New lower air dam for 2017 Chevy Full size van

[EcoVan]

For steady state highway, weight doesn't matter. Unfortunately most U. S. cars are city drive, so mass would be best addressed.

On the highway, I wouldn't expect a hybrid to be much good..for 48% better mileage it would need to get 33 x 1.48= 49 MPG. Remember I am running a diesel, not gas on this van. I can get 33.3 mpg @ 68mph with a 7,500 lb vehicle on midwest flat highways.
I have found that with heavy traffic moving at 55 to 60 mph, that the traffic appears to help push/pull the van along so even while in a lower gear than when running 68 mileage stays quite high, around 31, IIRC. The brick effect probably helps with this, I suspect....plenty of surface areas for vacuum to work on.

It seems most diesel vehicles get about the same highway as hybrid








S.

[freebeard]

Re: #19 -- I tend to believe this graphic.



25% front-- 20+30% middle -- 25% rear.

[freebeard]

My XFi is up on ramps right now as I paint the wheels. I had a look at the opportunity under the front bumper and the lower radiator bracket is 1.5-2" below the front bumper. So there's a part with a stepped, straight back edge and a curved front edge that's needed there.

That bracket is as low as the suspension.

[Vman455]

Originally Posted by aerohead

1) We'd be compelled to accept Ford's data, as specific to this particular vehicle.
2) A review of my sources, of which presented drag tables derived for the specific test vehicle, all indicated for 'sweet-spots', where drag curves for both airdam drag, and underbody drag intersected, constituting the configuration of minimum total drag, as a function of airdam vertical 'size,' just as with streamline bodies of revolution vs fineness ratio.

Yes, this is correct--but you are missing my point, which is that this "sweet spot" does not necessarily correlate with the height of underbody components, and thus that height should not be used as the determiner of air dam height; testing should. Go back and re-read Hucho; if it's the same as later editions (I'm looking at the 4th and 5th editions right now, which have the same wording and graphs on front spoilers) you should notice that the overall drag curve for an air dam depends on "equivalent hydraulic roughness" of the underbody, and not the height of specific underbody components. Moreover, the drag reducing effect of a front spoiler derives not from its "shielding" the stuff hanging down under the body so that flow avoids it (or whatever people think is going on when they regurgitate the height rule), but in fact--

The drag-reducing effect of a front spoiler is based on the fact that it diminishes the air speed under a vehicle, thus attenuating the contribution of the underbody airflow to overall drag.

Where that sweet spot is cannot be intuited, and definitely not with a simple rule such as "no lower than the lowest-hanging component." And, as pointed out in the Ford paper, drag reduction from a front spoiler also depends on how it changes the flow through the cooling system and pressure at the back of the vehicle.

An example. Base:



4.5 inch dam: -6.7% change in drag



9 inch dam: -9.0% change in drag



The shorter dam sits well above the lowest hanging components under the truck and yet, you get 75% of the drag reduction as with the larger dam (which still sits an inch or so above the lowest part of the skid plate and front differential). That's a huge range of height to get similar drag reduction (2.4% is at the limit of what's measurable on the road).

We're in agreement about testing, but sadly I still don't see much of that going on around here. People want to build and then hope when they should go the other way: test and then build.

[aerohead]

1) In Hucho's 2nd Edition, he used ' Equivalent roughness' in his first graphic presentation, Figure 4.80, page 166, and only qualitatively.
2) 'Sweet spots' occurred with all actual production vehicles investigated and reported with graphical drag data.
3) Twelve different configurations of four different airdams were presented for the Brazilian, Volkswagen do Brasil 1600X. A blueprint with dimensions would allow any interested party to reproduce the dimensional interplay leading to the twelve Cds, 1973.
4) Same for Max Schenkel's test vehicle, 1977.
5) Ditto: H.J. Emmelmann's OPEL Corsa,1982.
6) Ditto: Feysal Ahmed Adem's pickup truck, 2009.
7) Ditto: USN Lieutenant Nathan A. William's 1997, Dodge RAM, 1500 Pickup, 2003.
8) Ditto: Don Sherman's Ford Pinto, March 1974
9) Ditto: Don Sherman's Datsun 240-Z, May 1974
10) Ditto: Dennis Simanaitis' Volkswagen Scirocco, August, 1982
11) Ditto: ITworks Project CRX, USFRA, Bonneville Int'l Speedway, 1990.
12) Ditto: ITworks Project CRX/ CAR and DRIVER, Chrysler Proving Grounds, 1991.
13) Ditto: ITworks Project SPIRIT-I, DARKO, 2014
14) Ditto: ITworks Project SPIRIT-I, USFRA, Bonneville Int'l Speedway, 2014.
15) Ditto: ITworks Project SPIRIT-II, DARKO, 2017.
16) Ditto: Subaru Engineering Division, FUJI Heavy Industries, Ltd, SAE Paper 860216
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17) The airdam investigations would note airdam drag, Underbody drag, total drag, lift, volumetric flow through the cooling system.
18) Since pressure drag dominates the aerodynamic drag, and the forward stagnation point pressure is the static pressure, the differential between the nose of the F-150, and the average mean base pressure behind the tailgate/valance/rear bumper would be of great interest.
19) I'm comfortable 'owning' the comment about frontal area, as a first-principle precaution. Hucho's Figure 4.17, page 123, image (D) bottom of the table drives issue home.
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20) As to 'not shielding', other messengers, including R.G.S. White, at MIRA, took the actual complexity of underbody structures, and their impact on drag, such that he created a five-tiered rating system, comparing what one could perceive as the degree of aerodynamic torture chamber they presented to the oncoming flow, from best-to-worst.
There are structures which, un-shielded, can separate the flow to such a degree that, the entire underbody becomes turbulent, with no possibility of harvesting any of it's kinetic energy for pressure recovery.
The 1969 Chevrolet Camaro Z/28 was so nasty that it created 375-pounds of front lift at 115-mph. Adding the optional airdam cut that by 150-pounds.
The 'shielded' chassis components were now 'drafting' in the wake of the Cd 1.19 airdam.
In the absence of an airdam, belly pans and a diffuser knocked 70-counts off the drag of the Audi 100-III.
21) As to your quanta, I don't have any confidence in your testing methodology. For reasons mentioned elsewhere.

[aerohead]

I previewed the paper.
The thought crossed my mind that, as the airdam descends close enough to the ground that the frontal begins to increase beyond the original OEM value, that from that point, all Cds need to adjusted to agree with each new frontal area-based CdA.
Also, the F-150 is a body-on-frame, ventilated body-to-box configuration, and phenomena associated with it would be limited to ventilated bodies, leaving unibody pickups like Ridgeline, Cybertruck, Rivian, etc. in a different category.
The bed length isn't specified.
The fuel tank size isn't specified.
Standard cab vs Supercab.
There could be some variability in F-150 underbody aero performance depending on specifics.
Under full load, on unimproved ranch and farm surfaces, I'm uncertain how much suspension travel might occur during impacts, and if a 5-inch ground clearance airdam ( the lowest drag for the moving ground tunnel ) might experience a ground strike.
Do they actually publish a Cd for the truck?
The 2014, EcoBoost, 6.5-foot bed, 4WD, Supercab was Cd 0.402 and 36-sq-ft Af.
It looks like the F-150 constitutes 20% of Windshear's nozzle area. ( that's at zero yaw. I have no idea how large its aerodynamic footprint is at 7-degrees yaw ).

[aerohead]

1) I spent the last two days picking this paper apart.
2) So far, Levon Larsen and Sudesh Woodiga have created more questions than they've answered.
3) If, frontal area- based coefficients of aerodynamic drag are still defined by frontal area, then all but one Cd reported in the drag table are extremely dubious, as all are associated with a different frontal area.
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4) The February, 2015 edition of MOTOR TREND shared most of the 2015 Ford F-150, 3.5-L, Super Crew dimensions, and four views of the truck, from which scale dimensions could be derived and extrapolated to full-scale, as verisimilitude provides for.
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5) I ran calculations for all airdam face sizes, from 5" - to - 9", as the OEM pickup is already fitted with a five-inch airdam.
6) From the 'rule of thumb' metric, a 6.35" airdam would have been selected.
7) On a CdA basis, the drag minimum actually occurs with a seven-inch airdam. So I missed it by 0.65"
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8) The F-150's EPA HWY, 22.00 mpg occurs at a constant 100km/h ( 62-mph ). I reported this back in 2015, after AeroStealth and I tested his F-150, out in West Texas.
9) The lowest drag, seven-inch airdam returned an estimated 22.255-mpg, with a drag force of 139.989-pounds, aerodynamic horsepower of 23.143-hp.
10) The 'rule-of-thumb' airdam returned an estimated 22.200-mpg, @ 140.447-pounds drag, 23.218- aerodynamic horsepower.
11) The eight-inch airdam returned 22.211-mpg, @ 140.341-pounds drag, 23.203-hp.
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12) Without any 'measurement', the 'rule-of-thumb' airdam happened to come in at 0.247% lower mpg than the 'optimum.' ( a quarter of one percent ).
13) The 'rule-of-thumb' airdam came in at 5/100 % lower mpg than the eight-inch airdam ( Ford's claimed minimum drag example ).
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14) Larsen & Woodiga's work is ambiguous. I'm not going to speculate as to the reasons they chose to present their data in the fashion that they did. It doesn't speak well of the Ford Motor Company.
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I have the starting assumptions, premises, and more quanta if anyone's interested.

[EcoVan]

Is this the rule of thumb simply " at most covering the low hanging component of the vehicle" or some other mathematically derived number? I could certainly see in my mind's eye that a shorter air dam would shield the low hanging components from creating drag if the air stream is sculpted downward by the shape of the air dam or valance. This would allow greater ground clearance but still maintain the gains possible. The cross sectional view of the airdam/valance suggests it is creating downward velocity with the sloped section and then the smaller flat air dam portion is at the bottom.....Maybe the blast fence as described previously.

Unfortunately, its going to be a few weeks before I can test this installation, as the van needs a new sensor and some front end work. However, I have nearly 80,000 miles of experience already with it, so a base is already established. Also, with my experience with my 93 escort, the air dam by itself did'nt seem to be particularly beneficial and neither did smoothing out the front underside. However, when the belly pan was attached to the bottom of the air dam, that was when the magic happened. I suspect that this configuration allowed the cooling air being pushed/pulled into the engine compartment filling the vacuum behind the air dam, greatly reducing it drag.

I already started mocking up a belly pan that will attach to the bottom of the van's air dam and close up the front of the front wheel wells.

[aerohead]

Originally Posted by EcoVan

Is this the rule of thumb simply " at most covering the low hanging component of the vehicle" or some other mathematically derived number? I could certainly see in my mind's eye that a shorter air dam would shield the low hanging components from creating drag if the air stream is sculpted downward by the shape of the air dam or valance. This would allow greater ground clearance but still maintain the gains possible. The cross sectional view of the airdam/valance suggests it is creating downward velocity with the sloped section and then the smaller flat air dam portion is at the bottom.....Maybe the blast fence as described previously.

Unfortunately, its going to be a few weeks before I can test this installation, as the van needs a new sensor and some front end work. However, I have nearly 80,000 miles of experience already with it, so a base is already established. Also, with my experience with my 93 escort, the air dam by itself did'nt seem to be particularly beneficial and neither did smoothing out the front underside. However, when the belly pan was attached to the bottom of the air dam, that was when the magic happened. I suspect that this configuration allowed the cooling air being pushed/pulled into the engine compartment filling the vacuum behind the air dam, greatly reducing it drag.

I already started mocking up a belly pan that will attach to the bottom of the van's air dam and close up the front of the front wheel wells.

1) In science, we're cautioned not to use them.
2) Over 45-years though, one observes certain design behaviors.
3) All my sources for airdams reveal inflexion points for drag, as a function of 'face height', whereas, at some 'height' drag begins to increase again, as also seen in SAE Paper 2018-04-03, if you'll look beyond the superficial presentation as presented. Context. Conditions. Caveats.( world records live between the lines ).
4) Over the decades, you'd be hard-pressed to find a 'street car' in which the airdam extended below the 'belly'.
5) If you acted upon Larsen & Woodiga's apparent conclusion, you'd be shooting yourself in the foot, dragwise.
5) Wolf Heinrich Hucho, in his 2nd-Edition, cautioned us about doing anything which would aggravate frontal area. I happen to believe that it's a good recommendation, even for the billionaires.
6) The F-150's minimum drag airdam does not occur where SAE 2018-04-03 implies.
7) And it just happened, that the 'rule-of-thumb' solution ended up within a fraction of a percent of the optimum, without spending one second on testing. Just sayin'.
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By the way, I brought a level and tape measure with me this morning and measured curbs and parking space, pre-cast concrete minicurbs. If the F-150 'settled' as much as 1 millimeter in ground clearance when boarded, then, there's not a place you could park in Denton, Texas where Larsen & Woodiga's airdam wouldn't scrub, be damaged, or knocked off.

[freebeard]

However, when the belly pan was attached to the bottom of the air dam, that was when the magic happened. I suspect that this configuration allowed the cooling air being pushed/pulled into the engine compartment filling the vacuum behind the air dam, greatly reducing it drag.

The first problem with the 'rule of thumb' is that the air won't move straight back from the bottom edge, as you say.

Secondly, lowest point is ambiguous. The tow hook on my Superbeetle is 1.5" lower than anything else. it's frontal area is miniscule.

A while back I tried to get 'first approximation substituted for 'rule of thumb' but it was like King Canute ordering back the tide. [We jest]