Splitter design function and effects. Experience, Design knowledge?
 

I have been playing around with a prototype splitter for a couple of race seasons now, after putting on a "real" wing (GT3 cup) that I got out of a garbage can (to match my WCGT toyo tires). With the wing, at a moderate inclination, i got almost undriveable push. by the time i "0"ed it out it was back to prior handling, BUT, it was only making the same downforce that the stock kick-tail wing was making at 10degrees inclination. (about 15 degrees angle of attack based on the downward angle of air off the roofline)

Now, i added an old cup car CF splitter up front, and had to add to it with aluminum sheet to make it fit the wider 928 nose and come out some to be effective. I also cut hood vents so that the air from the radiator, could travel up out the hood to the lower pressure zone out on top of the hood, rather than get forced under the car to add to the lift component of air running under the car.

Ive also done some extensive testing with a Sunx sensor (a very accurate pressure sensor that measures pressure and vacuum to 1 bar, or 15psi).
I was able to confirm that the splitter is doing its job and has helped handling a bit. After the last weekend and watching what the faster guys are doing with splitters, I asked one that seemed to have his grow from the last weekend, how it helped. He replied, "night and day. We had to move the wing rear ward and upward to match the effects". well, that was enough for me. I added 1" to mine and did some test drives on the uncrowded freeway to 120mph. the splitter self destructed. so I bent it back, fixed it and added 4 cable supports. Tested to 130mph and video verification, and it worked fine.

Now, the question is, optimization.

Lets hear from the list, what the basic theory is of a splitter and what its effects are as well as trade offs.

mk

When the air flow hits the front of the car, there is a point on the nose where theoretically every molecule of air above that point goes up over the car, and every molecule of air below it goes down and underneath the car. All that a splitter is doing is lowering that point on the nose so that more air is forced up over the car and thus creates more down force on the nose of the car. The splitter is not creating downforce on itself per se, to the degree a rear wing does, but rather affecting the aerodynamics of the entire nose of the car.

Trade-off? I guess the same as a rear wing, more drag the more you ask it to do?

Edit - I guess also too much front downforce leading to oversteer...

That now marks the limit of my ability to provide useful information.

I'm not sure that's really the way a splitter "works". Splitters do act as a wing, by creating a low pressure area underneath and a high pressure area on top. Some good basic info here:
http://www.e30m3project.com/e30m3per...r/splitter.htm

Your splitter would probably be a lot more effective if you plugged up all of those big slots adjacent to it on the bumper (assuming you don't need them all for cooling). A splitter works by creating a high pressure zone that acts down on the lip, right now a lot of your pressure will be escaping through the slots instead of acting on the splitter...

I was just confirming what Ive read and experiemented with. I agree with you . I think you got it. However, there is an amount of air flow traveling under the car that gets squished into a venturi situation where the pressure under the car actually will go down if the undertray is ducted properly. If that same air goes over the car, under Beroulli's principles, the pressure mght also go down, but if their is moer air flow, the downforce over the nose might go up. also, those air molecules add to the downforce that the rear wing will make too. Like you say, the main function is to route the air that builds at the nose of the car, to over the car, or splill off to the sides of the car where the pressure is less. (usually ambient or lower). Then we get into ground effect, because if intuitively we would just jump to a shape of an upside down wing and put wheels on it. (like an oscar wiener dog car). So, obviously, there is an advantage of having a datonal prototype shape, but having it skim along the ground so very little air goes under the car. all mostly ambient air flow pressure, and then ALL the air travels over the top or to the sides for maximum downforce, matched with rear wing settings. also, notice the big air inlets at the nose. those take in high pressure air and then vent them just behind or at the front wheel area , to vent to a very low pressure zone on the front of the body.

anyone care to guess what kind of pressures I measured at the nose, below the nose, at the splitter 1"from the edge and about 1" high, and then under the car just under the nose? Or,how about the hood, near the hood vent?

Basic calculations show flat plate pressure is around .11 at 80mph and .44psi at 160mph.

At the front of the car there is the stagnation point. This is the point where the air velocity goes to 0 and the pressure becomes highest (stagnation pressure). This point is located somewhere on the front bumper/airdam. Remember that pressure acts in all directions. By placing a horizontal sheet underneath the stagnation point, the high pressure that is in this area acts on top of the splitter. This creates downforce. You can approximate stagnation pressure by 0.5*density*velocity^2. Take this stagnation pressure and multiply it by the plan view area of your splitter to get a ballpark of the force on the top of the splitter (this assumes atmospheric pressure underneath the splitter - see below).

Additionally, the splitter helps to create low pressure underneath it by forcing the airflow into a narrow gap between the road and the car. By forcing the air through the gap the speed increases. Net effect is you have high pressure above the splitter and low pressure below. However, the pressure below is very dependent on what the bottom side of the car looks like downstream of the splitter...

Basically the splitter is just using the high pressure that already exists at the stagnation point in front of the car and turning it into a downward force. To take full advantage of, and create low pressure underneath requires much more careful consideration.

Either way - putting on a splitter is a very easy way to pick up appreciable front downforce. The bigger the planview area the more downforce. It does not have a significant effect on drag until you start making it ridiculously long. A rule of thumb for a "big" splitter is they usually extend 3-6" beyond the front bumper looking from the top.

Since it appears he has added additional cooling openings in the front bumper (see the mesh insert in the posted picture compared to his avatar, I suspect that's not an option.

There was a Race Car Engineering article a few years ago that showed pretty substantial increases by simply closing up unneeded openings in the front bumper and using ducting to make sure the air that does enter your bumper is channeled to where it's needed.

Well, thats the debate. pressure tests seem to contradict that line of thinking. Intuitively, it makes sense, but the splitter, as the first poster points out, doesnt actually push down on the car as its main function, but routes the air flow through the radiator and to the sides of the car, so that air (those molecules)dont go under the car. the vent, right above the splitter is actually a benefit. It actually routes air to a lower pressure zone than to the sides, that of the air going over the car, causing lift. if the air flow goes in to the vents and out the hood as I have it set up to do, then you add to the downforce on the nose.

Ive already done sometesting with the pressure guage at 130mph on the nose of of the car and under the splitter.

See any simularities with the pro builds with regards to the splitter and vents?

Thats basically how it seems to work, he is right. (so far I believe)

Thats why I opened the discussion.

I thnk there is even a better link to a paper I saw a few years ago.

Ill see if I can drum it up.

Mk

Something tells me the ducting on those cars is a little more elaborate than yours :p. So are those slots coompletely sealed off so all of the air goes up through and out thru the hood? I assumed the slots were open and much of the air went under the car. Would be easy to tape them off and test it out...

yes, the highest pressure on the car will be right at the nose. try and guess what I measured as far as pressure in the nose area,

yes, the pressure builds in all directions and moves to differnential pressure. If there is a nose vent, it will vent to the engine bay, and then under the car if that is its next lower pressure. However, if you have hood vents and ducts, the pressure OVER the car as bernoulli shows, is the lowest pressure in the flow. mid hood and the roof. if you have a hood vent at the mid hood area, as you can see from the pics of the daytona or GT race cars, that air goes out the hood and not under the car, or even to the sides of the car. (ideal) the pressure on the splitter is a side benefit, as the air it now routes over or to the sides creates the most change in pressure over the car. the air that goes under the car from the nose is mayb 3-4 ", the air that goes over the car goes 1 or 2 feet. that both maybe below ambient, but the air over the car is FAR under ambient. I have the data to support it too.

Now, ground effects are also something to look at. I think the entire concept there is to channel the air so that it speeds up under the car, with tunnels and then opens out into the defuser where the pressure suddlely raises behind the car for lower drag and greater downforce in the rear. facinating concepts for sure. Im still not confortable with the ground effects and flow under the car. I believe that without the duct work or flat pannels, you end up with a lot of drag and some stagnation points to raise pressure slightly, but not by much. mostly drag I would imagine.

The proof in downforce that the splitter actually contributes is in the think aluminum I used. its 1/16" aluminum sheet with rivets. Maybe there is 100lbs of downforce due to the differential pressure. (on top vs under the splitter)

How far back does your splitter panel extend under the front of the car?

Much less efficient for sure! :) BUT, i have the air flow values and position for my chassis, and that vent is a lower pressure area. Ive been able to put tufts on the vent and they stand up at 100mph at a 45 degree angle, and lay flat as pankake with the vent taped off. But I think that the flow is crude ,but effective. obviousy, how great would it be to have full duct work venting out the hood instead of the air trying to find its way out by itself based only on pressure differentials. :)

stilll wide open. I need the cooling on the oil pan. (no oil cooler) ;)

so, i figure there is a lot of drag gong on down there BUT, ive measured the pressure down there under the car too. everytime I take it out, i go 120mph and see what another point on the chassis tells me.

Here is that website I was talking about:

http://images.google.com/imgres?imgu...%3D20%26um%3D1