motormechanic

iTrader: (1)

downforce would increase since you are tapping the low presure region even moreso with a larger splitter. It would also probably shift the aero bias to the front

bbundy

Put on a bigger rear wing to balance, add more power to counteract the drag, don’t slow down for corners.

Double O 86

Have a look at these two pages, and keep in mind that this is a winning car.

Help me understand why it appears to be bottoming out, but is still successful.

emilio700

iTrader: (3)

The curse of the internet. One guy did something that is counter to common knowledge of the subject. The question then comes "how come this is true?".

You are assuming the following:

Blue C5Z splitter config is optimized
He won because splitter was optimized
He won despite the splitter not being optimized

I don't know the answers and anyone here, even the experts, would be guessing too. My guess it still works just fine even though there is no air flow under half the splitter at max cornering G.

No offense intended, I just got a chuckle out of your response. I fall victim to the same thought process all the time. I force myself to assume nothing, dissect and deconstruct before coming to a conclusion. Keep in mind, while NASA nationals has some outstanding drivers and machines, it ain't F1. One can win there running far lass than optimized equipment.

I was down almost 20whp peak and ran my splitter about 2" too high and won in 2012. We ran the splitter high to make it more crash worthy and allow me to cut corners over the kerbs. My reasoning was that I'd have to pass in the technical sections because I was down on power. That meant potentially passing offline and using the grass, thus the really high splitter. In the end, I didn't need that ground clearance and could have used the extra downforce. This year we'll run it down at 3" static, 1" fully compressed.

plucas

The great thing about science is it is true whether you believe it or not. When there is no airflow under the car (splitter hitting the ground), the pressure gradient drops to 0. No pressure gradient means no downforce.

emilio700

iTrader: (3)

Sort of. The pressure differential from top to bottom surface is reduced but there is still positive pressure on top because of the general high pressure zone on the nose of the car. Running the splitter on the ground renders its effect null but it is still a functional air dam, which still increases -lift over a stock nose.

So yes, no negative pressure creating negative lift but the air dam still works. You have to be careful here as some readers may take your (precise and accurate) comment out of context and assume that the remaining nose config with air dam provides no downforce

jmann

As shown by Nascar which runs their splitter on the ground.

triple88a

iTrader: (2)

So you're telling me that a splitter increases downforce AND lowers drag? Definitely installing one in the spring.

ZX-Tex

iTrader: (15)

Look at DTM (German Touring Cars) which have highly developed aerodynamics and are damn fast, hard core pro level cars. They also run their splitters at very low ground clearance, and they appear to be nearly touching the ground in the corners, much less than 30mm anyway (gotta use metric, it is a Euro series after all).

emilio700

iTrader: (3)

Splitters in and of themselves do not reduce drag when compared to a bluff nose. Around our shop, we call the ABS sheet flat style airdams an "EP Nose", as popularized by a decade or so of E and F Production SCCA racers running them.

plucas's screen caps and data infer a drag reduction with splitter (#6) over a comparable nose without splitter (#5) but that's counter to current understanding of the application. I don't have an answer for that or care to speculate. Maybe he can shed some light on the possibly anomalous data. CFD is complex stuff and it's easy to get errors. He mentions the underside was simplified, maybe the tires weren't spinning, etc

It is likely possible to lower net drag (whole car) over a full OEM nose compared to a #6 config, but deleting the splitter altogether will generally have lower drag than an EP dam/splitter.

nitrodann

However the dam and splitter on a Miata specific application has a lower CD than stock.

But the car now has higher frontal area, so I wonder what the net result is.

Dann

ZX-Tex

iTrader: (15)

Good point on the area... But here is what I am thinking...

Except for velocity, all of the variables in the fluid dynamics drag force equation are linear:
F = 0.5 * p * v^2 * Cd * A (p = density of air, v = velocity, Cd = drag coefficient, A = projected area)
According to the CFD analysis from the previous page, Case 6 has about a 24% lower Cd than Case 1. Therefore, the projected area in Case 6 would have to increase by 24% compared to Case 1 to negate the reduction in drag coefficient. With Case 2, the margin is even bigger.

Though not exact, when you consider the projected area profile viewed from the front of the car, and that the tires come into play, that the total increase in area (even with fatter tires, since they are mostly covered by the air dam) is not that much, and that worst-case it is probably a wash, and total drag force is worst-case about the same. So you get lots of down force without a drag penalty.

Based on what I understand from what Emilio is reporting, this is consistent with his testing. So theory and practice indicate it's a free lunch. Plus it looks bitchin, it is relatively cheap and easy to do, and makes it a hell of a lot easier to get an oil cooler and a big radiator stuffed into a V8 swap (in my case), not to mention easier nose ducting.

FWIW, I refer to this mod as the 'Mad Max' air dam design. It somehow reminded me of 'The Gimp' from Pulp Fiction with his zipper mask but Mad Max is far cooler. I've done it on one car, and am doing it again on my current build.

plucas

Nascar runs just an air dam and it is on the ground to keep air from under the car. Nascars do not have a flat underbody/undertray. Keeping air out from under the car decreases the drag.

If it is on a normal street car and you take it back to the front axle centerline, then yes. Covering the area from the front of the car to the front axle centerline will decrease the drag around 0.015.



Running them very low is good. When it hits the ground, massive amounts of downforce is lost.

Looking back at the models, the difference between them is the splitter smoothed into the underbody like a diffuser while the damn just ended. That would cause the higher drag seen by the dam. The boundary conditions were set to have the wheels rotating.

plucas

Yes this is true but the higher pressure at the top will be the same (if not considering temperature changes like track temp) no matter how high the splitter is. If you are 10 inches off the ground or 2 inches off the ground, the top will still have the same high pressure. The underside of the splitter is where the distance from the ground makes the difference in downforce. I know you know this, just clarifying my previous statement I guess

mr_hyde

iTrader: (4)

In the splitter application, how much or an air gap is optimal for maintaining downforce? We know a splitter hitting the ground eliminates the pressure gradient but that would only happen under heavy braking where downforce on the front wheels isn't scare. Is an inch under max braking enough to keep the air moving? 2"? More?

The other scenario a splitter may drag the ground is under heavy cornering where the outside corner rolls down. How does the wedge shaped profile of the front effect how the air is moving under the car and is it enough of a concern to drop heavier springs under the car or do anything stupid with the sway bar to counter roll? Obviously, there is a balance (no pun intended) to be found.

Fascinating discussion - thanks for the contributions!

Leafy

iTrader: (1)

It is kind of a new development, I think the COT came out like 4 years ago. I wont fault you for not paying enough attention to nascar to notice. I really only watch it if they're on a road course.

mr_hyde

iTrader: (4)

Wasn't COT scrapped because they liked to get airborn when the tail beat the nose through a corner? All the cool kids are tinkering with their decommissioned wings in the other thread.

I agree real racing involves turning left and right as well as braking, but there are many more aero similarities to what we are doing vs. F1.

Leafy

iTrader: (1)

They scrapped the cool wing because it made the cars too easy to drive and made the races a bit boring. They used the air born excuse when getting rid of it because they don't want to openly claim that it was really about making more money. And from the few races I caught it seemed like the COTspolier catches air just as much as the COTwing. Until they like opened up the flaps on the roof more or something.

plucas

See that is a question with no right answer since it just depends on the cars whole setup. Setups are all about tradeoffs. Add more downforce, but then you might have to increase spring rate (might hurt mechanical grip). The best way I know of to do this is through vehicle simulations like ChassisSim (which is awesome) or lots of track time changing setups. Neither is cheap but track time is more fun for the driving enthusiast.

Yeah I do not watch Nascar because I have very little interest in cookie cutter cars

Der_Idiot

iTrader: (2)

I was thinking of doing a diffuser and underbody work, but adding a wing might be a little too over the top for a daily driver. Hmm..

Edit: As an aside, where is arguably the best location to place inlets for ducts that cool the brakes? Most use the sides, but is that optimal?