Pickup truck aerodynamics

[TheTestPilot]

Just had a look at the 1997 university tonneau study(post 46) it says tailgate down has a positve effect ,I thought it was the other way around?

[aerohead]

Originally Posted by TheTestPilot

Just had a look at the 1997 university tonneau study(post 46) it says tailgate down has a positve effect ,I thought it was the other way around?

In Sport Truck Magazine's aero issue,at least one of the truck gurus interviewed mentioned that in one instance,a pickup might see lower drag if the tailgate were swung from the top,allowing a hole between the bed floor and the bottom of the gate.
We're talking about 1% drag and 1/2% mpg.
Every truck should be considered on a case specific basis.
The clincher for me was the lift issue.I believe that lift increased 30% when the gate was lowered.
Correction:
SAE Paper# 881874 claimed that the half-tonneau reduced rear lift by 30%.
The combination cab-wing and half-tonneau maintained the 30% rear lift reduction.
Lowering the tailgate of an open bed increased rear lift by 16%.
Sorry for the late correction.

[TheTestPilot]

That poses the question of, what are the results of tailgate down or removed with a hard tonneau.
I would like to try this but my 4y hilux ate no. 4 sparkplug at 556000kms , I think it's dead.
Alas poor Hilux.

[freebeard]

My guess would be that you'd have a box cavity, with base pressure touching every part of the bed. But the vectors all cancel except the one pointed out the back, so the tailgate's drag is transferred forward to the front wall of the bed.

So not much difference.

[aerohead]

One of our members in Europe (Belgium I believe) tested his bedcover both ways and reported that mpg was better with the gate installed.
Today's pickups have faux spoilers on the gate,hoping that drivers will be reminded to put the gate up after unloading cargo.This infers that aero is improved with the gate in place for any pickup which sports one of these 'spoilers.'

[aerohead]

I've added the wind tunnel data from the 'Spirit of Ecomodder.com' visit to Darko Technologies facility in Ogden,Utah on September,4,2014.
It's at the thread header on page-1,permalink #1.
You-Tube video is in the works as well as some still photos.

[PaulB2]

Today's pickups have faux spoilers on the gate,hoping that drivers will be reminded to put the gate up

I suspect it makes for a stronger tailgate too.

I have a 2013 Ram 1500 with a "rambox". This is a good news bad news thing. The bad news is that it seriously limits what you can do in the way of a cap, if not eliminating it entirely. The good news is that it is hugely useful and leaves the bed nice and square, along with adding extras like movable hookup points on a rail. The rail made my half-tonneau easy as I just slide the tonneau in the rail from the back. When I need it out of there I can pull it out and flip it over and throw it in the bed, where it takes little room.

I was just thinking of the half-tonneau + wing combo. I've been reading a lot of these threads and not yet seen something like I am about to describe.

Seems to me with that vortex spinning in the back, anything that will smooth the path and encourage the rotation would help, which may be why the wing helps. But can't we do better? The space is a square one and the vortex is cylindrical. Maybe creating a cylindrical area between the half-tonneau and the rear cab window, including the bottom surface of the wing, would make that rotation cleaner. For example, hanging some flashing material from the front edge of the half-tonneau and from the front of the bed, leaves you with a half-cylinder. If it is flexible like heavy plastic it would not get in the way any worse than not having one, if you have to take the half-tonneau down for cargo. Likewise the bottom part of the wing could continue the cylindrical contour. Ideally you'd end up with 3/4 of a cylinder for the vortex to rotate in.

If this is the way to go, I first need to shorten my half-tonneau a bit to give larger area for this rotation. The open part of the bed is now smaller than the height of the cab above the bed.

The width of the wing should be just the bed width I think, in my case 52 inches (inside the ram box).

It's a little hard on the front of the bed to smooth things out because the window is offset forward from the bed interior a few inches It makes me think the entire wing section should be built on the front of the bed, rather than mounted to the cab. A small port in the cylindrical section, with an acrylic panel, should give rear visibility. Or if acrylic can be bent into that curve the entire surface above the bed could be acrylic. I think this can be done with heat; some guys make fairings for their motorcycles this way.

Has anyone around here tried to give that vortex a nice cylindrical space to rotate in, as I describe?

[aerohead]

The existing locked-vortex is already 'streamlined' in form,looking just like an aeroshell.
Allowing the vortex to 'communicate' under the half-tonneau to the inner face of the tailgate is also of some importance,as it provides a pressure differential from front to back across the gate,allowing some thrust and is part of the streamlining criteria of the wing/half-tonneau system.

[PaulB2]

I'm guessing this effect is pretty miniscule, and anyway if there is a pressure differential across the tailgate, the same pressure differential exists when an intermediary form such as a box is there, and has the same effect.

The back of the vortex "sucks" the tailgate (or whatever) forward, and the front of the vortex "sucks" the front of the pickup box backward, so all that would seem to cancel too.

I'm guessing also that if you have a round vortex in a square hole, the main vortex may be streamlined but there might well be mini counter-rotating vortexes and other turbulence in the corners. If we take care that the outside of the vehicle is streamlined to avoid turbulence, maybe we should in that box too.

I'm just wondering if anyone has investigated along these lines.

[aerohead]

*evidently,their exists a force vector across the closed tailgate associated with the low pressure 'centroid' of the vortex which is common to the atmosphere exposed in the open configuration.This differential has been attributed to some of the drag reduction which is not present when a bulkhead is placed between the gate and leading edge of the tonneau.An inspection of GM's patent drag curve might quantify the difference.Whether the affect is major or minor is not emphasized in the literature.
*It would be good to have an illustration of what you mean by box.This could be interpreted many ways.
*A 'blister'-half-tonneau has been tested by myself and John Gilkison on the 1994 Toyota T-100.It demonstrated higher mpg than without it.