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I keep thinking about this pitch sensitivity on splitters and I've come up with an idea, and I guess I'm looking to see if anyone has done this before or knows in advance why it wouldn't work.

 

Here it is: As an example, imagine a 6" deep splitter mounted fairly high off the ground, maybe 4" or 5" high, with an air dam skirt on the leading edge that hangs down basically on the ground. I was looking at Competition Car Aerodynamics and it's clear that with both an air dam and a splitter you have a low pressure on the underside/backside. In theory the splitter have less pitch sensitivity and still have a nice pressure differential between the top and bottom of the splitter. As a side bonus it would be a lot easier to load a car on a trailer with 4" of ground clearance in front.

 

I know the pressure differential doesn't look as high for the air dam in CCA's diagram on p 65 so the splitter may be less effective, but I still think this might work reasonably well and solve the pitch sensitivity issue and might be an improvement over a traditional splitter on a relatively low speed car that is relatively softly sprung. I see guys putting splitters on cars with relatively soft suspensions, and I can't help but thinking that the splitter is probably doing as much harm as good as it bounces up and down at the front of the car.

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I have experience with two Z cars that ran splitters. Both were quicker with them than without and neither had problems with porpoising. The level of downforce that we generate is a lot less compared to a sports racer that has a flat bottom and front and rear diffuser sections.

 

I think your idea will work but I'm not sure I'd spend too much effort on it based on my past experience. In both XP and EM you can't run a diffuser section on the splitter and this is what seems to increase the pressure gradient and cause the bouncing.

 

I would recommend that you make the splitter so that it is hinged and can move up when it comes into contact with the ground. That will save a lot of wear and if you do it right you can even leave it on while loading on a trailer.

 

Cary

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So I guess if you hinge it you need to run cables instead of rod ends at the front. I know about Dave's car. What was the other Z that you ran with a splitter?

 

You can run cables or you can build a frame that pivots at the back and everything bolts to this. You can even use that idea to independently set ride height and splitter height. I have a really cool video somewhere of the tripple 8 Ford falcon's setup (aussie V8) and how it worked.

 

The other car was Morgan Smith's. He also road raced his car and commented how much more planted the nose was in higher speed corners. It was solidly mounted and would occasionally rub the ground fairly hard.

 

Cary

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I'm outside anything I have first hand experience of but have derived Bernoulli equation back in school .... so that make me an expert right ;) Anyway, I think it depends on how you are trying to use it. It seems there are two potential phenomenon. First is creating a stagnation point right above the splitter while preventing air from going underneath the car. This creates a high pressure area above the splitter that effectively pushes the splitter down thus creating downforce. Your idea would accomplish this. I'm guessing you would want the splitter to be flat or even slightly down for this. The more advanced but difficult to implement function would be to use the splitter to increase the speed of the air going under the car. As the air speeds increases the pressure goes down (Bernoulli) effectively 'sucking' the car down. This is where flat undertrays and diffusers are required to make this more effective. This also makes the angle of attack critical to get the air to accelerate like you want it to. If the angle is too steep it may actually cause lift due to mechanical forces of trying to push extra air down. Then again I could just be all wet.

 

Cameron

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I'm outside anything I have first hand experience of but have derived Bernoulli equation back in school .... so that make me an expert right ;) Anyway, I think it depends on how you are trying to use it. It seems there are two potential phenomenon. First is creating a stagnation point right above the splitter while preventing air from going underneath the car. This creates a high pressure area above the splitter that effectively pushes the splitter down thus creating downforce. Your idea would accomplish this. I'm guessing you would want the splitter to be flat or even slightly down for this. The more advanced but difficult to implement function would be to use the splitter to increase the speed of the air going under the car. As the air speeds increases the pressure goes down (Bernoulli) effectively 'sucking' the car down. This is where flat undertrays and diffusers are required to make this more effective. This also makes the angle of attack critical to get the air to accelerate like you want it to. If the angle is too steep it may actually cause lift due to mechanical forces of trying to push extra air down. Then again I could just be all wet.

I think you basically have it right. This would give up on the idea of the speeding up of the air underneath, but you should still get a pretty good low pressure area under the splitter due to the air dam, and you still have the high pressure on top, hence a pressure differential to work with which would be greater than just the force on top pushing down. The difference I guess is that the air that doesn't go under the splitter has to go around the car, which is why I would think it would be less effective than a perfect splitter setup.

 

I'd love to see that video, Cary.

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I thought the primary purpose of a splitter was to efficiently separate the air that flows under the front of the car from the rest of the air that flows around and through the radiator. If you add an air dam below the splitter, what is the benefit of the splitter?

 

I can imagine that the pressure above the splitter might be a little greater than below it, but I'm wondering if that would amount to a significant amount of downforce. Seems like there wouldn't be much difference if you ran the same front end but without the splitter part.

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I'm outside anything I have first hand experience of but have derived Bernoulli equation back in school .... so that make me an expert right ;) Anyway, I think it depends on how you are trying to use it. It seems there are two potential phenomenon. First is creating a stagnation point right above the splitter while preventing air from going underneath the car. This creates a high pressure area above the splitter that effectively pushes the splitter down thus creating downforce. Your idea would accomplish this. I'm guessing you would want the splitter to be flat or even slightly down for this.......................................................................................................

 

Cameron

 

Thats the way mine is set up, its a road car so there are limits as to what can be done. All I can say is that by 150KPH I have no understeer even with a rear wing. Much under that and there can be understeer, perhaps the wing starts to work before the front does.

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This may be of interest, I recently made a new front splitter and under tray with a diffuser for my track 240Z.

On the first outing it worked so well that it pulled the bottom half of my G nose down and started scraping on the ground!!

I only realised this after my engine blew up and I got out of the car after stopping on the track!!

Some pictures of the spoiler and also of the hole in the engine block, the engine is a RB26.

 

cheers,

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post-10772-12679975209566_thumb.jpg

post-10772-12679975580396_thumb.jpg

post-10772-12679975763692_thumb.jpg

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Have had my gnose up to 130mph with no effect (ie., nothing fell off) after I installed this Roostmonkey made splitter but I realize that it really needs to be several inches lower to work effectively. Oil spots are from my old Ford Exploder (can you say "clunker").

 

Poor road conditions and the fact that I trailer it on occasion limit how low I would place it. I'm thinking of a lower FG skirt that would be removable but that's just in the planning stages at this point.

post-311-12680203419067_thumb.jpg

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Good to see some more posts on this topic, my car has developed a high speed understeer problem so I've been looking for some aero answers. Perhaps some dive plates on the front but they do not look like they would do much. This linked site has some basic aero info http://www.superhachi.com/theory/downforce/

 

For those who trailer their race cars and are worried about low front clearance, I have seen removeable splitters/air dams used. They can eg slide on to a couple of solid pin mounts and be retained with a few Dzus fasteners.

 

23zed, whats the theory behind your recessed undertray design? I'm thinking of doing something like that but with the recess immediately behind the splitter. Am not sure which way the floor should slope though. I was thinking from high at the front to low at the back but yours is the other way around.

Edited by 260DET

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23zed, the sketch below, taken from the link I posted, appears to recommend a high front, low rear floor. It may be just the way he has done the sketch but I think he is illustrating a narrowing volume for the air as it moves rearwards. Which would speed up the air and lower its pressure.

 

If that is right yours would still work but the effect would be limited to the area where the air comes under the splitter in a high speed/low pressure state.

post-185-12680286300347_thumb.jpg

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260DET, in the drawing you refer to he is only talking about the function of the splitter.

Take a look at the final drawing, in it he shows a diffuser, notice that it opens up as it goes back.

Do a search for a picture of a V8 Supercar front splitter/diffuser and you will see what I mean.

My splitter/diffuser is a copy of a early version VR Commodore.

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There's some terminology confusion here. The term splitter generally refers to the horizontal lip extending forward from the bodywork/air dam/spoiler that divides the airflow between the upper body and lower undertray. From there you have three main underbody sections:

 

1. The front undertray which can include a diffuser(s).

2. The mid undertray which is generally flat.

3. The rear undertray which can include a diffuser(s).

 

In general racing practice the front undertray has two diffusers which exhaust to the front wheel wells - which are designed as low pressure areas. The rear undertray generally has one diffuser exhausting out the back of the car with the rear wing positioned to act as a pump on the rear diffuser increasing its efficient by a significant amount.

 

Splitter angle (up, parallel, or down) depends on the front undertray design. If there is no front undertray then the splitter needs to be parallel or pointing down a couple degrees. If there is front undertray then the front undertray design and intended function will determine the splitter angle.

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Front diffusers aren't legal in many SCCA classes, including the one my car will be in, X/Prepared. The increase in drag from the front diffuser shown above may be worth the extra downforce it helps to generate even if there isn't a full undertray, I suppose you would have to measure that on a case by case basis. That splitter is pretty high off the ground. Wondering if it might be working under braking when the suspension compresses to get it closer. Not sure on that, but it would be interesting to see if it worked in the wind tunnel at the ride height shown. I would suspect that it doesn't.

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260DET, in the drawing you refer to he is only talking about the function of the splitter.

Take a look at the final drawing, in it he shows a diffuser, notice that it opens up as it goes back.

Do a search for a picture of a V8 Supercar front splitter/diffuser and you will see what I mean.

My splitter/diffuser is a copy of a early version VR Commodore.

 

I understand how a diffuser works, the final drawing concerns the rear of a car where air flows out the back. If it couldn't the diffuser would not work. With a diffuser under the front of and in the middle of the car the air has no clear exit path like it does at the back, which is why I'm wondering how your undertray section works, the splitter part looks fine.

 

Can't find any VR SC info on the www, did you personally see one and copy it to make yours? I'm just trying to square aero principles with your design to satisfy myself that it should do whats required if I use the concept to make one for my Z.

Edited by 260DET

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.......................................................................

 

In general racing practice the front undertray has two diffusers which exhaust to the front wheel wells - which are designed as low pressure areas...........

 

Ah that info fills in a blank, have seen them and wondered what they were for, they often incorporate strakes.

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Having got my lent copy of Competition Car Aerodynamics back, finally, there is a mention of using a diffuser design in the front undertray, will have to study that more to see if its a practical design for the Z.

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No interest? To complete the subject for anyone searching later, I'm going to look at a double diffuser design venting into the wheel well areas. The single central design has a venting problem due to it being in front of the engine, there is no low pressure area there like there is in a wheel well.

 

The pic below illustrates what I have in mind but note I don't want to vent in front of the wheels, just between them and the engine. There are high pressure areas in front of wheels, I'll be looking at some sort of curved air deflector in front of them.

 

34h9rnl.jpg

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No interest? To complete the subject for anyone searching later, I'm going to look at a double diffuser design venting into the wheel well areas. The single central design has a venting problem due to it being in front of the engine, there is no low pressure area there like there is in a wheel well.

Unless you have a flat undertray and a diffuser in the back of the car. It is interesting and I was just using that same CFD image to prove a point elswhere about a week ago, fig 4-14 if memory serves.

 

The pic below illustrates what I have in mind but note I don't want to vent in front of the wheels, just between them and the engine. There are high pressure areas in front of wheels, I'll be looking at some sort of curved air deflector in front of them.

The diffuser would be illegal for most SCCA classes, but certainly worth doing if you don't have those constraints. The high pressure in front of the wheels isn't so bad on a sedan, and really isn't bad on a lowered race car. On an open wheel car I seem to remember they're responsible for a majority of the drag. If you just get the air directed at the wells the low pressure will do the rest. Unless you have your diffuser quite a bit lower than the frame rails, I don't think you'll have too much room to raise it up where it won't be in front of the wheel.

Edited by JMortensen

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If the wheels are in front of the diffuser exit I was thinking that when turning they would interfere with the air flow a lot. A deflector should help isolate any variation in air flow from that, as well as do something to reduce wheel induced drag.

 

The thing I like about having a diffuser instead of a venturi is that it should work fairly constantly during variations in ride height and roll. A venturi on the other hand relies on the presence of the ground to form its shape so any variations there would vary its effect quite significantly.

 

My Z presently has a front venturi setup, some times for no obvious reason the car will understeer at high speed in corners, other times it is rock solid.

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