Foamie aerodynamics.

[wagondude]

What you have found is what other diesel owners have found. You can tow quite a bit of weight without causing a drop in milage, but you won't get much better not towing than towing. Just the way they make power.

[atahoekid]

I don't really think it's going to 'help' your mileage: you still have to punch a big square hole in the air to get truck through.
To see that it didn't hurt your mileage much is a good thing.
Imagine if your were pulling a commercially-built travel trailer (a big square box)...

EEK!!! Also thinking that 14" tires might help a little with rolling resistance? Key word may be "little"

[Wobbly Wheels]

Maybe, but you would burn more because they would accelerate and decelerate more. My Ranger (2wd) came with 14s and the first thing I did to it was to put 15s on with 235s. It takes a little more to get them rolling, but once there it takes less to keep them at speed thanks to momentum. The 14s were better for the 'hole shot' but weren't the greatest on the highway
There aren't really any 14" truck tires anyway...

JMHO and all that tho

[atahoekid]

Agree with you that larger tires are harder to get rolling but are also harder to stop. Design also plays into this thinking. I designed and built Road Foamie with 14's in mind. The 12's on it now are an emergency temporary solution. I think the 12's look awkward on it. The 14's will give it a more balanced look. It's probably the dumbest reason for making the change but I can't help myself!!!

[Wobbly Wheels]

Hehe... I misread what you wrote. I was trying to politely discourage you from putting 14s on the truck!


As to the dynamics of 12" vs 14" trailer tires, I can't see how it would affect rolling resistance, assuming the contact patch doesn't change significantly. It should give you a higher load rating, less bearing wear and more momentum tho.

I'm building on a tent trailer chassis that came with 8" wheels so believe me, I know how silly it looks! I'm going bigger for the same reason, except that I'm justifying it by saying it's to bring the coupler up to match the truck so the trailer doesn't tow nose-up

[WesGrimes]

Not exactly the same, but seams relevent:

[atahoekid]

... I'm justifying it by saying it's to bring the coupler up to match the truck so the trailer doesn't tow nose-up

UHHHH Yea, that too!!!

[atahoekid]

Not exactly the same, but seams relevent:

I'm assuming by reading those photos that curving the roofline creates smoother flow and lowers turbulence and therefore should decrease drag and therefore increase mileage. Wish I had known that before my build but definitely something to keep in mind as I think about the next possible build. thanks for posting that!!

[GPW]

If you think of Air like Water (a Fluid) , then it all becomes clear ... as a Fish !!!

[Shadow Catcher]

I have been thinking about this a good bit. Compass Rose is a good bit wider than the Subaru and because of the tongue box is separated by a good distance and an almost perpendicular galley area. I have said before that it might be an idea to build a wind tunnel and do some model work and the numbers/potential savings are intriguing.

[GPW]

SC, having gone through all that wind tunnel testing business with the glider guys , it all ultimately only proves the things you knew already by sticking your hand out of the car window into the wind ... “flying” ...
All comes down to the cross sectional area of the trailer presented to the relative wind... a fixed constant. as that much cubic air must be displaced (like a fluid) . Bigger the area , the More Drag ... the Faster you go, the More drag ...
That and how the air Flows around that area - shape ... That is the variable part , going from flat plate presented directly to the wind (RV) or something streamline like a bird or fish ... Nature figured this out some time ago ... One only has to look around to see what works and what doesn’t (RVs)

From there it gets ridiculous with consideration of parasitic Drag (vents , door handles , window frames , etc... basically , anything sticking out ( including your hand ) ... and then there’s Surface Drag ... Friction of the air passing over the surfaces it contacts ... all adds up ...

In a vacuum , all this goes out the window ... the Air is the problem , it has “substance “ , you can feel it !!!

[atahoekid]

I think most of us understand the basics. It's the little things we don't know or didn't consider that drive the thirst for more knowledge. I knew the sides sticking out would present an issue. Wish I knew curving the roofline instead of flattening it out would make a difference. Oh well, put this in the memory bank for next time! And I'll keep reading these threads to learn from guys like GPW and WesGrimes who know this stuff already! Thanks for sharing!

[GPW]

Mel , sometimes the differences are so small ,not to bother ... flat roof vs, curved roof is only a few % . I think yours came out just fine !!!

[wagondude]

Mel , sometimes the differences are so small ,not to bother ... flat roof vs, curved roof is only a few % . I think yours came out just fine !!!

The more inportant part of the illustration is the size of the gap between the cab and trailer and how the curved roof of the trailer follows the line of the cab deflector. Not easity reproduced with a car and a bumper pulled trailer. I think GPW is right. Any curve you could have put in to keep your design would have been of very little benefit.

[ghcoe]

I have often wondered if you can reduce drag by trying to make the trailer ride height as close to possible as the tow vehicle. If it is higher I would think it would create a suction of air, creating more drag. If the trailer was lower it might be better by making the air flow over the top. Air dams on the front of cars actually increase aerodynamics even though it presents a larger frontal area.