Will you be getting a picture showing how far forward the panel reaches. I'm curious to see how effective it might be at stopping air from getting on top of it.

 

There is a picture posted of my car and how far they go. You can see where it ends where the two big mounting bolts are for the rear diff support. The sides of this have 45 degree angles that turn up to help catch the air by the axles/control arms.

 

Here are the pictures of the diffuser on my RS. The distance they go to the front are exactly the same.

1st picture. This is the diffuser as seen from the back of the car. You can see it is just flat and straight. Looks like a "line" in the picture. I put the diffuser under the cover so any air that does make it over the diffuser can get out.



2nd picture. This is to show the break (bend) I put in the diffuser in the upper area near the rear sway bar. This break is there to keep the air from the control arm area from getting above the tray.



3rd picture. Side view of the completed/installed diffuser. The diffuser is installed with 3/16" aluminum rivets. You will need to buy a rivet gun capable of installing a 3/16" rivet to do the installation. You will also notice the DEI heat shielding that is stuck onto the undertray to keep the exhaust from burning it.



4th picture. This is just a shot of how the diffuser uses the rear two bolts to hold it up and secure it, also to show the cut out around the hanger.



About fitment of the diffuser. IF you are running someone else's exhaust or any style other than what is pictured here then the tray will have to be cut to fit around the muffler.

5th picture. The diffuser off the car and cut how it is shipped. The rivets and adhesive heat shield are also included. If you need to cut the diffuser to fit with a different exhaust the material is easy to work with. Just about any saw will cut it or even good tin snips.

 

Trinydex: Yes i agree on point one...but have read some articles where a slightly uneven surface creates a boundary layer and therefore reduces drag. Swimmers use suits specifically designed to do this to allow less drag reducing the effort needed to maintain a pace. IIRC Sharkskin trademark

Point two Yes that's why i was asking the question regarding if the area under the car could be considered to be a venturi area. Pressure drops when air speeds up inside a venturi IIRC this is just past the narrowest point..this allows a carb to actually work to "pull fuel" into the throat area. If you look at the side view of the Evo, it is raked down at the front and is slightly higher at the rear. This is like one side of a venturi..or also similar to a ground effects tunnel bult into the bottom or on either side of the center tunnel of a ground effects car.

Does that illustrate better what im saying?

However the fact that rear diffusers work well when angled ~ 15 -25 degrees to the bottom of the car tells me that the air in front of the diffuser is not really low pressure. What the angle does is move the pressure point similar to the narrowest point in a venturi further to the back of the car, reducing the pressure there and decreasing lift at that point.

So taking what others have said, side skirts would isolate the flow ( yes I was around when they were used by Jim Chapparal and then used by Lotus in F1 and banned especially the sliding variety ) My opinion would be that the air would be slightly lower in pressure under the car than the flow along the sides and overall higher than the pressure above the car (lift). So the skirts would be to prevent air spilling in, preventing large scale tumbling, and disruption of the flow.

It seems to me that the type of side skirts used as style items could also be used by inverting and flipping over so that the narrow edge is pointed down but farther out to the edge of the car, allowing the flat edge to mount to the bottom of the car.

Milburn

 

David: Nice fifth pic.. Cant wait to get it in, fit it and try it out.

To the other contributors in this thread..are strain or movement gauges the best way to tell the amount of deflection? Is there a low buck method that may provide a rough measurement at speed? Chad@CBRD can you chime in here?

Milburn

 

Sneak peak at the new Cyber Evo setup... Looks like Nakajima is at it again

(Is the Cyber Evo going even wider )





New Cyber Evo undertray sneak peak:

 

^Holy cow!

 

If you want to look at the car as a whole, its actually like a big mishapen aerofoil, which creates lift. im not sure if thats what youre getting at, because the only thing simmilar in creating downforce under a car and a carberator is bernoullis principal

Actually, the diffuser is angled up to exhaust the airflow under the car into the wake (vaccume) behind the car, which "sucks" the air from under the car speeding it up. max velocity and lowest pressure is seen right at the point that the diffuser angle and underbody meet.

In a properly set up car, pressure under the car is lower than anywhere else. it only needs to be a fraction of a psi lower, but spread out over the entire underbody that can translate to 1000lbs of downforce or more.

 

Does the cyber evo use the voltex rear fenders?

 

im soooooo jealous.... flat bottom evo's are so impressive... that new car is going to freaken ROCK OUT!

will any of these parts make it into production? i might need to sell a kidney!

 

i guess i'll have to agree with _your_ first point to a degree. i think the phenomenon you're speaking of is the raising of the reynolds number with a roughing of the boundary surface. you see this in the cosworth cylinder heads. but the thing is the roughing is regularized (much like the swim suits i imagine). and this will produce some acceleration of air which in a well engineered way can help flow. the motions of air under the car are probably less regular and more importantly create huge deviations in flow, presenting large cross sections and more importantly again, unidealized flow shapes. if you check out some of the lmp cars on the mulsanne type you see some cars use an almond shape to cover the tires even! presenting an idealized geometry to flow around (even while presenting significant cross section) is important.

as for a raked car it does definitely help downforce as it turns the entire car into a wing shape (i hope this is what you're talkin' about) the problem is of course that the car is not an idealized geometry to flow around so you get plusses and minuses.

side skirts mounted so you have a transverse protrusion is an interesting idea.

 

i actually want to talk about this a little more. i was trying to avoid the subject with talking about a car that is raked... one of the minuses of that situation is as soon as you get flow under there it will expand (or the car will draw air from the sides) but this expansion is not controled and i'll call it violent.

the flow at the tail through a diffuser is controlled, the slats keep the vortical (is that a word?) flow from getting too large (wide).

i have a bit of a question though, i see more benefits to an angle of attack on a diffuser section than just the ability to exhaust flow into the wake void, but i do recognize that as significant. i guess i'm thinking in general terms but there has to be some innate benefit to turning flow.

this is just my thinking but there should be some basic necessity of the flow to unangle the diffuser (creating a downforce or downtorque i guess), this is a semi obvious when looking at a wing. a wing angled like this \ the flow must try to make it like --.

 

the main advantage to running a flat bottomed car with a rake is votex generation. the vorticies are formed from the air being sucked around the sharp edges along the sides of the car. The suction comes partially from a rear diffusor and partially from the diffusor effect of the raked floor. Even cars that are running full underbody aero still rely heavily on vortex generation for their downforce. Thus you see all kinds of vortex generators under and around the car. The strakes (not slats) are there to keep the air flow going in the right direction. When a car is turning the air isn't nessasarily flowing along the centerline of the car which results in the loss of downforce. They also serve to isolate each section from each other so that small disturbances in one section don't kill the flow in the others.

The overturning moment assoicated with a diffusor is quite small referenced to the overall downforce generated, due to the relatively small pressure differences across the surface of the diffusor/underbody and the low camber of the airfoil (it's basically straight).

 

Mr. Buschur,

You seem do be the only one here actually interested in drag reduction. If you're looking for the biggest bang for your fabrication buck in terms of drag reduction, I would recommend installing a front air dam that extends as close to the ground as you can make it. As others have said, the air flowing under the car is highly disturbed by all the crap down there. Short of making the whole bottom of the car flat, if you can keep as much air from going under the car as you can you will reduce the drag produced from air flowing over all the "crappy" stuff.

 

Thanks griceiv, my car is as close to the ground as I can get it in the front and still drive it. With the ride height I have set and the front lip I can barely fit my shoe under the front of it. Doing my best