Civic grill block, front undertray, tire deflectors

[LeanBurninating]

I am interested to know, first of all what materials you used to make your tire deflectors out of, and also, how has that plastic held up being so close to the exhaust?

[binarycortex]

Originally Posted by daqcivic

My 2nd mod was a grille block:
{image removed for brevity}
The radiator is tiny, and covers only the passenger side of the grill opening, so the grill block leaves about 30% exposed. I have not measured any significant improvement with this, which is frustrating and confusing, especially since the whole driver's half of the grill was just letting air straight into the engine bay where it slowed down and exited underneath.

I am wondering, and somebody correct my if I am way off, but wouldn't the grill block work better if the block was on the outside rather than inside the grill. It would make the front end smoother and easier for the air to flow around the grill rather than get trapped in it when it hits the block.

[Daox]

You are correct binarycortex.

[daqcivic]

I would guess that, because the grill is located within a flat, front-facing profile of the bumper, the 1-2 inches the grill block is recessed will not add any quantifiable drag. That flat section will create a high pressure/low speed pocket of air around which the surrounding airflow will divert.

Again, this is my guess, but the small recess of the grill will not measurably effect how air moves under or over the front edge of the bumper. But I could definitely be wrong!

However, mostly for a different reason, I am working on a new grill block that I will actually attach to the front of the bumper instead of within the grill. Along with this I am making a rectangular duct that will extend from the grill block to the edges of the radiator in order to maximize the efficiency of the remaining grill opening.

Also, I've since extended my front tire deflectors inward to divert air around the wheel wells, I'm working on a new section of the undertray that will add more coverage, and I also made small rear tire deflectors.

I've not had any melting/warping issues with the undertray being close to the exhaust. I initially lined the edges of the undertray where the downtube is with aluminum flashing, but discovered there really wasn't much heat build-up, so I removed it.

Anyway, when more pics to come soon!

[Otto]

When designing front wheel deflectors, you might want to consult the Hucho book: In that there is a pic of a car (Calibra, perhaps) in a wind tunnel, and it shows that centerline airflow deflects at about a 60 degree angle outboard toward the front tires. So, the wheel deflectors should be canted inboard correspondingly, i.e., pigeon-toed so as to meet the incidence of the air as it actually impacts the front wheels. In other words, an air molecule in the free stream may splatter against the inner sidewall of the front tire at about a 60 degree angle, meaning that the effective frontal area of the tire is much greater than if the air were hitting it squarely on the front tread. So, the deflector should be installed so as to fair the flow as it actually travels to and past the tire. Otherwise, the tire acts as a wall to partly block the airflow from escaping from under the car.

Easy way to check on your car: Tuft testing, with the tufts mounted on inverted Ts, showing flow 1" to 6" below the level of the car nose. Then, have a friend with a camera shoot pics from an oblique position at highway speed.

[LeanBurninating]

prepare yourselves for the unbearably awesome spectacle of graphics



Is this kind of what you mean? sorry I am a really visual learner. I don't quite get what you are saying.

[Otto]

Originally Posted by LeanBurninating

prepare yourselves for the unbearably awesome spectacle of graphics



Is this kind of what you mean? sorry I am a really visual learner. I don't quite get what you are saying.

That's kind of what I mean. Your very nice graphic shows only a 30 degree angle from the long axis of the car, but as I recall, the Hucho book picture showed the smoke flow toward the tire/wheel more like 60 degrees as measured from centerline. Maybe somebody here with Hucho can look it up and post it, as I last read that a year or so ago. In other words, the deflection was a LOT more than one might intuitively suspect. Until I saw it, I would have thought the air went more or less straight on back, with parallel bow waves and wakes around the front wheels. Perhaps the shock wave off the nose impacts the ground, creating some sort of curtain that the air moves laterally around. Best to do tuft tests on your car, see what angle the flow really is, then add rounded leading edge deflectors accordingly. Seems like I saw a pic of a baby Benz with such pigeon-toed deflectors.

And, the leading edge of the deflector should be radiused or rounded, like the leading edge of a wing.

[wwkayaker]

I agree with binarycortex on the GB. Your current block will create a ram air effect (or increased pressure) on the radiator. My GB's do the same thing and I am working on getting the parts that I need to move my GB's to the outer edge of the bumper along with coast down testing.

Is your linoleum black or did you paint it? I have some tan linoleum in the garage and am cureous if it could be painted black.

[lunarhighway]

Is your linoleum black or did you paint it? I have some tan linoleum in the garage and am cureous if it could be painted black.

than big rounded dam is painted while the experimental flap in the side is not.

i used a satin gloss acrylic black spray can... on a slightly sanded surface... given the abuse the area sees i'd say it holds paint very well, although the original color would hide small chips better than a more off color, but it all looks quite good
however in contrast i've had coroplast and other plastic in the same area with similar paint on it that would just flake and chip after a few weeks. even with flexing there's no apparent cracks.

[daqcivic]

Originally Posted by Otto

When designing front wheel deflectors, you might want to consult the Hucho book: In that there is a pic of a car (Calibra, perhaps) in a wind tunnel, and it shows that centerline airflow deflects at about a 60 degree angle outboard toward the front tires. So, the wheel deflectors should be canted inboard correspondingly, i.e., pigeon-toed so as to meet the incidence of the air as it actually impacts the front wheels.

I remember when I first started looking at the designs of factory deflectors I was puzzled by their "pigeon-toed" placement, and wondered if it was because of what you found in the Hucho book; oncoming air flow will be directed outward as it nears the nose of the car because there will be greater pressure underneath the car than alongside it. Since then I had come to think, based on what others were saying, that the placement inward of the tire profile was to divert air from being sucked up into the low pressure wheel wells and creating turbulence as it cycles in and out. But if the airflow is angled outward, esp. as much as your windtunnel picture suggests, there would be even more air, and more turbulence, created by air interacting with the wheel well cavity, as well as with the inward face of the tires/wheels!

So, it seems to me that the placement of the front tire deflectors should be as I had planned, but with less concern to cover the outer side of the front tire profile (as this could actually increase the wake by diverting air further out from the sides of the car than it would otherwise travel), and with the shape of the deflectors pointed at an inward angle toward the front, instead of facing directly forward.

The McLaren SLR has front aero-shaped deflectors that exhibit this design strategy. Also, the deflectors on the current Civic and new Insight (they use the same pieces) seem to bear out the outward-facing flow phenomenon as they use a dual-deflector design (see attachment) that seems to first deflect as much of that outward-facing flow as early as possible without creating a lot of high pressure build up underneath the front of the car, and then deflect the remaining air away from the tires/wheel wells. The aerodynamicists seem to be trying to divert air w/o adding drag by doing so in a two-tiered fashion, much like some of the barge board designs on F1 cars in the last several years.

Given all this, why are factory deflectors always flat, forward-facing tabs? My guess is that these are cheap, require less fine-tuning in the windtunnel, and allow air to be diverted to either side of them equally so that a car will be more stable in crosswinds or when traveling through the wake of semis. Personally, I will go with a more 'optimized' and sculpted shape for my deflectors because it's more challenging and has potentially greater benefit, and I'll live with more instability on the highway if that results.