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Diffusers and belly pans


rustrocket

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Air will always move from a high pressure area to a low pressure area. Low pressure areas are created where air is accelerated (relative to the surrounding air) or heated in an unconfined space. That movement from a high pressure to a low pressure area creates a force on a surface that impedes flow from high to low.

 

Engine compartments are designed to draw air in through the front grill opening and exhaust that air down, under the vehicle. If the engine bay became a significant high pressure area then air couldn't come in through the grill opening, it would all stack up in front of the car (which it does to some extent anyway). Adding an undertray without allowing for an air exit from the engine compartment will create all kinds of problems.

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The advantages of the belly pan are a reduced positive pressure increase under the car. This can be about the best that can be expected in a street car, especially a front engine, rear drive set-up. Any small amount of pressure increase, over a large area, will creat a large effect. Obviously in full racing applications, the pan is capable of a negative pressure, creating downforce.

 

Stability and downforce, or, reduced drag? They are oppisite of each other

In the sense of wings and ground effects, this is true. Increasing the downforce, and stability (by creating a higher pressure on top of the car) results in higher drag, which is what you are saying (can't have one without the other). Reducing the high pressure results in decreased drag, which is what you are doing under the car with the pan (flat pan, no diffuser, etc).

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  • 1 month later...

Its been a long time coming, but as usual, things never work out the way you thought they would. It is alway harder. For me anyways.

 

But, its finished.

 

My rear diffuser is finished. I have decided not install the side fences...yet I think they may give me streetability issues with such a low car.

 

The rest should work, my rear angle is set between 7-9 degrees, as per Johns C's info.

 

It goes all the way from the back of my diff to the back of the rear valence. I have used mesh at the back to allow trapped air out as well. This was not originally part of the plan..

 

It is one piece, and can be unbolted and removed from the car to work on the gas tank etc. It was pretty tough to design it to be removable. It would have been much easier to just make it and put it on.

 

I have pictures of the pan, I will post them hopefully this afternoon.

 

I'm pretty busy today cleaning the car for the Zfest car show tommorow morning.

 

Scott

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Great! I would like to see the pictures. The closer you can run the diffuser to the ground, the more effective it will be. Best case you can figure about 150 lbs. of downforce if you do a good job controlling air spilling under the car from the sides.

 

If a real, working wing is mounted at the back of the car you might see an increase of about 25 lbs of downforce thru the diffuser. The wing needs to be positioned as close to the diffuser exit as possible, but that might make the wing less effective and negate the increased efficiency of the diffuser.

 

Ultimately, the rear of your car needs to be shaped exactly like the rear of the Audi R8s... :D

 

http://www.mulsannescorner.com/audir8-01-6.html

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Ultimately, the rear of your car needs to be shaped exactly like the rear of the Audi R8s...

 

I'll have my body man get right on that, and i'll try to have him shape the rest of my datsun like the rest of the R8 too. :D

 

Forgive my ignorance...but wouldn't downforce the back of the car make the front of the car lighter at high speeds?

 

Scott - i'm anxious to see the pictures.

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Forgive my ignorance...but wouldn't downforce the back of the car force t highthe front of the car to be lighter at high speeds?

 

Yes, if the downforce generator is behind the rear wheels. An example is the rear spoiler I built for my car and ran in the last OTC. It generated more downforce then the BRE rear spoiler I was runnign for years before. Without making any changes to the front I ended up with aero understeer in corners at speeds over 90 mph. The car was very well balanced at speeds under 90 mph so mechanical grip was well sorted.

 

Overall lap times were the same so the aero understeer offset the increased grip at the rear and the net effect was a more difficult car to drive at speed. If only my splitter idea had worked...

 

btw410paddock1.jpg

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Here's my two cents. If you made a front belly pan that stop right before the tranny tranny tunnel and then out a up sloping section to huide air in the the tranny (thus creating the hourglass shape, increasing airspeeds) then directed the air to the rear diffuser using angled fences leading to the vertical slatts on the rear diffuser, I think air flow would be better. Also have angled fences up front to direct air to the tranny tunnel. In my design the outer inlets of the front facia would direct air to the angled fences to the tranny tunnel and my rear fascia along with an actual bellypan/diffuser would finish the hourglass aerodynamics. AS for the vortex on the sides, I dunno, that's beyond my knowledge. What do you thing of the rest of the idea though? Ideally th eunderside would act like wing right in the middle of the car (tranny tunnel) Also I thought about little inlets (cowl style) on the front fascia design to allow high pressure cool air (less debris) and have it directed to the front brakes.

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hourglass aerodynamics.

 

If you mean an hourglass shape under the car, that kind of shape is less effective then a flat bottom with front and rear diffusers. The old ground effects designs induced huge amounts of drag. The new flat bottom designs generate equal or more downforce with almost no drag.

 

Lots and lots of great info and pictures here: http://www.mulsannescorner.com/

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Do I understand your "hourglass" design to mean increasing the area, or depth, in the mid section of the car?. This is similar to what many jet engine designs are based on in that increasing the area (in this case, the clearance from the car to the ground) slows down the air speed (anti-ventury effect), which increases the air pressure. It would seem this would increase the pressure under the car, which is a no-no.

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I too always thought that making the bottom of a car as flat as possible was to your benifit. I too don't quite understand the hourglass deal. The Enzo for instancehas a complicated undertray with motorized diffusers. I think a flat surface with rib guides with the 7* departure rear diffuser would be perfect. Maximum surface area, minimal drag. Of course to properlt achieve this I think you might want a custom chassis, but it could be done with the stock one I think. To my understanding flattening the bottom allows you to make fine aero adjustment by directing the air going over the car versus under. Just simplifies things.

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The angle of the diffuser is somewhere between 7-9.. I'm sure it has flexed slightly as it was bolted tightly to the car.

 

The diffuser actually is higher closer to the diff..it angles slightly down, then levels off, then starts to rise as it exits. I did this to simplify the installation at the diff mount, and to reduce the amount of air that may get over top of the diffuser from the tranny tunnel.

 

If you look at the previous page of this thread, I have a pic of the front belly pan (red Z before it was painted black)

 

The hard part was designing it to be removeable from the car, and to function properly. every part was prefabbed from card board, and assembled on the car...duct tape was very helpful. Once I had measurements, it was fairly quick to hand cut each piece. We used a vice with angle iron to make a metal brake to making clean bends.

 

Including all the redesigns, and hashing out ideas, and problem solving mounting and asthetic issues that arrised at each stage of building, I figure it took myself and a friend 20+ hours to do over a 3 week period.

Materials were approx 150.00, for SS bolt hardware, aluminum sheet, flat bar, and angle braketing.

 

As for a kit?.. way to hard.. too much custom stuff..too much rework would be needed, as different body kits, exhausts, gas tank styles, etc would make it too hard to do.. some of the MSA rear skirts would make it very easy to a pan if you wanted... finding something to attach it to is the hard part...especially it you want it to be removeable.

 

Rims. Simmons 17x8 and 17x11

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