would look like a 747.

 

zamdar1
class dismissed. ?? The post was a good refresher. Thats old school theory/facts thats been around longer than me and I been towing/haulin'/driving trucks pullin' trailers for over 50 years. Ninty percent of your post makes sense but aero and hydro theroies don't mix. 5ers/TTs don't float or even get up on plane worth a dang.
........JIM

 

Any guesses as to how much better mileage a low profile car hauler would get vs. a wedge setup for a truck that has to clear the bed rails.?

 

xyzer you are absolutely right that the right gear ratio is important.and yes it would look like a 747, after all a teardrop(round end in front) is the most aerodynamic shape. our goal is to get as close to that as possible.

J&L aero and hydro theories do have some aspects in common, mainly slipstream, you're right that lift are not the same between the two, it's accomplished two entirely different ways. airfoil vs. trapped air.

i guess what i'm trying to say is if you can get the air to bypass the drag imposing items on your rig the better fuel mileage you'll get and the less that gust of wind is going to affect you.

Patrick i think if you get a trailer that doesn't sit much lower that your truck and you can keep the frontal area of the trailer about the same as your truck,i.e. width and height, either trailer i think would be real close to each other.

i'd also like to add that if i offended anyone with my original post i'm sorry, that wasn't my intention. all i was trying to do is point out an overlooked part of aerodynamics.

 

Hey, I'll go along with that, and I apologize for getting snotty. I guess something about being addressed as a "boy or girl" ruffled this old mans skirt.
I still disagree with the statement that more drag will raise your rpm, but who cares, and ya made me inspect the bottoms of the model Indy cars given me by the Laziers, so I guess you can say "MadeJaLook!"
Happy New Year.

 

zamdar1....no offence taken! We are all looking for the "Magic Bullet"! I do know most "open wheel race cars" create downforce at high speeds at the expense of horsepower and fuel efficancy. At 200mph they can drive on the ceiling. They sit high while sitting because the suspension isn't "loaded". It takes a bunch of horsepower and fuel to do make them "squat"....

"Gypsyman" said "I run a wing when towing heavy and it helps to the tune of +0.9 mpg on average. That same wing COSTS me 2.1 mpg when I'm not towing. More food for thought..."

Now that would be an aerodynamic improvement! If !!!! You take it off when not in use! So I guess it is a bunch of compromises.....personaly I would swap the 42" tires when towing or buy a set of 4:10's or both. The wing is definatly the cheapest solutation! I know I know it's an aerodynamics question!

 

the 42" tires would have to go just not good for towing, in my opinion. not to mention that i would have to haul my butt up into that rig everyday and that ain't gettin easier as the years tick by.