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2G Rear Diffuser Design

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Atuca

Supporting Member
1,140
292
Jan 6, 2007
Central Valley, California
There were less then 10 threads about rear diffuser design, and the ones I saw did not look to be anywhere as functional as I would like to make one. I have spent the last few weeks designing one specifically for the use of function in a stock body 2G DSM. This is not a universal one size fits all solution, I want to make one specifically for the 2G and have it work. Even if the function is small and even if it is nearly negligable in the end (in the sense you have to be over 100 MPH for it to have an effect), I want the design to be functional and not just be fins sticking out the rear of the car; I want it to be designed to work.

So first off, I am posting this thread in the Road Course forum because that is what I am building my race car for. The design has been based off research I have done on the Time Attack Forums, images of race cars, and conforming to NASA rules. In respect to the last point, a rear diffuser is defined as (page 28 of 2012 rule book):

http://www.nasaproracing.com/rules/time_trial_rules.pdf

Add rear diffuser (note: additional points must be assessed for any vertical panels
incorporated into a rear diffuser that are greater than five inches in height---G.8) below) +2

As I understand, the rear diffuser cannot go past the rear axel without being considered an underbody tray, which accounts for 5 points in itself.

Taking that into account, I have designed a proto type rear diffuser kit that has 3 pannels and the rear diffuser. There is a panel that attaches to the rear sub frame to give a flat bottom to an otherwise uneven vertical mounting location for the flat diffuser, along with two rear inner fender well liner "splash/mud guard?" that attach to the stock points, but also extends down to meet the rear cross member/subframe and with its flat bottom, gives a completely sealed uniform seal for a rear diffuser to attach to. The panels block off 95% of any of the open area that used to exist north of the bottom of the 2gb bumper. Some air I guess could escape over the diffuser in front of the axel, but with the gas tank and the floor of the vehicle, air would have to be very determined to travel up and over the axel and not just follow the path of least resistance, however that is what I am here talking about.

I tried to think of a way to also go under the muffler to have 98% of the air sealed, but to be honest there just wasn't a way to do it without having to block air flow to the intire driver side of the rear diffuser. Our exhausts come out from the center of the vehicle's rear crossmember/sub frame, and in an atempt to have no bends, most aftermarket exhausts shoot from the rear of the car at the rear cross memeber 45 degrees out the back of the car. If I had a stock exhaust or a aftermarket exhaust that came out the bumper straight on (and not angled) the two 90 degree angles would effectively take the same space, blocking all the drive side of the diffuser.

When trying to base mine off race car diffusers, the exhaust in most exotic cars and other race cars either exited the car ABOVE the bottom of the bumper, or in the center of the bumper where it didn't interfere with the aerodynimcs. In our cars, thats just not possible. We'd need to make an exhaust custom to fit the rear diffuser and to make space, remove the spare tire/trunk floor with flat metal stock. Not quite your "bolt in solution". :p

So in an attempt to make this diffuser work with all 2G exhaust systems, I decided to just leave the diffuser flat and requiring it to be to cut it to fit the exhaust. There's a million different exhaust systems, I only want to design this once.

I am already working on a fiberglass proto-type and will show pictures once I get them sanded out and have something to show, right now I just have a few layers of fiberglass for the fender well extension and the rear sub frame panel, once I decide on the angle the flat diffuser connects to the new panels, I'll cut the panels horizontally at the right height to give the proper angle for the diffuser.


On to the questions.

Aside from the fact I am looking for design input in the most general sense of aerodynamics, I havn't discussed the question of the fin design. Should it be something that is part of the diffuser design, or something attached to a flat diffuser? If it is to be part of the diffuser design, which is fiberglass, how do I make a fiberglass mold with little fins? Would I be left to make the fins 1/2 to 3/4in thick to allow room for a roller? Or would it be better to try and create some 1/8" metal fins and attach it after the fact?

What angle from front to back does the diffuser need to be? I believe I read on the Time Attack Forums by Andrew (Gixxerdrew) himself stating 7 degrees was the perfect angle, but maybe he was talking about the fins? I can't find the thread for the life of me, but I want to make sure the flat area of the diffuser is angled right.

How many fins and what angle should the fins be at (completely straight front to back?)

The NASA rules says nothing vertical more then 5 inches can be added. Would making a panel that attaches at the stock rear fender well cover splash guard thing be considered part of that 5 inches or are they talking just about the fins and diffuser itself?

I am sure I'll have more questions but this should get the conversation rolling.
 
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As far as truly designing a correctly functional rear diffuser you need to have access to a wind tunnel. By placing a tray you may reduce the drag from the rear bumper but you can also cause turbulence and air being forced into different under car areas that may generate lift.

The main thing for racing is the drag from the rear bumper. not knowing all the technical rules a panel just blocking from the diff back past the bumper would seem the most beneficial. To me i would want to start the piece tight to the bottom of the car so wind couldnt catch the tray and cause more drag. And by extending it past the bumper will allow the airstream from the top of the car to assist in pulling air because of the low pressure area directly behind the car. I would also still add a set of diffuser holes to each side of the bumper to further assist in evacuation air the rear bumper traps.

on most f1 cars and high end sports cars that race at the track near me this is how the fins are layed out pattern wise
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sorry for how crude the drawing is. the tires site in from of the outer lines and the turn out is behind the tire to assist in removing air and turbulence from wheel wells. The diffuser normally extends past the bumper about 3-6 inches depending on the car.

obviously with the wind tunnel you can figure out the most efficient shape, height, angle, and fin count.
 
To that point, you reminded me:

I have seen in some race cars the area behind the rear tires had it own channel in the diffuser out the rear of the car. Right now with my "revised" fender well liner, I just block off any air completely instead of letting it escape into the bumper "parachute".

Would opening up that area instead and just giving it a flat area out the rear of the car be better then blocking it off? I watched some land speed photos of Andrew's car where the area around the fenders and tires really gets the air (dust in the pictures) all mixed up.

I had planned on tucking as much as possible the rear diffuser to the rear subframe, but from front to back I thought I had to have a particular angle for it to work at all. There is maybe an inch or two below the rear subframe that does not go lower then the gas tank and floor pan of the car body in front of the sub frame/axel and I assume I can go at least that low before I am creating drag.
 
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sorry was editing the last post. its hard to say anything at all about aerodynamics and its effect without a way to test any changes effect being real time and recordable. with a windtunnel/smoke trail all of this stuff is instantly obvious. only problem is the cost per hour for the wind tunnel. its why so many people just build something figuring its better than nothing and drill the holes in the bumper.

companies spend millions on this kind of testing when they design the car in the first place. to me it would almost easier testing wise to do like a standing mile without one. bolt it on and do it again to see if the reduction in drag netted you a faster time.
 
Here's the posting from Andrew on the Time Attack Forums:

My $.02:

Basic design questions for a rear diffuser: Target downforce, targets for drag? (power production, track to track optimization).

The design of strakes is greatly influenced other underbody aero (or lack of it) so there is no rule of thumb there. The angle is highly dependent on the shape and location of the rear deck, rear wing shape and location, total rake of the car underbody and many other smaller factors.

About the only basic rule of thumb I can think of would be a flat plate at 7 degrees to ref plane. Seven only because its in the middle of a range and you could end up doing more harm than good if you got it wrong. Its the most likely number to do something good but if you do it right, for which there is no rule of thumb, the downforce could be 4 times as high or more than the simple flat plate at 7 degrees.

So failing that you can get a pro aero guy, I think the best you can do is make your main plane AOA and your strake placement/quantity adjustable or easy to change, get a sensitive driver in the car and test them.

rear diffuser basics? - Time Attack Forums


and here is the photo of the ducting from the rear tire I was talking about:

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That makes sense. worst case scenario is it causes more drag and gets ripped off the car. LOL I think anything would help considering the bubble ass 2g has a parachute for a rear bumper on it..

The other issue to consider is strength to weight ratio of the construction materials. because of this it always points to fg or cf.
 
As of now, I am using fiberglass for the contruction of everything on this. Wether the design of the fins is fiberglass or thin metal, that is up for discussion.

Right now, I don't see how I could create a mold to make the fins encorporated into the diffuser:

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Either the gel coat would be on the wrong side or the fins would ahve to be thick enough to allow a roller to get in there, unless there is some other way to do fiberglass I dont know about.

Thick like this(or thicker maybe even):

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or I have to go with attach on metal fins after the fact:

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I get what you are saying about wind tunnel designing would net a better product in the end, and at this point, aside from tuff testing, I cannot afford or am interested in that. I am aiming to design a diffuser the best I can, and my thought is if it is designed to work well, at a minimum I am not hurting anything. I feel though that with some thought and design put into developing, I end up with something that does more good then bad.

About all I can do and the point of me making a thread is to take into account all the rules the govern the racing organization I intend to go through, and to follow all the design philosophies of aero dynamics of rear diffusers.
 

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As for your question for how to form the fins in fiberglass, you need a vacuum bag. Resin infusion would be best, but even if you wet lay up your mold and bag it, it'll be 1000x better than just slopping resin in there to fill the voids that will form from the tight curvature. If you can do basic fiberglass work, vacuum bagging isn't much harder, but it is a LOT better.
 
you could mold the fin side and the bottom plate separately then resin them together
 
I thought more about what questions I needed to ask to get the answers I was looking for. I think I am mixing up my questions between features of a diffuser and design requirements. I'd like them all answered but the one most prominent is the ideal angle for the diffuser to attach to the bottom of the car. That angle dictates all the mounting locations of the diffuser to the sealing panels, and is basicly the point I am at.

I've also seen it keeps fairly flat (or small amount of degree of angle) and then swoop up the ass of the car real sharply. What's the design difference of keeping the angle steady out the rear of the car and flat and then angling it up sharply?

Compare the attached pictures.
 

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This is most likely due to pressure zones made from the shape of the rear of the car. The shape of the car is going to make completely different zones in the bottom rear and the decision to make the diffuser curve up or remain flat is most likely to take advantage of a better pressure area. With a better pressure area the air is going to flow out from the bottom more easily and stay where you want it to go. Sometimes this will be closer to the ground and sometimes this will be slighly higher up the bumper, hence the curve leading up so the air will flow more easily.

This is all going to be dependant on the turbulence created under the car, the overall shape of the bumper, and any other diffusers in place under the body.

Without a wind tunnel this next statement is completely speculation: I feel like having the diffuser with the curve would be better. Just like a hood vent you want there to be a little bump before the opening to help create better pressure. I think that having a curve would "force" a better pressure situation. That is just a guess though. I would just assume it would work that way.
 
I've spent most of my day at work reading other car forums and trying to piece out the common answers that come up from the varying theories on rear diffuser design.

Some of the things that stood out, was on nearly all stock body production vehicles without any other aero work, the "7 degree" rule applies to the angle the diffuser has from front to back. The extra little flip at the end gives a lower pressure area for the air to move into. If air moves up to fill the low pressure zone, then down force is created under the car.

The issue though, with just adding the higher angle to the diffuser is that in a vehicle with a spoiler designed for down force, the low pressure area behind the car can be varied from what one would expect. You might be able to get away with a 15-20 degree at that point, but without the wind tunnel testing it would be hit or miss, and can actualy cause less downforce. I don't unerstand how the rear wing can mess up the rear diffuser low pressure zone, but if its a known fact it does, I think I am best to not mess with it.

Since I said I was designing for a stock body, I think it is a safe bet to just go with the 7 degrees. Unless someone comes up with a really good reason not to, I think I can check that off my list.

Another thing i read on several threads was about the fins. The outer most fins are generally there to stop the influence of the pressure changes and disturbance of air flow from the wheel wells. So I think instead of just blocking off the fenders completely, I will incorporate it into the diffuser and have chambers escape out the rear with fins parallel to the inside of the wheel fender wells. Then there will be a fin on the inside of the exhaust muffler. 4 total I think will be the number.

Although I read that the fins should start as far forward as I can get them, but with the exhaust piping coming out from nearly the center of the car, its going to impossible to get much use out of the fin if I keep it parallel to the side of the vehicle. I would have to angle it to keep the fin long, or I will have to start the fin probably half way back in the diffuser ceiling.

I'll keep digging around for more info, but that's what I've come up with so far.
 
Just to throw in my .02 but if it was me I would make a bottom of the diffuser, under the fins, flat as well. Like seen in this picture of the Corvette car

24 Hours of Le Mans 2012 - Corvette Racing

This design they cut the bumper to make it shorter and pushed everything up. This also probably helps keep the rear end planted like an extra spoiler but I bet it would really make a difference with the fins in there as well. It should be able to help with not only downforce but also help keep the rear end check around corners and such. I don't actually have any proof of this but I don't think we can have those results without a wind tunnel. I wish I could build something like this and test it, but I have 2 other big custom projects going on with the car and this will have to wait until I have more time.

Can someone tell me what the fins have to do with air flow? I just picked up some sheet metal mysef..Not sure what im doing with it yet.

The fins help with the stability on corners and at high speeds. This keeps the car predictable which helps keep the driver having consistent lap times and not crashing the car.
 
Ahhhh.So same principal as a spoiler..Direct air away from the car as effectively as possible...While keeping the tires planted on the ground by directing it away...


Like splitters on the front,to add downforce on front tires...Diffuser,is taking that air and moving it away from the car.

I may have just said the same thing 3 times,my bad.
 
Ahhhh.So same principal as a spoiler..Direct air away from the car as effectively as possible...While keeping the tires planted on the ground by directing it away...


Like splitters on the front,to add downforce on front tires...Diffuser,is taking that air and moving it away from the car.

I may have just said the same thing 3 times,my bad.

That is exactly right. It is called a diffuser because it diffuses air...
 
As for your question for how to form the fins in fiberglass, you need a vacuum bag. Resin infusion would be best, but even if you wet lay up your mold and bag it, it'll be 1000x better than just slopping resin in there to fill the voids that will form from the tight curvature. If you can do basic fiberglass work, vacuum bagging isn't much harder, but it is a LOT better.

I've done alot of research on vacuum bagging and resin infusion, and that looks like something I could invest in (if I can solve this fin integration issue), the process undoubtedly looks very nice and they say it makes for a better piece.

However, if I were going to try and make the fins thin, less then 1/3 inch or so thick, I still don't see how I would make the mold, and how I would lay the fiberglass/carbon fiber. My concerns are the mold would not release the fins (pulling verticle to get the flat diffuser up but the fins would be stuck horrizontally) and I don't see how I would be able to form a piece of cloth up and around the fins. Would I just lay a flat piece that covers the whole diffuser and cut slits for the fins, lay that, and then cut pieces sized and shaped for the fins and lay them seperately?

I've seen (as pictured) a few 1/8 thick (looking) fins or so and they are either metal bolted on after the fact or they are molded in what looks like plastic. All the carbon fiber ones look to have fins that have some girth to them. So it doesn't look like.. anyways.. that the thickness of the fins is that important.

Please correct me if I am wrong.

p.s. what are rakes? Are rakes another name for fins? I keep seeing the word but google doesn't just give me a simple answer. I know about the tool rake and how you can "rake" leaves with it, so it would make since it's another word for the fins, but can I just clear that up :)
 
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So in reference to the diffuser, it would be the angle at which the fin base and fin tip? am I drawing this side view right? And if I am, what is the rake angle supposed to be?
 

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I pm you my phone number 2 days ago at your request for my suggestions

I think I cleared up for you the deal on the plug and the mold.

The fins or strakes are easy to make on the plug and then to incorporate on the mold.

Call me and I will clear it up for you, you are making it difficult on yourself.

Make the thing and maybe I wont have to make mine.

I built an undertray for my corvette back in the 80s, to stop the nowdays called the parachute effect, had no idea of diffusers or anything similar it was only common sense and I did not want to put big holes on the bumper. Did it help, yes, Did I get more downforce, I dont know, but driving behind my car, the back bumper was not blowing up like a balloon . so I know i gained something.

arrowhead

Direct the air straight out the back and not let anything hinder that, which will cause a disturbance and make the diffuser less effective.

You want disturbance in the flow coming into the diffuser that is why they are now dumping the exhaust into it, something to do with laminar flow.
 
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I pm you my phone number 2 days ago at your request for my suggestions

I think I cleared up for you the deal on the plug and the mold.

The fins or strakes are easy to make on the plug and then to incorporate on the mold.

Call me and I will clear it up for you, you are making it difficult on yourself.

Make the thing and maybe I wont have to make mine.

I built an undertray for my corvette back in the 70s, to stop the nowdays called the parachute effect, had no idea of diffusers or anything similar it was only common sense and I did not want to put big holes on the bumper. Did it help, yes, Did I get more downforce, I dont know, but driving behind my car, the back bumper was not blowing up like a balloon . so I know i gained something.

arrowhead


I'll be giving you a call soon as I am not working, which is wed-sat. I can browse the web at work but I can't make person phone calls, and I really didn't want to bother you on the phone the day I reached out to you via PM. I will call to say "Hi" this weekend even if I get everything I am looking for online, but I had already guessed if I called you our conversation may very well last longer then a few minutes :)

You know us young whipper snappers and our technology, rather use the internet and over thinking it maybe when all I gotta do is call up someone on the phone LOL!

If this project goes well I am considering making a few of these, hense the need for a proper mold and integrating the fins into it. That just makes it easier on me.
 
In the same sence they "engineered" it for the 3kgt, I am looking to "engineer" one for our car. A big part of what I am doing is more then just a flat diffuser, it's sealing up everything under the car rear ward of the axel. The design of the diffuser is just part of the journey I am taking to engineer the perfect bolt in setup.

Although the actual diffuser itself may be the toughest part of that journey :) But because the whole bottom of the car is sealed, a universal fit diffuser really doesn't achieve what I am after. They only (from what I've seen) fill the center of the car, and the mounting solution would not seal on our cross member. It just ends up extending the bottom of the bumper and still allows air around the sides of the diffuser to get caught and stuck in the bumper anyways.

I'm sure a universal helps, but it isn't perfect. I am trying to be close as I can to achieving perfect. :thumb:
 
So are you planning on making a belly pan or tray to get the most out of the diffuser?
 
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