extra wind resistance?

[tflux]

Hi all!

I'm getting closer to start my build, but I don't want to spend a lot of time and what little extra money I have, and end up making a mistake. So here is my question...

How much extra wind resistance will I encounter if I add a foot of height to my trailer?

I want to be able to tow this with my car (Honda Accord, manual transmission). The height of the Accord is 4' 9". The minimum height of the trailer will be 5' 6", but I would really like to make the trailer 6' 6". The outside shape of the trailer will be very similar to the "weekender", so there will be a roughly 30 degree slope for the extra height above the roof of the car.

How bad will this extra foot affect my mpg while towing 65 mph?

If there is a big difference in mpg, I will definitely want to keep it at 5' 6".

Thanks.

[atahoekid]

I don't know if anyone can actually quantify your loss, but in designing my TTT, I figured the more roofline of the trailer that sticks up in the air, the more deficit you will get. There's a discussion about airfoils and wings on the back of trucks, etc. on this site that might answer your question, but I'm not so sure a wing works on the back of your TV. Personally, I plan on keeping the height of my truck as close to the height of my truck as possible.

[madjack]

...about a foots worth of extra wind resistance...I know that is a "flip" answer but as the atahoekid sez, there is no way to quantify this...unless you have access to heavy duty computer modeling software and a windtunnel...if all the added height is contained in the sloped area, that'll help some...how wide is this monster going to be????..............
madjack

[atahoekid]

Why the need to stand up and the extra head room?

just decide if you want a teardrop or a rv. plain and simple.

with a rv you will pay more in gas and get more in luxury and convince.

with a teardrop you will sacrifice luxury for the basics but will get the best gas milage and the cheapest way to go for maintence and towing with out problems.

So decide which desinge you want.

Slow.

I can't say that I agree at all with Slow's statement. I know of plenty of people who want enough head room to put on pants or stand up in a galley or whatever. It's not either TD or RV. It's whatever variation you're wanting to build. There's plenty of others on this site that would agree with me. You're the one who is designing, building it and "living" in it.. Do it anyway you want. I understand your want for more headroom. I want the same. Luckily though I have a full size PU as a TV. The taller height of the truck allows me to build a taller TTT without too many concerns about mpg loss. Do whatever you wish.

[tflux]

I guess I should clarify.

I still would not be able to stand up, and I don't want an rv. The 6' 6" height is the outside height. Subtract the 18" floor height for the trailer and this gives you 5 feet of interior space. But if mpg suffers too much with the extra height, I could probably get away with 4 feet of interior height.

I REALLY like the "lil diner" setup. I want to be able to sit down at a table and play cards/watch movie/eat dinner if the weather is not cooperating. The "lil diner" is the perfect setup for me except, like atahoekid, I want to keep the top of the trailer as close to the same height as my car as possible.

If I drive sensibly, I can get 36-37 mpg with my car unloaded (in the summer). I am hoping to still get 28-30 while towing my lightweight trailer (again, driving sensibly). If I can get 28 mpg with the 5' 6" height, but only get 20 mpg with the 6' 6" height, then I definitely want to stick with the shorter height with better mpg. Anybody have any experience with a similar situation?

[tflux]

The width will be 5 feet wide, same as the width of the car so I can use my factory mirrors.

I know the answer to this question is very hard to quantify. I guess I was just looking for someone with a similar experience.

Like... "my first trailer was 5 ft tall and I got 25 mpg. Then I built a new trailer that was 6 ft tall and with the same tow vehicle, I only got 19 mpg."

Any thoughts?

[atahoekid]

If I drive sensibly, I can get 36-37 mpg with my car unloaded (in the summer). I am hoping to still get 28-30 while towing my lightweight trailer (again, driving sensibly).

Driving downhill, with a tail wind without the trailer, I'd never get 28 mpg, never mind 37. Those kind of numbers just boggle my mind. I don't drive my truck to and from work too often because going up and over an 8800 foot summit limits my mileage to 16.

I now have very little sympathy for your dilemma

Just kidding, I hope you find your answer.

[myoung]

Another possibility that you might consider would be a pop-up. The interior height of my TTT is about 60 inches but the electrically operated pop-up adds another 18 inches over the galley and extending back over 6 feet into the trailer.

My goal was a trailer that could fit in my garage but still have standing headroom in the areas that need it. The sitting and sleeping areas don't need the added height.

[48Rob]

TF,

The numbers are a little off.

18" Off the ground with 6' overall height does give you 5', but you still need to subtract 2-4" for the floor, about 2" for the roof/ceiling, and if you have a roof vent, a couple more inches.

A dropped floor is a popular option for those of us who need more headroom, but don't want to build a taller trailer.

You mention that an extra foot in height would be the determining factor in your build because of the drop in fuel mileage.
I would question if you have done the math to determine the mileage drop with the smaller version, and why you feel it is the line that must be drawn?
While indeed the extra height would create a little more wind resistance, your tow vehicle has already lost the majority of its mileage by having to pull the weight of the 6' trailer through the air at speed.
If an extra foot of height would break the bank, so to speak, can you really afford to invest several thousand dollars into the trailer to begin with...and is it worth all the effort to build something that is okay, but not really what you wanted.


Rob

[bobhenry]

shirt sleeve answer is .3 lbs per sq ft times MPH

OR your 1.75 foot of exposed frontal height times your 5 foot width is 8.75 sq feet of exposed frontal area x .3 = 2.625 now take that times your 65 MPH and you have 170.625 pounds of wind resistance. The sloped front will help a bit but there are still no conciderations for eddy currents or the vacuum drag at the rear of the trailer.

[GPW]

Drag on trailers , relates mostly to Frontal area... the area that is presented to the "relative" wind .... Stick your hand out the window .... that's what, about 40 sq/in. ? You can" feel" the Drag ... So multiply that by the area of your trailer that faces the wind... Simple to visualize that way . And you'll also find out that Drag increases with Speed ... exponentially ... so if you want to just save on gas (and who doesn't these days ? ) just Slow down ... I did a test with my TT which has almost 80 sq.ft. of frontal area and by slowing down from 60mph to 50 mph , it saved me 10% on the gas consumption... I think that's enough to justify spending a little more time on the road , enjoying the passing view...
Streamlining helps a lot , but there's always some "frontal area" causing Drag ... Just something that has to be allowed for .. as with the parasitic drag of awnings , side mirrors , lights , door handles , etc. that BH mentioned and the basic skin drag (friction ) ...
My current feelings are that something that matches the height and width of the Tow vehicle is about optimal ... anything bigger is just a "parachute " to tow , any smaller becomes uncomfortable for the human body ...
I see a lot of "utility " trailers here that are very low to the ground , and wonder why we can't replicate that ... whatever you save on the underside can be added to the trailer height (and inside)...

[hugh]

I can only give a "seat of the pant,s" impression. My TTT has an interior height of 5'3" which allows me to stand up if if I slouch and makes getting dressed in the morning a lot easier. I have set it up 2 different ways, with the leaf spring both under and over the axle at different times. With the axle over the springs it has lots of ground clearance and I am able to take it to some very rugged off road trails. It sits about 8" over the height of the Jeep Cherokee set up like that. When I set the axle on top of the springs it lowers the roof line just a bit over 5". My XJ has a 232 cubic in inline 6 and you can really feel the difference in wind drag and also notice the difference at the pumps.Plus in the higher set up the cruise won,t hold speed at anything over 65 mph. With it lowered the cruise seems to work just fine. The Jeep has 35" tires and the trailer is running 32" tires at this time. Gas just shot up here to over $1.25 a liter which works out to $5.70 a gallon so the trailer is going to put permanently in the lower style.

[schaapveld4]

I suggest you make the lower side profile out of cardboard lean it against the wall, and gauge how comfortable you will be, will you bump your head getting up in the morning? can you sit at a table if thats what you want. don't forget to include height of the mattress. then if you think you want more headroom add some cardboard to the bottom until you are comfortable. that way you have a better idea weather or not the added wind resistance is worth it, or you can compromise and only add a couple inches instead of a foot ect.

-Matt Schaapveld

[Pete S]

The way I understand the flow of air over a sedan is that it gets pulled down the roofline and back deck of the car. This is part of the reason why sedans get good gas mileage. Once again, "as I understand it", this is also a problem for towing trailers with any low-powered/high MPG sedan.

The wind is headed down the back of the car and runs right into the full front of the trailer. I'm sure factors like the slope on the back of the car and the distance from the car to the camper body will determine exactly where the wind hits the camper.

There is an image in this thread: http://tnttt.com/viewto ... 5&start=30 from the folks at VW that shows what happens to the air over a sedan. I'd say the lower edge of the air-stream hits the front of the camper not far above where the tongue meets the camper body. That surprised me, and I think it spells out the challenge for those of us who tow with a sedan and care about aero-drag.

I was also surprised to see that a slant front on the trailer didn't help the drag coefficient AT ALL. I get the sense that basically the front of the trailer takes the full blast of wind, almost as if there was nothing in front of it. Maybe even worse, maybe it is compressed or speeded up from the trip over the top of the car. The aero-design legacy at VW goes back to legends like Ferdinand Porsche so I tend to think what they have to say is pretty valid.

We tow with a little sedan too. While I think it would be cool to get better gas mileage I'm really more concerned about the automatic transmission overheating and taking a dump. I'll try the deflector wing because of the 15% reduction in coefficient of drag (Well, that is what the VW diagram would lead us to believe). At 70 MPH that could be huge.

I've also started looking at converting to a drop axle or a torsion axle. Either one to lower the trailer, also to reduce drag (though it also just LOOKS sinfully cool!)

So, I wonder how much of that downward airflow wants to continue down the front of the camper body? Maybe it needs/wants to go under the trailer instead of doubling back to go up and over? How much of that flow is blocked by our tongue boxes and assorted crap that we strap on up there?

Ok, there is my "as I understand it" answer.

Pete

[john warren]

well, i wondered about it myself when building my trailer. i went with 4 foot side walls, and with a 12inch thick airmatress i can sit up no problem with out hitting my head, and i'm 6 ft tall. most matresses are only 4 to 6 inches. true , you might not be able to wear a cowboy hat in there,,, but heck,,,the spurs will just have to do.