Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - RV rooftop air deflector (DIY: Toyota Highlander pulling 4Runner/Apex travel trailer)

slowmover · 09-03-2012, 10:14 AM

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)

.

09-03-2012, 10:14 AM	#25 (permalink)
slowmover Banned Join Date: Oct 2009 Location: Fort Worth, Texas Posts: 2,442 2004 CTD - '04 DODGE RAM 2500 SLT Team Cummins 90 day: 19.36 mpg (US) Thanks: 1,422 Thanked 737 Times in 557 Posts	From the Net 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) . Last edited by slowmover; 09-03-2012 at 10:35 AM..