Crepitus

What is the black material and where can I source it?
Thanks.

LiveandLetDrive

Late to the party as ever and somehow I never manage to be fashionable. Ah, well.

So you know where I'm coming from, I had a year of experience in NASCAR aero before I went to get my postgrad in Racecar Aero (yes, that's a real thing) but have since been involved in OEM vehicle thermal CFD rather than aero directly.

First the lecture: I will make some speculations from theory but (carefully collected) data trump every time and I have gathered none. (Though I really want to get a CAD model to run some CFD on, and a pressure sensor that can talk to USB to change that fact...) I learned in the NASCAR days that the ideas you are 100% positive about produce a measurable benefit about 10-25% of the time. For the underbody divide that success rate by 5-10x. It is just extra messy and weird down there. ( ) There is simply no replacement for experiment. There are very, very few universally applicable tricks, and even the safe-bet airdam has its subtleties.

So anyway, here are a few miscellaneous thoughts:

I think everyone here is right in arriving at the air dam as battle #1 to fight. It's the first thing the air sees and your first, best chance to control it. Any airdam is your friend. However I think we'd do well to notice what happened in '80... the air dam moved forward. The pocket of high pressure area under the forward nose is what we want to remove and while I believe a pacecar style air dam is definitely a net gain, it is kindof two steps forward and one step back as the downward facing section of the forward nose with nothing under it becomes even higher pressure, thus local lift.

And here come the world's worst Paint sketches using my couch as a mousepad...


First, although I write "drag" for the forward-facing surface I just wanted to point out that pressure in that area "resolves" to forces in different directions on different surfaces. The high pressure spike at the front of the airdam is returned to you many-fold in lower pressure behind.

Okay, so my point is this. #3 is significantly more efficient than #1. The high pressure in the forward "triangle" of the nose doesn't matter to lift if it is not open to the ground. An inflated balloon has high pressure everywhere inside, but it resolves to zero force. #2 gets you most of the way to #3 without the aesthetic penalty of drastically altering the nose or fitting an >'80 nose (sorry guys . #2 will be slightly higher drag as it jets the air out sideways at the ends whereas #3 can align the flow along the side of the car. #4 is best case and what you'll see on modern racecars where the high pressure at the nose is fully leveraged for downforce on the "splitter" and inlet pressure for cooling ducts of all sorts. Also why virtually all modern cars have vertical noses down the the ground.

For the same reason I think the black rubber strip as gkull posted above is not ideal as it increases the pressure ahead of it effectively decreasing the effectiveness of the splitter but 1- putting it at the front may be unattractive or physically difficult and 2- it is probably still a net gain due to being another restriction to air trying to flow under the car.



Actually... that Mercedes is doubly clever by having the cooling inlets up high where they won't relieve the high pressure on the splitter...

Stretching to a new post before I accidentally delete this one somehow.

LiveandLetDrive

So, airdams are good, we already knew that. An airdam alone is going to be good for the front lift especially and drag overall.

Now, imagine putting your hand in a flowing sheet of water, like a broken wave rolling in at the beach. As DaVinci noticed, you get this:


This is your airdam. The more you keep out, the more it wants to rush back in behind the front wheels into that low pressure zone. There are basically 3 ways to reduce this which will decrease overall lift, with some emphasis on the rear, and decrease drag.

1. Use crazy vortex generators like an F1 car to spiral down the side of the car and effectively seal the underbody. Sorry, we have to forget this one without really good CFD and years of wind tunnel time.
2. Lower the entire body. This both reduces the open area for air ingress and decreases the total volume of air under the car. Less air ingress means sustained lower pressure under the car. Volume is important as it is the energy that the car puts into a volume of air that causes drag. Unfortunately lowering the car is limited by suspension travel, necessary for mechanical grip and the realities of the street. Also even with stiff suspension street cars need some clearance to survive.
3. Side skirts are an alternative to #2. Solid or better yet soft sacrificial side skirts can give a similar benefit in preventing air ingress. Something like hard rubber conveyor belt material sandwiched between the rocker trim and frame seems like it could do the trick nicely and bend over speed bumps and touchdown on compression without unloading the wheels or causing damage.

As the pioneers of racing aero discovered, the more you rely on underbody aero the more sensitive you will be to ride height changes, bumps, etc. So that bears consideration.

And I should have mentioned from the beginning that we need to remember the order of magnitude of the changes we're making here. If we're putting 2" strakes down from our flat underbody panels and the suspension is moving through 6" of travel and the airdam is 4" off the ground, the air is just going to laugh at us as it sails by unimpeded.

And this has become the aero thread but suspension play, geometry, and alignment need to all be spot on before you even think about messing with the aero, as gkull alluded to earlier.



Only after adding side skirts would I think about a rear diffuser as the air pouring in diagonally will play hell with its effectiveness. A diffuser wants straight, attached flow to work. Unfortunately, while filling in the "parachute" is a good idea, making a diffuser that looks good is a lot easier than making one that works. Even the Ferraris with moderately clean underbodies have pretty mild diffusers because the air is so dirty by the time it gets back there that it is hard to extract any useful work from it. If you go bang into a steep incline the flow will just separate and say seeya later. F1 and prototype cars spend the whole length of the car preparing and protecting that bottom-center flow. Unfortunately with the halfshafts and cargo boxes as they are it seems unlikely we're going to be able to fit a functional diffuser. Simply boxing the area off may be the best we can do. Better yet, drop the fuel tank as low as you can (for CG) and just box in that space. Not too pretty on the street though.

LiveandLetDrive

Last (maybe?) but not least, the beloved rear deck lid spoiler/wing. For '68-77 notch-back cars, I'll give it to you straight: You're wasting your time with anything less than 6" or so up from the deck. The air is separating off the back of the "notch" like a brick and almost surely never reattaching to the deck. The usual "wings" that have nothing but an inch gap to the deck are dead weight. They're totally shrouded in turbulent air. Maybe the outer tips are seeing a bit of well-oriented air but that's the least efficient section of the wing at the best of times. For flip-up spoilers the story is similar. Without some serious prominence they're sitting in dead air.

The story is better for the '78-on fastbacks, probably. It's hard to say whether the air stays attached as it is very dependent on the exact geometry, and may vary with speed in borderline cases. The Vette fastbacks are pretty gradual though so they may work. 1-inch-off-the-deck wings are still worthless (the boundary layer of slow air is still far too thick, you need significant flow under the wing!) but a few-inches tall spoiler or a properly angled wing a few inches off the deck might be beneficial.

That wicked looking race Vette earlier had the right idea. Stick that wing up in the clean air where it belongs. Even then most of us would be hard-pressed to set the angle of attack properly so it's actually functioning as a wing and not just another "parachute." Extensive and careful tuft-testing could get you in pretty good shape. (And/or oil surface-flow visualization.) This may all be worth it though as beyond the direct benefit of downforce, wings and spoilers increase the low pressure behind the car and effectively "pump" the underbody, further lowering its pressure and so decreasing lift.

So in conclusion for the vast majority of us, slapping on a pacecar airdam is going to be the safest bet and the biggest bang for your buck. After maxing out the airdam, then side skirts and underbody cleanup. Then maybe think about a rear wing if you're willing to do it like you mean it.

I haven't even talked about vents but as SkunkWorks points out, you really need to understand not just what is probably low and probably high pressure, but really how pressures on the opposite sides of a solid surface compare to eachother. For instance with underhood venting, the engine bay is a "relatively" low pressure area (especially with aggressive airdam sealing) whereas the hood varies from extremely low pressure (farthest forward around the curve of the nose as the air accelerates) to quite high pressure (at the cowl.) Where is the crossover? I couldn't tell you. The wheel well flow is crazy-complicated. It's somewhat high pressure, but relative to the hood? Maybe? Apparently it is in some cars like the highly developed racecars that have fender vents, however those cars have sealing to the ground ahead of the wheels that is drastically different to ours and I would say there is no guarantee there. Engine bay venting to the gills? Again, possibly a wash between two somewhat low pressure zones. Especially as tuft tests in a post above seemed to indicate negligible flow.


Obviously a few of us weirdos enjoy this stuff so let's keep at it. My '70 is unfortunately dead in the water for at least a few months still but I'd be interested in any ideas anyone has for finding a pressure sensor that can talk to a computer program and record. Trying to read a fluctuating analog pressure gauge while driving 160mph seems a little dicey without a suicidal/brave passenger. My Accel DFI has at least a MAP sensor, so maybe it has spare channels you could attach another pressure sensor to? Maybe a cheap USB-connected weather station or something intended for HVAC testing? I think you will want at least two channels so you can always have one "pitot tube" pointing straight ahead at the nose, measuring total or dynamic pressure and thus an accurate speed which you can use to scale the second channel that you move from place to place on different days in different conditions.

pauldana

LiveandLetDrive...... I think I will have to reread this several times..... wow.... really really good write.

gkull

Many many years ago I entered my first Nevada Silver state open road race. I had a stockish front 79 front end. I used a lowered front end to create body rake and an extended front chin spoiler right down to the pavement.

In clean air it was okay. But during the prerace practice days I went out with some other really fast later model Vettes. It was a whole new driving experience being passed or coming up behind drafting another car. Just stay away from the wake turbulance of other cars.

I went back to the thinking room and my answer was why not buy what GM used for Daytona over 200 mph racing?

The black rubber is home depot rubber wall molding. It gets ground away and I just replace it

My front end is so long that it touches down and gets broken. So this is new fix and paint last year. As you can see through the upper vents my black air dam is the second line of defence against air under the car. The front end is kind of the pre splitter technology

69427

I like the styling of that front end.

TheSkunkWorks

A most excellent post LiveandLetDrive. Thanks very much for sharing. I've wondered about coming up with some sort of affordable automatic aero data acquisition for the very reasons you've cited.

IMHO, by the very nature of the original question posed by the OP, aero was necessarily going to be front and center. Speed is always a good topic.

69427

L&LD,

Good stuff! I'm in agreement with your thoughts there.
I've been putting some restrictions on myself regarding these modifications. I like the basic styling of the early C3s, so I haven't been making any major cosmetic changes.
With the above self-imposed limitation, anything under the bodywork is fair game, hence my suspension and aero mods. The suspension change has worked out well. Now I'm just trying to reduce some of the deficiencies of the car's underbody aero qualities.

damoroso

Me too...damn it!!

myoriginal77

gkull and everyone else, sorry I was away from the internet a few days,

The Salt Flats event I go to is run by the Utah Salt Flats Racing Assoc.

http://www.saltflats.com/

September 8 - 11,

a little more low key than the other events, they have a 130 mph course (short) and the high speed course (long) for the 150 + cars.

For the 130 you need 2 runs between 130 & 140, if you go faster you won't get a timing slip.

Was fun to run and watch, like I said, low key, I recommend it,

Frank

LiveandLetDrive

http://forums.corvetteforum.com/c3-t...aser-aero.html

FYI for anyone "subscribed" to this thread. I'm taking first steps toward doing some CFD.

vetteguy22

I am really interested in the results.

Taijutsu

I have always felt that the rear spoiler was way too low to do any real good.
Now the question is how is the best way to add a few inches to get some real downforce w/o too much drag. The eternal comprimise! LOL

I hate the rear of my 74 and will replace it w/aftermarket, suggestions?

R

TheSkunkWorks

As I've already mentioned in the new thread, don't forget there were five variations of the C3 body during the model run ('68-72, '73, '74-77, '78-79, '80-82). The PC rear spoiler was designed for '78-79 fastbacks, and almost certainly works better on them than likely on pre-78's, but the '80 & up rear bumper w/integral spoiler was made even smaller than the PC one. That said, factory aero kit for racing was available back in the day, prior to '78.

While not entirely scientific, I've observed that the back end of many cars with properly functioning rear spoilers tend to get dirty rather quickly after only a few miles of spirited driving, as does my '78 which most certainly works.

In any event, when contemplating a larger spoiler than the PC I'd be cautious of going past the point of diminishing returns on spoiler size, or you could be adding drag without benefit of further rear lift/downforce improvement.

Taijutsu

Is it only me or does it seem like the factories avoid letting a spoiler extend much behind the car?
Thinking of the roll-over point of diminisding returns and increased drag,
what if the spoiler was higher with less angle of attack?
ie: rather than a 60* spoiler, 6" tall,
use a 45* spoiler, 10" tall?

Reminds me of a hobby stock car raced locally.

R

eagle275

I have been thinking about adding this to my front.....



Corvette Air Dam, Front, 1968-1972



http://www.ecklerscorvette.com/corve...1968-1972.html




Can cool the brakes or add cold intake to a system like the Spectre?

Spectre Performance 728 - Spectre Performance Musclecar Cold Air Intakes



http://www.summitracing.com/parts/SPE-728/

TheSkunkWorks

That's a classic front dam design born of racing back in the day that's definitely worth a look for earlier C3's, bearing in mind that even a dam configuration such as in fig. 1 is far, far, far better than none at all.

FWIW, I'd use those ports for brake cooling ducts, as originally intended, and stick with the L88 CAI setup.



Oh, and thanks for not letting this fairly meaningful thread (relative to most) not age out only get locked down. Seriously, mod guys, some topics deserve a better fate than that. Sticky?

eagle275

Skunkworks - do you mean this one? Thanks!


http://www.thel88corvette.com/inquiry.htm

TheSkunkWorks

Yep, that's what I'm doing, with matching air box in the hood.