extra wind resistance?

[atahoekid]

you might not be able to wear a cowboy hat in there,,, but heck,,,the spurs will just have to do.

SO.... inquiring minds would like to know. "What exactly are you planning to do with spurs on in bed?"

[tflux]

All great information, and I appreciate your replies.

I really like that vw airflow diagram, however, i don't like the actual results of the wind tunnel study. I assumed there would be an "air pocket" created between the car and the trailer, which would mean the only extra air resistance would come from the amount of trailer above the height of the car. But the diagram shows otherwise. I was relating this to the fact that a pickup truck with the tailgate up gets the same or better mpg than with the tailgate down. The tailgate creates an air pocket that does not increase wind resistance. I would bet that this "air pocket" effect would be more accurate if towing with an suv vs. a sedan.

Hugh's response makes me think I need to stay as low as possible. I was suprised to hear that 5 inches of height made that much of a difference in wind drag.

And with the formula from bobhenry, if I cut my height down to 5.5 feet, my exposed area will be 0.75 feet x 5 feet wide = 3.75 sq ft. x.3 = 1.125 x 65 = 73.125 lbs of drag. Even if these numbers aren't very accurate, it still gives me a comparison. 170 lbs vs 73 lbs.

A pop up portion is very appealing, I may have to think about that. I like the idea, but it sounds like a lot more work with more ways for the trailer to leak.

It's sounding like i will be keeping the height down to 5.5 feet.

Any other thoughts?

[bobhenry]

Not to drive ya nuts but a vertically rounded front and rear really helps redirect the air flow. This is the logic behind the softly rounded vee nose cargo trailers. The formula I found is for a flat nose directly into the wind.

I started on a trolley style with a round nose and tail. ( Got sidetracked and sold the frame) I think this design style might be a nice compromise while retaining the height you want.

or ya could slow down

[Pete S]

See, I would have assumed that there would have been an aero-shadow behind the tow vehicle too. According to the diagram from VW that just isn't the case with modern design sedans.

I was thinking some more about my comment that the airflow speed might actually be INCREASED as it comes down the back of the car. On an airplane wing the top of the wing is where the speed of the airflow is the greatest. That is a big part of what creates lift (Bernoulli...). Assuming that the back of our cars behave much like the top of a wing the airflow would be somewhat higher speed than ambient air. That wouldn't work in our favor.

Also, because of the way airflow comes down the back of the car I think the "exposed area" starts much closer to the area where the tongue meets the camper body. If that is the case the front of our Ben-Roy type would be nearly the full 4 foot by 4 foot frontal area. Maybe it is actually worse because of the Bernoulli thing I mentioned above.

You would still reduce the "exposed area" by building shorter but if most of the front of the camper is the actual "exposed area" than the PERCENTAGE of gain would much less than your previous calculation might suggest.

OK, I'll shut up now...

[whitefishpoint]

I can answer this one from the experience of my 3 different size trailers.

Towing with a Vibe (car=5ft w), I got the following:
31 mpg, no trailer

26 mpg with 4ft w x 4ft h teardrop ( height about 6 inches higher than the car.)

21 mpg with 5ft wide x 4ft h teardrop.

15 mpg with 6ft wide x 5ft h breadloaf style trailer.

[tflux]

31 mpg, no trailer

26 mpg with 4ft w x 4ft h teardrop ( height about 6 inches higher than the car.)

21 mpg with 5ft wide x 4ft h teardrop.

15 mpg with 6ft wide x 5ft h breadloaf style trailer.

PERFECT! I knew somebody would have this kind of info. Thank you!

I am surprised with the results. Very drastic. Look at the difference between 4 feet wide vs. 5 feet wide (same height), 5 mpg. Then only 15 mpg with the 6 x 5??!!!! You could do better than that with a full size pickup!

Do you have any additional info on your towing? Did you tow them all at the same speed? Did you tow the 4 x 4 in the same gear as the 6 x 5? Or did you have to drop a gear to tow the 6 x 5? Any major weight difference in the 3 trailers?

[rowerwet]

some of that drag is gonna change with the shape of the tail of the trailer, a teardrop is very much like the tail of a more efficient car shape or slightly better

[KIDZAGN]

Here is a simple minded suggestion from a simple minded guy....
Could you rent a small U-Haul type cargo trailer about the same size your design would be, check the weight and add a little if you need to and do a couple hundred miles and see what happens. I'm sure any design you come up with will have less wind drag than a cargo box.
At least you would know just how bad it could be.

Just a thought

Mike

[pete42]

With that kind of gas mileage build it any size you want.

when it only gets 8-9 mpg then worry about how tall it is.

If you want one you can sit up and play cards or watch TV then by all means build it that way.

don't settle on something you don't want and be sorry for wasting the time and money.

pete

[GPW]

Kid , Great idea ... a test !!! Those U-Hauls are Heavy too , so you get a better idea what would work with your tow vehicle ...

I've got a paw paw camper top ,same height as the cab roof , on my pickup , so the air flows better off that than circulating all around ... and I suspect the airflow would be better over a similar height trailer , otherwise that's just extra frontal area sticking up ... Guessing the best performance could be obtained with a cover between the trailer and the tow ... so the area in between wouldn't leak air ... Silly idea , but like a windproof stretch sock ... that just snaps on when you're towing ...

[bobhenry]

Kid , Great idea ... a test !!! Those U-Hauls are Heavy too , so you get a better idea what would work with your tow vehicle ...

I've got a paw paw camper top ,same height as the cab roof , on my pickup , so the air flows better off that than circulating all around ... and I suspect the airflow would be better over a similar height trailer , otherwise that's just extra frontal area sticking up ... Guessing the best performance could be obtained with a cover between the trailer and the tow ... so the area in between wouldn't leak air ... Silly idea , but like a windproof stretch sock ... that just snaps on when you're towing ...

That's funny, I was thinking of the nylon covered slinky like they use in the dog trials ( only bigger)as a tunnel. Extend it out and clip to the trailer front.

Silly silly me

[john warren]

slowcowboy,,, just recolect what the old hands always tell the tenderfeet.
"never squat with yer spurs on"

[Miriam C.]

While you all are having all this fun remember that each driver may experience different results. I get 15 mpg on my truck and only 14 miles per gallon when towing if I don't stand on the gas pedal. Now if I forget to switch to overdrive on the flats it might go to 13 or even 12. Love me a truck.

[Pottercounty]

Here are a couple of links for Vortex generators...

I have used these along the leading edge of aircraft wings as well as applications on trailers. They may be unsightly however, they work well and reduce drag. I've seen many semi trailers employ these on the end of the box all around the rear doors...


http://en.wikipedia.org/wiki/Vortex_generator


http://www.amazon.com/V-Force-Vortex-Ge ... B002YK9GZY

[whitefishpoint]

31 mpg, no trailer

26 mpg with 4ft w x 4ft h teardrop ( height about 6 inches higher than the car.)

21 mpg with 5ft wide x 4ft h teardrop.

15 mpg with 6ft wide x 5ft h breadloaf style trailer.

PERFECT! I knew somebody would have this kind of info. Thank you!

I am surprised with the results. Very drastic. Look at the difference between 4 feet wide vs. 5 feet wide (same height), 5 mpg. Then only 15 mpg with the 6 x 5??!!!! You could do better than that with a full size pickup!

Do you have any additional info on your towing? Did you tow them all at the same speed? Did you tow the 4 x 4 in the same gear as the 6 x 5? Or did you have to drop a gear to tow the 6 x 5? Any major weight difference in the 3 trailers?

Yes actually we went camping at the same locations with all of them. They did weigh different I'm sure. I never weighed the first 3 trailers but I did weigh my breadloaf trailer and its 950 lbs empty.

It is true that a full size pick would do just as good towing, however, after I unhook and am just driving around seeing the sights - I'm back to the great gas mileage of the small car. Whereas, you're at 15 mpg unhooked on a pickup truck. And most of the time on vacation, I'm not towing. So it makes a big difference on the gas bill.