Front bumper aerodynamics

[Madact]

The requirements are quite detailed:

Acceptable 'eyebrow heights' (the measurement from the centre of the wheel vertically upward to the edge of the mudguard) or 'bump rubber clearance heights' (the clearance between the bump rubber and corresponding contact point) for most passenger vehicles can be found on the Passenger Car Track List for 1970 and later vehicles .

Other requirements for lowering a vehicle include:

either front or rear, the suspension travel must not be reduced by more than one third of that specified by the vehicle manufacturer
the vehicle must also retain 100mm ground clearance which is measured between any part of the vehicle and the ground
all measurements must be carried out with the vehicle on level ground in its normal operating but unladen state
coil springs must not be shortened by cutting, and no suspension component is to be subjected to heating
lowering blocks may be used on leaf spring suspensions provided they are manufactured from steel, aluminium, or metal and to be positively located to the axle spigot hole and the spring centre bolt. Note: the axle mounting 'U' bolts are not permitted to be below the lower rim height.

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 but maybe it has been - or the springs are a bit old & tired . 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)

[freebeard]

Acceptable 'eyebrow heights' -- That sounds bizarre, an administrative rule that requires a lookup table. 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.

[Madact]

Originally Posted by freebeard

Acceptable 'eyebrow heights' -- That sounds bizarre, an administrative rule that requires a lookup table. 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]?

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 (it reads a bit under right now - within legal limits, but legal limits is 10%, so that's not saying much).

Originally Posted by freebeard

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, 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.

[Madact]

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

Air Curtains.
Consistency between form and function is crucial to engineers at BMW M, who seek perfect balance of various aerodynamic factors in the wind tunnel. As a result, the 1 Series M Coupe has been equipped with BMW’s latest aerodynamic innovation. Air curtains, used here for the first time in a BMW production vehicle, improve air flow around the wheel arches with the benefit of significantly reduced turbulence.
This innovative aerodynamic feature consists of openings in the outer section of the lower front fascia that route high-pressure air through ducts at each front corner. The ducts are approximately 10 x 3 centimeters wide and are designed to channel air to openings at the front of each wheel arch, where it is discharged through a very narrow opening at high speed. The escaping air stream covers the side of the front wheels like a curtain, thereby reducing aerodynamically unfavorable turbulence around the rotating wheels.
This feature has a key role to play in the development of future aerodynamic innovations. An example of the air curtain principle was first presented in the concept study BMW Vision EfficientDynamics at the International Motor Show (IAA) in Frankfurt in 2009, and the developers of the BMW 1 Series M Coupe took on the pioneering task of applying this element to a production vehicle. To do so, they utilized the unique capabilities of the new full-scale “rolling road” Aerolab wind tunnel at the BMW Group’s new Aerodynamic Testing Center.

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

[Madact]

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.

[Sven7]

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.

[freebeard]

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):



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.

[Madact]

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

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

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...

[Madact]

Originally Posted by freebeard

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

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.

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...

[freebeard]

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.