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Old 10-09-2014, 10:15 PM   #1 (permalink)
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Front bumper aerodynamics

Well, I picked up a set of 2nd-hand bumpers (front and rear) for my 1997 Civic CXi hatch (I believe this is the Australian equivalent of the US Civic CX hatch, but with a 1.6L SOHC non-VTEC engine & power steering). So I can go mad with bumper mods and still have a mint car while working on it

So, I'm planning to do a 'neat' bumper mod, which will involve a full lower grille block and air dam of some description, with a bit of fine tuning before getting out the plastic welder and putty. As far as starting points go though, I have a couple of options for fabrication, possibly incorporating commercially available 'lips' in the construction. Most of what I've seen on this site and others falls into the category of 'simple grille block' or 'straight air dam', but I was wondering if anyone has any experience with or input on the other options, either on this particular model of car, or in general.



All of the options obviously lower the stagnation point by varying amounts, but will have different effects on under-body airflow. And of course there's the option of having either a front under-body pan connected to the edge of the air dam, or a diffuser (actually there are a few options there). I'm aware splitters are generally used for downforce, but *can* also have a beneficial effect on drag - I think my favourite aesthetically is the angled lip (c), but drag is the main concern of course.

Other restrictions:
- Minimum road clearance for body work (unladen) is 10cm (4 inches) here, and I'd like to leave open the option to lower the car slightly, so the new air dam should only extend downwards maybe 7-8 cm (3 inches) from the existing height.

Disclaimers:
- Car is currently un-instrumented, I have an MPGuino which will be installed before I start adding aero mods (NB: Australian civics didn't get OBD until many years after the US models did).
- Yes, I'm aware that a kammback will probably yield larger results, and I am planning on doing that, but that's for another thread...
- Things are pretty busy at the moment, I won't be starting on fabrication for a little while

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Old 10-09-2014, 11:00 PM   #2 (permalink)
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And some relevant links (the first one seems most relevant):

Splitter or Air Dam – Which Design is Best? | Hancha Blog

Aerodynamics
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Old 10-10-2014, 12:21 PM   #3 (permalink)
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Quote:
Originally Posted by Madact View Post
And some relevant links (the first one seems most relevant):

Splitter or Air Dam – Which Design is Best? | Hancha Blog

Aerodynamics
These links are great! The only thing i see in the Miata is that the most aerodynamically efficient bumper is not very pedestrian-safe, but the second most efficient (4) looks like the best solution, even giving some good downforce. I have seen that most manufacturers use solution number 3.

I wonder if the third design (or the fourth one), used in conjunction with a partial/total grill block would be as efficient as the other designs without losing that factory look a lot (talking about the Miata)

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Old 10-10-2014, 01:17 PM   #4 (permalink)
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It's documented on this site that the (b) option will have the lowest drag. Splitters have increased drag compared to straight vertical air dams; this is why companies use as close to vertical air dams as the designers will allow. Check out this Tahoe- Chevrolet Tahoe picture # 01 of 19, Front Angle, MY 2015, 800x600. I guarantee you the aero engineers wanted to move that air dam forward, but couldn't due to styling and approach angle.

"Solution 3" is a compromise for usability, but if you know how to drive a lowered car, use the same principles to avoid damaging a more forward-mounted air dam.

(d) may actually increase your drag, because it will be forcing air under your car. Whether you have a belly pan or not, you do not want to be compressing air like that.

I would do something like this if I were you. Image taken from EM user johnlvs2run's Garage.


PS- It seems like you're concerned about having downforce as well- do you race? If you need downforce at the expense of efficiency you'll want to go with a splitter. But if you're looking for fuel economy, the aforementioned mods will actually help you with highway stability as well as efficiency.

PPS- I know the Miata link says their CFD showed lower drag with the splitter, but both land speed racing and fuel economy challenge vehicles are usually modified to have vertical air dams.







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Old 10-10-2014, 10:50 PM   #5 (permalink)
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Well, sounds like (d) is out at least. This option is used on solar cars etc, but then they have fairly high ground clearances (in proportion to body thickness) and negligible high pressure zones at the nose, so they wouldn't get the compression effect...

I'm not concerned with downforce at all (though I'd rather avoid generating *too* much extra lift ) - just acknowledging the 'conventional wisdom' that 'air dams are for drag and splitters are for downforce', which is what I'm questioning here, based on a few studies I've seen (including the one linked).

The vertical air dams shown all extend very far down, which is not an option - a vertical air dam which ends 10-15 cm (4-6 in) above the ground (as per local road-worthy regulations) will also force air under the vehicle, simply by means of the high pressure zone at the front of the vehicle - a situation a splitter or vertical dam with an angled lip *may* improve.

On the speed record cars, downforce is a huge concern (hence the full-height dams), but there may also be other reasons to avoid a splitter (salt tracks can apparently be rougher on the small scale than bitumen). Plus the speeds are quite a bit different, I'm looking at 60-110 kph (45-65mph) not the 300kph / 200mph range. I suppose what I'm saying here is that building a salt-flats car that goes over 300kph doesn't imply you've done careful comparisons of air dams vs splitters and then optimised it for the highway at 100kph... there will no doubt be similarities, but the best solution may not be the same. By the same token, studies I've read showing drag improvements from adding a splitter to a vertical dam are also from race car studies, though at lower speeds. Which is why I'm looking for "passenger car" experience...

One big factor, as pointed out, is that splitters do "look" dangerous (I imagine the plywood or aluminium ones referenced elsewhere on the 'net would be quite deadly, in fact) - one reason option (c) was included.

Does anyone have links to FE comparisons of, say, vertical vs vertical with splitter on this or similar sites, at realistic on-road clearances? I did look, but couldn't find any references to actual results from anyone trialling a splitter or angled lip in a FE context.
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Old 10-11-2014, 12:20 AM   #6 (permalink)
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I would block the upper grill completely, and cover much of the lower grill and then a straight down spoiler that extends only a bit below the lowest protrusion under the front part of the car. Do not go closer to the ground that needed to "cover" the lowest portion.
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Old 10-11-2014, 02:28 PM   #7 (permalink)
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air dam

I'm going to vote for 'b' also.And for the same reasons mentioned by Sven7.
The 'Spirit of Ecomodder.com' nose is like (b),with a bit of layback and is already producing induced drag (downforce).
FIAT research shows the same Cd for 'straight' and 'layback',so there's no apparent drag benefit to the layback.
The forward-projecting angled shelf or splitter would increase the induced drag,so unless you're going to track race the thing,there's no point.
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I would mention also,that if the distance from the nose, to the face of the radiator is less than the height of the radiator,then you might consider moving the nose forward to that dimension.
It will allow the minimum entry opening (if you do an airtight duct ),and also allow some plan curvature/radius which would help in crosswind drag reduction.
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Old 10-11-2014, 02:45 PM   #8 (permalink)
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Hi
am already running a "b" style front air dam. made no difference to mpg but has increased stability in cross wind and passing trucks. This may be due to it being fitted to a motorhome, from a safety point of view its staying but will make it from lawn edging instead of parts lying around. just wanted to see if it work at no cost apart from my time
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Old 10-11-2014, 11:24 PM   #9 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
I would block the upper grill completely, and cover much of the lower grill and then a straight down spoiler that extends only a bit below the lowest protrusion under the front part of the car. Do not go closer to the ground that needed to "cover" the lowest portion.
Good point about blocking the upper grille. Something I should have realised from human powered vehicle experience, the best place for a low-velocity ram intake is always the stagnation point, as power lost is a product of pressure and velocity - the upper grille would have higher air velocity, thus higher losses. Should be easy to fabricate, too - in fact I'm tempted to go out and tape a bit of clear plastic over the upper grille right now

Quote:
Originally Posted by aerohead View Post
I'm going to vote for 'b' also.And for the same reasons mentioned by Sven7.
The 'Spirit of Ecomodder.com' nose is like (b),with a bit of layback and is already producing induced drag (downforce).
FIAT research shows the same Cd for 'straight' and 'layback',so there's no apparent drag benefit to the layback.
The forward-projecting angled shelf or splitter would increase the induced drag,so unless you're going to track race the thing,there's no point.
---------------------------------------------------------------------------
I would mention also,that if the distance from the nose, to the face of the radiator is less than the height of the radiator,then you might consider moving the nose forward to that dimension.
It will allow the minimum entry opening (if you do an airtight duct ),and also allow some plan curvature/radius which would help in crosswind drag reduction.
Induced / parasitic drag is a good point - but only if extra induced drag of one option > extra profile drag of another . The bottom edge of the common (b) option also looks a heck of a lot like the edge of a flat plate perpendicular to the wind, too - or a sharp corner, if combined with a belly pan, neither of which tend to be great for drag. The sharp corners with the other options are no better, of course. What about something like this instead (call it option (f) )?



I like the idea of lengthening the nose, but I'm not sure how I'd go legally speaking there -apparently an engineer's approval is needed for 'lengthening a vehicle' here. The intent of this is to do with changes in wheelbase and gross changes in hood/boot length but the wording is ambiguous and cops looking to meet their (allegedly non-existent) ticket quota are known for 'generous' interpretation of the rules at times. A subtle increase (maybe 5cm) might be doable, though I'm not sure it would be worth the effort. The plan view of the bumper is a bit 'flatter' in the middle than near the edges though, so that much nose extension, by continuing the curve established at the sides, could be made to look very natural indeed...
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Old 10-12-2014, 03:27 AM   #10 (permalink)
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How do they define 'bodywork'? My Superbeetle will mostly clear 3 1/2", but there is a front tow hook on the right that is at 2 7/8". The law whereabouts is no lower than the wheel rim, and I have front tires with a 2 1/2" sidewall. Most people think it's low, but it's not hoodride low.

I would think in Australia it would be important to look imposing, what with road-trains, Mad Max and all.



Polished steel, even if it's only a beaten skin over the donor bumper. You could go with a step in the center for extra clearance for gravel roads, or widen it until the outer sections are a spat over the front wheelwells. Seven 2" round holes at the stagnation point.

Drop it, then size the airdam. Satin clear Plastidip for low maintenace.

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aerodynamics, air dam, bumper, grille, lip

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