Raising a Trunk?

[cRiPpLe_rOoStEr]

You might find some inspiration checking the design of the hatchback versions of some cars which used to be available in Europe, with a bubble-shaped rear glass. Remember the early Ford Mondeo, Euro equivalent to the US-spec Ford Contour and Mercury Mystique, which had the hatchback option in contrast to the sedan-only approach in the USA.

[aerohead]

Originally Posted by JacobLeSann

A ducktail/lip spoiler is interesting. I see this on newer Chevy Cruze/Malibu models, Chrysler 300 models, teslas, etc. I never really did know how lip spoilers work, but seeing Aerohead’s CRX makes me wonder.

Adding a rear wing that extends the whole length of the rear into a boxed cavity (sort of like a Porsche K35 rear) is the main idea I’ve been looking at. I trialed a quick n’ dirty bonneville style trunk extension on my long distance trip recently, which really did help the mileage.

The fastback is indeed a superior design, it’s just that I’m trying to retain the stock practicality/aesthetics/vision. I plan to give the car to my mom once I fix up my hatchback. That one will get more radical mods. The hitch mounted tailbox is likely gonna go on that one, when traveling.

I think there is relatively stable flow down the top half of the rear glass. If you look at a side view of the FG2 chassis (as freebeard said), you can see that the transition to the rear glass is very gentle, even though it’s a steep angle.

Dr. Wolf H. Hucho spoke of spoilers being almost as effective as just raising the body contour up to where the top of the spoiler would ultimately be located.
On a notchback, the drag will be affected by:
* The length of the roof in relation to total body length.
* Any aft-body roof camber/curvature.
* VGs strategically placed.
* Backlight angle.
* C-Pillar curvature.
* Boot height.
* Boot length.
* Addition of a 'wing' type spoiler.
* Addition of a flip-up spoiler.
* The 'effective' 'angle' across the backlight, to the trailing edge of the car.
* Aft-body camber/boat-tailing.
* Diffuser ( if paneled upstream ) [ I personally do not subscribe to non-belly panned diffusers ].
PS A spoiler, by itself, cannot provide a pressure recovery function as would be experienced with a box-cavity, or proper boat-tail. The elongation is mostly wasted.

[JacobLeSann]

I understand this thread is like a year old, but I don’t want to start a new one.


I came back to read this thread after I was looking at pictures of a custom Walkinshaw Commodore from the 80’s. It’s featured in Julian Edgar’s book on aerodynamics and various other automotive sites. Supposedly, the body kitty reduced drag up to 25% less than the stock body.

Is this spoiler at the rear the main culprit? I think so. I noticed that the deck-lid has been raised quite a bit, and you can see a rising angle below 30 degrees starting from the rear windshield. Looks to be a very exaggerated version of what’s been described in this thread.

What do you guys think?





(P.S. I think there’s a 911 in there, but same idea.

[freebeard]

Is this spoiler at the rear the main culprit?

What do you guys think?

It's the spoiler/front airdam combination. That was learnt with the original 911 spoilers. Balance in the force, or something.

[cRiPpLe_rOoStEr]

Originally Posted by JacobLeSann

I think there’s a 911 in there, but same idea.

Not to mention some newer models of the 911 still resort to a duck-tail like that, yet in the 911 it also had something to do with the air flow for the engine bay.

[aerohead]

Originally Posted by JacobLeSann

It looks like flow reattaches at the boot and separates cleanly off the top end of the trunk. I’ve posted some tuft-test photos here:
https://ecomodder.com/forum/showthre...tml#post675015

The 2022 civic coupe has a very neat rear end. It’s a very true-to-form fast back and has some sharp separation edges at the rear. My issue being that my rear end isn’t really like that at all. I can’t really get it to take that shape without a lot of modification, hence why I’m looking at raising the lid. The 2022 Civic sedan is a bit of a better model. You can see the progressive raising of the trunk’s height over the different model years.

I think I might go ahead and test out some prototype aero mods. I want to raise the lid toward the base of the windshield, and drop it down towards the car’s rear,, flowing out into a rear bonneville-wing type extension. I think that would work very well as a fastback substitute.

Of the three predominant body styles for production vehicles, the notchback is the most mutilated, and does not lend itself to aerodynamic geometric relationships enjoyed by fastback and squareback vehicles.
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Kamm's last K-Car, constructed in 1958, at Battelle Institute, Germany, was an Auto Union/DKW notchback sedan, converted to a 'kammback', via a composite integrator, directly overlaying the OEM notchback afterbody.
Once 'converted' to a K-form, additional streamlining is simply a modular, plug-and-play exercise, reducing drag as a function of elongation.
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Wolf-Heinrich Hucho's 2nd-Edition is full of afterbody optimizations involving :
* geometries as a function of roof length as a percentage of total car length
* roof camber-contour if any
* backlight angle
* C-pillar geometry
* boot height saturation
* boot length saturation
* adhered vorticity
* attached counter-rotating longitudinal vorticity
* separation-induced downwash ( false-attachment )
* vortex burst
* spoiler undershoot
* spoiler overshoot
* spoiler saturation
* etc.
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As to the 'Bonneville' spoiler, you might be interested in the 2007, Ford Fusion notchback, 999 LSR car, which was aero-modded, from Cd 0.34, down to Cd 0.21.
A technical image of this car indicates the 'coincidence' of a perfect alignment of the tearing edge of it's Bonneville spoiler and the AST-III.
You may discover the same 'coincidence' for Chevrolet's Cobalt ECOTEC LSR's Bonneville spoiler tearing edge.
The Opel Calibra's rear deck optimization also indicates a 'coincidence' of it's tearing edge aligning with the AST-III.
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There is some probability that HONDA has already optimized the rear of your car, for what it is, in the context of the product committee's design specification. And Honda is very tight-lipped about aerodynamics.
The 2022 Coupe afterbody silhouette is likely just a concession to more stringent CAFE standards.
You can do better, but anything significant would require 'losing' the notchback.
The best images for the Cd 0.1716 M-B EQXX, Cd 0.175 Lightyear Zero, and Cd 0.1764 Think Flight Subaru Impreza, indicate the coincidences for exact matches to the AST-III.
It's the only thing I know to be a sure thing.

[aerohead]

I'm thinking that everything they did to the car contributed to the modified performance.
That said, since aerodynamics is about the rear of a vehicle ( Hucho, 2nd-Edition, page- 119 ), and we're given that the entire rear of the Holden was in separated flow, that the C-pillar modifications, along with the 'bird bath on the boot' contributed significantly to the drag reduction.
It's my opinion though, that, since we're talking about a race car, low drag did not dictate design, and the team strove to strike some tradeoff between drag and lift ( downforce ). The rulebook from the sanctioning officials probably has some 'say' in the matter as well.
A 'blueprint' of the car would tell us a lot about what they actually did.

[JacobLeSann]

I appreciate the info and ideas. I quite like that Ford Fusion.

Tired of my own guesswork and theorizing, I broke out the yarn and tape. I made a spoiler from cardboard that follows a shallow angle starting at the base of the trunk lid and ending at the rear, that has a height of maybe 3 inches.

I noticed a lot more pressure on the rear (the tufts are flowing much faster), and the tufts spun and fluttered all over the place. I’m not sure if that’s good or bad, but that seems bad.

The stock rear end seems to have pretty good flow. The C-pillars have a poor transition to the rear windshield and from what I can tell that is causing two vortices to fall off them; culminating in a separation bubble at the base of the trunk’s centre.


So, maybe that’s the wrong way to go. I have considered extending the sides up to create a wall all around the spoiler; like the “bird bath” on the Walkinshaw.

But given that flow seems to be attached for most of the windshield, I think I’ll try out the spoiler on that 999 LSR Fusion. Very similar to the first spoiler I ever built for this car.

[JacobLeSann]

Here’s some better(ish) photos of the rear airflow after I put the spoiler on. It seems to me that there’s a lot more turbulence back there, and more tufts that are separated. I’ll try raising the height of the spill plates and put a lip at the end.


If I’m making a huge mis-observation, let me know!

[JacobLeSann]

Here’s my quick mock-up of a Bonneville style spoiler. I just eyeballed the length based on the image of that Ford 999 LSR. This seems to do a very good job cleaning up the airflow back there. On the other hand, there is still that separation bubble, but now it’s diminished in size quite a bit.

Stupid phone, froze while I was recording. When I tried again, it was too dark to get any good images. Now I get to drive to work in the spaghetti car, so I can get some better pics after work tomorrow.

I still want to try raising the trunk height. This Bonneville spoiler works but I wonder if raising the whole trunk 1.5-2” would do anything about the separation bubble. Have to see.