RV rooftop air deflector (DIY: Toyota Highlander pulling 4Runner/Apex travel trailer)

[botsapper]

That shape looks so good, it looks like a 1998 -2005 VW Beetle front hood.
>>> aftermarket fiberglass suppliers or junkyard time.

[slowmover]

Originally Posted by aerohead

In plan view APEX' corners have zero radius.
I've been collecting large radii structures to use as molds for wet layups.
An awning company could also roll square aluminum tubing and heli-arc it into an Airstream-esque birdcage framework onto which tailored awning fabric could be attached forming a very light,hollow nose cavity add-on.
Awnings stand up to Texas thunderstorms all the time.I see no reason why they wouldn't do fine on the face of a trailer.
Anyway,when time avails itself,I'll get into this one.
We're also going to do a fairing for the rooftop AC unit as orbywan has done.

I think this lays out the future in re trailer towing quite well.

[KamperBob]

Saw this a month ago in MA.


Less monstrous than Franken Hauler and better than stock flat front with all sharp edges.

[aerohead]

Originally Posted by KamperBob

Saw this a month ago in MA.


Less monstrous than Franken Hauler and better than stock flat front with all sharp edges.

Bob,that's a little sweetheart.Soft edges everywhere,especially up at the top,which seems to be the coup de etat when it comes to blow-overs.
And the footage from Texas Tech showed the 'Nose Cone' to be the best with respect to attached flow.The air just goes around the radius and hugs the downstream surface.
Not quite as good a leading edge as on your rig,but a pretty good add-on.
The fenders would be the thing to look at I guess.

[slowmover]

Here is the White Paper quote:

To minimize drag resulting from crosswinds and turbulent air, tractor-trailer gaps should be minimized or aeroskirts should be used to smooth the airflow. Beyond approximately 30 inches, every 10-inch increase in tractor-trailer air gap increases aerodynamic drag by approximately 2%.

Did some more digging around last night on the subject of rooftop air deflectors and came up with these from the RV world:

The OP: " . . I need to add here that I used this very same software for recreating an engine explosion on an aircraft while in-flight. This was presented to a group of aviation professionals. They all agreed it was accurate in the way it simulated air flow . . . . "

(Accident re-constructionist)

RV.net: An Animation


Animation, Revised


Continued


From a FORD forum:

Deflector Roof Rail Mounting Clamps


Record-keeping is the real bug-a-boo of the RV world. Inconsistent results, overall, and "general impressions" predominating. Plus, attention to hitch rigging, vehicle steering alignment and proper tire pressures make air deflector gains/losses not much more than "noise" (at present).

But, as with AIRTABs, the wind deflectors are nearly always credited with greater stability in the face of crosswinds, especially as generated by other traffic (semi-tractor trailers).

How well they work is ironically better credited with the "mistake" of leaving a wind deflector in place once the trailer is unhitched (ha!)

For the purposes of this thread, the TT rig in question probably has noise in the system in re proper hitch rigging (applied leverage by the weight distributing hitch) that is, first, only solved by using weight scale data; and, two, proper steering alignment of both vehicles; three, proper tire pressures (weight scale data for the tow vehicle); four, the elimination of brake drag on both vehicles; five, elimination of steering wander by the tow vehicle.

This "noise" can change the FE results "achieved" by the wind deflector completely.

Posts by Ron Gratz on the Towing subforum at RV.net (I use the WOODALLS portal to same) are the best guide in how to set up hitch rigging. This is about the play of under & over steer (induced yaw) and is part and parcel of what the big truck manufacturers speak of when looking at Steering corrections per 100 miles as a factor in FE. Hitch rigging is formulaic. Straightforward, overall.

Quote: (from, Just Got Back From the CAT Scale)

A good rule of thumb for a range of TV/TT combinations is load transferred to steer axle = about 2 times load transferred to TT axles, and load removed from drive axle = about 3 times load transferred to TT axles. The theory does work!

Although that ratio is a decent approximation for rigs where the wheelbase of the TV is roughly half the distance from the ball to the axles of the trailer, it's only a rule of thumb where those measurements fit those proportions.

The theoretical relationships are:

Load Transfer to Steer Axle divided by Load Transfer to TT Axles = (Ball Overhang + Ball to TT Axles' Midpoint) / TV Wheelbase

Load Transfer to Drive Axle divided by Load Transfer to TT Axles = (Wheelbase + Ball Overhang + Ball to TT Axles' Midpoint) / Wheelbase

But 2:1 and 3:1 works pretty well for many TV/TT combinations --

for example A 130" WB Suburban with 65" Ball Overhang and 195" ball-TT axles gives those ratios exactly.

So does a 160" WB truck with 60" Ball Overhang and 260" ball-TT axles.

Ron [Gratz]

If a friction bar hitch is the present system, it also is a contributor to lesser FE (IMO) due to it's crudity (and needs to be disconnected when on wet roads, etc). The rig owner ought to look at the new ANDERSEN WDH as an inexpensive alternative to what is no better than a doorstop. Ease of achieving proper weight scale values is far higher, besides. This is by no means the "best" hitch, but it's a step in the right direction (and for purposes of adjustments to any component or system to try and filter for FE gains via aero changes). 90% of all travel trailers are incorrectly hitched (industry data). Verify that it is not (should be a first step).

The goal is in remaining lane-centered with no driver input despite extraneous influences.


(Will add some more to this post later)

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[KamperBob]

Ross, thanks for those links. Dustin's CGI is Flow Illustrator on steroids. The level of detail looks so realistic and plausable. But thanks to Lucas, Pixar and others we need to be more skeptical than ever. In Hollywood the immutable laws of physics aren't. Validate, validate, validate.

[slowmover]

I thought some of his asides (about air flowing back under the tow vehicle) were certainly of interest. I don't see him selling anything, or acting as a stalking horse. I added these links primarily as being representative of current RV world discussions. I saw no real reason to add ones from Class 8 trucks as that world is pretty well understood by its members (industry leadership).

The RV world tries in re FE, but is short on leadership. Air deflectors have been around quite a while (I certainly remember the discussions after October, 1973), but there has not ever been decent consensus due, again, to record-keeping and accounting for the system noise of a combination vehicle, where (unlike a 5'er or tractor-trailer) some time & care in achieving formula results in hitch rigging can "drown out" aero aids.

There are any of a number of hypotheticals where I believe I could show that an aero aid cannot be separated out where the gain is a small percentage.

It's easy to say it works (and I'm hopeful, not doubtful), but verification is difficult (unless one has worked the numbers necessary). Rigging, alignment, brake drag and steering wander are all to the point. (And new vehicles are not to be assumed correct).

Were I in Colorado, I would take the rig to Transwest Truck Trailer RV in Frederick to start sorting out trailer brake drag, bearing adjustment, trailer axle alignment after a stop at a CAT Scale to get weight scale tickets for the vehicles separately and lashed up; with and without WD activated. (I can provide a fill-in-the-blank chart for collecting all the numbers).

The same basics apply to the tow vehicle. New does not constitute proof of zero brake drag and zero steering wander, much less alignment. Small differences are magnified in towing.

I realize the rig owner may not want someone else spending his money, yet on the other hand the best performing rig may be to his interest. And a relationship with a talented shop is that first step.

My point to this second post would be that there is no downside to any of this. The difference between an "okay" rig and one that has numbers nailed down is night & day while on the road.

This is when the "feel" of an aero aid becomes palpable.

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