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 :D

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.

http://ecomodder.com/forum/attachmen...1&d=1412902572

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 ;)

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)

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.
http://ecomodder.com/forum/emgarage-...d07a89bdad.jpg

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.

http://image.gmhightechperformance.c...nationals+.jpg

http://image.hotrod.com/f/eventcover...cing_entry.jpg

http://img.photobucket.com/albums/09...insightlow.jpg

Coincidence?

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 :D ) - 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.

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.

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.

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

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 :D

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) )?

http://ecomodder.com/forum/attachmen...1&d=1413080574

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...

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. :thumbup: 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.

http://ecomodder.com/forum/member-fr...inspection.jpg

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.

The requirements are quite detailed:
So I just looked up the list and apparently the "minimum eyebrow heights" for my model are 356 F / 347 R. So I got out a ruler, and that's almost exactly what the car is sitting at. Pretty sure it's all stock (previous owner wasn't aware of any modifications), and it sure as heck doesn't *look* lowered :confused: but maybe it has been - or the springs are a bit old & tired :p . Anyhow... I've double checked my measurements, so maybe I *won't* be lowering it (or lowering it any further, as the case may be)...

Minimum ground clearance of any part of the car seems to be about 140mm, and the bottom of the front bumper is about 200mm, so based on the above advice, I should look at extending the bumper downward about 60mm (about 2 3/8 in)

Acceptable 'eyebrow heights' -- That sounds bizarre, an administrative rule that requires a lookup table. :confused: Are pre-70 vehicles exempt? Any rule about covering the tread width? Are fender skirts (front and/or back) disallowed? Re-arching the fender?

suspension travel -- dropped spindles OK?

Cutting/heating/lowering blocks -- This sounds commononsensical. What does you tire sidewall measure [at]?

While I like the shiny metal grille block/airdam on Bombshell Betty, there is a thread on an Aerodynamic Bumper that has currently dropped to the bottom of page 2 in this forum. He takes cooling air from in front of the wheelwells and it seems to be a good strategy.

Yeah, welcome to Australia, the Land Of Needlessly Complex And Obtuse Regulation Of Every **** Thing... I bet some bureaucrat just went through manufacturer "eyebrow height" specs, subtracted 20mm to allow for a bit of normal wear-n-tear spring sag, and went home that night chortling into his neckbeard :( .

Pre-1970s vehicles only have to comply with the 100mm minimum & suspension travel reduced by no more than 1/3. Dropped spindles are fine as long as the other requirements are met. Tread width must be fully covered between specified angles etc. As for re-arching the fender to allow for more drop with the same 'eyebrow height', goodness only knows. I don't see anything preventing fender skirts though - minor bodywork modifications aren't really restricted as long as there are no sharp projections (with allowance for OE mascots and pre-1965 car designs) - the above rules are to do with lowering.

I'm currently running 195/50 R15 (radius 288mm, sidewall 97.5mm) but next set of tires will be 195/55 R15 (radius 297.75mm, sidewall 107.25mm), which is still within the allowable deviation from placard (+/- 15mm diameter), but will give me *slightly* taller gears and bring my speedo closer to true :thumbup: (it reads a bit under right now - within legal limits, but legal limits is 10%, so that's not saying much).

Yeah, I took a look at that - long nose leaves plenty of room for an effective diffuser to slow down the intake air for the radiator, too. Having extra fender in front of the wheels makes sense too, though I'm not sure it necessarily follows that that's where cooling intakes should go.

Slightly off-topic from the previous discussion of air dam height and edge treatment, but here's another interesting thought from BMW, which might be interesting to try - 'air curtains' to reduce tubulence between bumper and the front wheel:

BMW Drops the Goods on the 2011 1-Series M Coupe (With Photos!) | The Smoking Tire
BMW EfficientDynamics
BMW EfficientDynamics : Aerodynamics

I see this has already been discussed here, but I might maybe give it a shot...

This front bumper from DuraFlex is actually pretty close - except you'd want to blank off most of the scoops, of course ;)
1996-1998 Honda Civic Duraflex JDM Buddy Front Bumper Cover - 1 Piece

Here's a quickly edited mock-up based images on one of the images from the above link. This version has
- Upper grille block
- Generous rectangular stagnation point intake (this is Australia after all, they reckon metro temperatures may hit 45C - 113 Farenheit - this summer. Not including heat island effects.) - not sure if this is the best shape, oval section with rounded edges to avoid separation might be better.
- Bump at the bottom as per "option (f)" ... well, pretty close, anyhow.
- Air curtain intakes - moved back a bit compared to the intakes in the source image, to avoid an unnecessarily long internal vent. Not sure if I'll do this yet.
http://ecomodder.com/forum/attachmen...1&d=1413360851

Do that bumper sketch, but do not try air curtains unless you have access to a wind tunnel. You know those ricer bumpers are not functional.

IMHO the rectangular opening is Okay, it needs to conform to the internal ducting to the rad/heat exchangers. An oval opening would need to transition to their rectangular shape. Radiusing the edge OTOH could be important to create a rectangular bellmouth.

If you do the air curtains, maybe a wind tunnel will find you. :)

There is some internal structure involved. The exit is a tall narrow slot that vents across the face of the tire, not onto the tread. And I'd move the outer edge of the intake opening forward to make the openings 'cross-eyed', increasing the apparent opening to the sideways-moving air. Intake larger than the exit as it's a convergent duct.

Here's a picture that I hadn't shown because I wasn't happy with it (and there're problems trying to reproduce it):

http://ecomodder.com/forum/member-fr...23-9-48-49.png

It's basically a lipped vertical air dam curved to match the bumper, with flat-faced wings to scoop the air. The shape of the junction of the dam and wings is not right; I struggle with the design tool. :(

Well, I don't have a wind tunnel, but I do have OpenFoam set up (once I unpack and set up my computers) - I could run a few simulations easily enough, all I'm lacking there is a dimensionally accurate CAD model of a Civic :D

Even without that, a 'basic' air curtain set-up is *fairly* simple in design (theres a bunch of parameters that need tuning of course) and doesn't change the profile of the bumper much, if at all - it would be very easy to block off the intake for ABA testing.

Thanks frebeard for the air curtain details - I'm familiar with the basic principle, but wasn't sure exactly where to point it - its unclear from the photos I've seen whether it was pointing at the edge of the tread or slightly outside it... I like the lip/scoop model btw, that looks nice :cool:

One thing I was thinking with the air curtain concept, is that it wouldn't actually be too hard to tuft-test a 'tape-on' version as a proof of concept - just need a few extra 'long' tufts on the front of the wheelhouse to capture flow further over the wheel, and a few tufts further back on the car to see the downstream effects at the side of the car... A tape-on version would increase frontal area slightly, so probably not a drag improvement in itself, but might be enough to show the effect... Or I could even do a corflute air dam and implement the air curtain on one side only... Hmm...

Just another thought there, looking at the picture. The leading edge of the 'flat faced wing' might not be pointing in quite the right direction - if there's still a bit of sideways flow at that point, the leading edge there would be at quite a high angle of attack, which could be a problem...

I agree it's suboptimal. It would take CFD or tunnel time to sort out. The thinking was that air moving sideways in the airdam is captured and forced back into the air curtain. Air on the outside of the base can flow straight back rather than being pushed outward.

Crosswinds would need to be investigated.

Random product found on the interwebs - a bit pricey mind you ... and it's for the facelift model. Still...
JoJo style EK front end (96-00 Civic) closed

http://product-images.highwire.com/9...934389419n.jpg