2G Rear Diffuser Design

[GofaST4life]

This is going to be sweet.

 

[eclipse420ags]

Any updates on this?

 

[Atuca]

Been dealing with some personal non car stuff, taking up alot of my time.

I need to finish sanding and body filler"ing" the sub frame panel, but I finally finished with the fiberglass aspect of it. I had to go back and fill in the holes ect so that the mold will be able to release from the plug, didn't think that through...

The flat diffuser panel is about done, just shaping it to fit the bottom of the 2gb bumper perfect. I'm a perfectionist, what can I say..

Next on the agenda is the strakes. Got to figure out how I want to mount them, and how (if at all) will I incorporate counter sinking the mounting points on the diffuser itself. I don't have an easy way to mass produce metal strakes.. I'd have to manually cut each one out of flat sheet metal with a angle grinder, and then have to sand/grind it to shape. Bleh.. I am open to ideas on this one...

I have a local supplier that would be able to get me high quality resin and fiber glass at about double what the cheap home depot stuff costs, but everywhere I've researched says the higher quality stuff is easier to work with and provides better results. Especialy as we go into the colder months of the year, its taking a while for resin to harden (usually the full 12+ hours) which real world time means one or two fiber glass layers a day. I don't feel (or have the space really) to create multiple molds to just start a side business out of this, so it will have to be a work of passion fueled by the interest it generates. It also alows me to have a week or two to myself in between making the things . Its already started to feel like a job with the amount of hours I've invested, but I'm the kind of person that once he says he's going to do something, he does it.

 

[snowborder714]

Not sure if they sell strakes that you could buy and bolt up. If not, see if you can find someone with a laser cutter, they'd be easily able to mass produce something you design.

 

[ed1380]

We also have a plasma cutter, so let me know if you want some sample pieces cut out.

 

[mitsumikey]

This is very interesting. A functional one definately interests me, and especially if the bumper doesnt have to be hacked. Good work so far!

 

[LeeSD1]

The best functional one would probably as stated above. To cut the bottom of the truck and have the exhaust run under it and out the middle area of under the bumper.

 

[Getritch]

Just read the entire thread (quickly) and correct me if i'm wrong, but I do not believe anyone wholly addressed the question of what the diffuser actually does. its first function, of course is to reduce drag, but how it does that wasn't really expanded upon and i believe it should be since it MIGHT have an effect on your design, if there's any wiggle room at all.

first, the diffuser, as mentioned, smooths the underside of the automobile. this is simple. smoother surface=less drag=faster air.

second, it DOES create downforce, but more important to that than the diffuser design is the ride height. if the rear lip of the car and the front lip have approximately the same ride height and thus, airspeed and trap area, the pressure that builds up in front of the car will not be able to exit with any significant velocity to create the "sucking" that produces downforce.
http://i1284.photobucket.com/albums/a580/getritch/airflow_zpsad7fac4b.jpg

to that effect, cars are built with the rear end higher than the front. this gives the escaping air a high pressure area in the front and a low pressure area in back.
http://i1284.photobucket.com/albums/a580/getritch/airflow2_zps81c76227.jpg

as you can see, as the area the same volume of air must fill expands, the air accellerates to fulfill this function. the faster air creates more vaccum and more vacuum is more downforce.

the diffuser itself assists this by removing the "parachute" area which results in a buildup of high pressure (red) air in the the rear bumper which actually creates lift.
http://i1284.photobucket.com/albums/a580/getritch/airflow3_zps1917ebeb.jpg

now, to address the turbulent air. As anything moves through the air, the air behind it must travel around it and contstantly fill the space behind it, lest it leave a total vacuum. this is called a pressure differential. in every car, this effect is parasitic to the vehicle's inertia when the air created behind it is turbulent due to various levels of flow separation.

the NUMBER ONE function of a diffuser in any car is to streamline the rejoining of the top and bottom airstreams. a teardrop shape would produce effectively no drag from the trailing edge, and while this is great for aircraft design, it isnt so practical or attractive for automobiles. you'll notice that the fastest aerodynamic cars have a rear fascia with 6-18 inches of flat surface and the rest is sloping surfaces designed to bring air together as smoothly as possible. this smooth rejoining of air is the primary focus of a diffuser and strakes in a wide, flat diffuser as it keeps the airflow as straight and with as little turbulence as possible when rejoining.

overall, a good diffuser design increases airspeed to the rear greatly, allowing more downforce, but it's most useful in eliminating the drag that hinders top speed. the easiest way to measure the effectiveness is actually the paper/tape method. if you attach (VERY securely) a cheap digital camcorder and LOT of 1-2 foot long paper trails to the underside of the car/diffuser and record a few standing miles you'll know plenty about which diffuser is most effective at straightening the airflow.

in our car, going for high downforce or high aoa diffusers is probably a moot point unless you cut into much of the rear bumper to allow a good, steep slope, maybe 15 degrees or so, and tapered to a smooth, round edge.

i'm extremely interested in your project, Atuca and you've done some immense work with the design with very little input to go on and no math that i've seen whatsoever. the CAD drawing in particular was awesome and very enlightening. if you DO make these i'd definitely buy one, since i have no idea how to mold fiberglass whatsoever, but have been doing much research into finding one for my GSX build.

also, my information comes from 2 semesters of fluid dynamics and several automotive aerodynamics books, in case you were wondering.

if you want to call me we can chat, too. i'd love to be a part of this project in any way i can. pm me.

 

[eclipse420ags]

You copy and pasted twice bro.

 

[Getritch]

You copy and pasted twice bro.

nope. site error. edited.

 

[Atuca]

Getritch:

TBH there has been alot for me to go on, and plenty of math. I just been keeping it from the forums because.. that's my hard work and design I've been on several.. in the dozens.. of different forums. I've found different racing forums, exotic car forums, fluid dynamic forums, down to regular civic forums. Researched, read PDF versions of racing books, there has been plenty of research in the design. I then designed it in an auto-cad program using measurements I had to take with medieval tools and due to their inaccuracy, did it over and over and over to make sure it was right. Deciding to just have a flat diffuser panel itself was not decided easily, placement of the strakes was taken into consideration, deciding on if the bumper had to be cut or not. The diffuser shape was one design concept that had to materialize, then the angle and mounting solution had to be devised. There's so much that was put into it that, some may never know or appreciate all my time, and that's fine. At the end of the day it just needs to do its job.

I am not a fluid dynamic engineer, so conceptually my design had to be based off the proven work of other designs. It's also part of what is taking so long, as I am unable to validate design philosophies myself. I am instead forced to search for my answers and that's not always easy as it sounds. Alot of what I was/am after is either vehicle specific, or a trade secret. Andrew's Aero Kit is a good example of that with reference to our car. Even if you see what you want to do, understanding the design philosophies and implementing them for your application can be quite tricky even with an end goal in mind.

If you read the full thread then you are aware that cutting the bumper and trunk is something that was discussed and may be done still on a future version, but this design is working with the constraints that your typical DSMer would want to work with. Hense street/race version. With the amount of time it has taken to design the first, it may be a while before I get around to a race version. However, I was happy to annouce that the street version would still be a viable source of added rear downforce with the angle/shape of my newest sub frame panel, and for 95% of DSMers, will probably be the only version they really would ever need, even on a race track. Street/Race may be a bad name to differentiate the two; it's just what came to mind first.


ed1380:

Obviously I'll have to make the first set by w/e means available to me. Depending on cost, and more importantly availability, I may be able to have you mass produce some for me. There should only be two different strakes, the inside 4 and the outside two. I hadn't quite figured out the design and mounting locations for the strakes.. so until I get to that point, this point is about moot. I like the idea of removable strakes though.

All:

I'll see if I can get some updated cad renders done. I am really poor at photo composition, especially with using a flat black color on the model. I'll try my darnest to get the diffuser panel done this weekend. At least that way you can see what it looks like on the car IRL if I can't get a render done properly

I been thinking about actual finished product details, would anyone object to a just a fiberglass with black gel coat and bare metal strakes, or are you guys expecting a carbon fiber top layer and anodized black strakes? I ask now because ordering materials is something I will need to start doing soon, and anodizing (or painting, or powder coating) would be another process I would need to out source properly. If people are expecting and opting more for professional end result, I need to take that into consideration. I'm really not sure what people are expecting here. I'd could guess the cheapest option as you are all DSMers (wait.. so am I..) but then again exterior modifications sometimes people are more apt to get something that "looks" good (I.E. carbon fiber body panels). Carbon fiber just means I need another resin and cloth purchase to take into consideration as well, but maybe because its mostly under the car.. you guys think its worth the extra money? I dunno..

 

[Purple_Demon]

<<<Tickled pink. In for results on a street setup. I would be ok with re-routing my exhaust if I had access to a welder. This is gonna sound a bit ricer-like, but I know diffusers have specific purpose and design, but I think they look damn awesome. Ive always wanted one XD

 

[eclipsegsx1736]

as you can see, as the area the same volume of air must fill expands, the air accellerates to fulfill this function. the faster air creates more vaccum and more vacuum is more downforce.

A given mass of air that transitions to a larger area will slow down, not speed up. This is what creates a low pressure zone. The diffuser should be designed to keep this slower, lower pressure air from mixing with other airstreams on the sides of the car.

I been thinking about actual finished product details, would anyone object to a just a fiberglass with black gel coat and bare metal strakes, or are you guys expecting a carbon fiber top layer and anodized black strakes? I ask now because ordering materials is something I will need to start doing soon, and anodizing (or painting, or powder coating) would be another process I would need to out source properly. If people are expecting and opting more for professional end result, I need to take that into consideration. I'm really not sure what people are expecting here. I'd could guess the cheapest option as you are all DSMers (wait.. so am I..) but then again exterior modifications sometimes people are more apt to get something that "looks" good (I.E. carbon fiber body panels). Carbon fiber just means I need another resin and cloth purchase to take into consideration as well, but maybe because its mostly under the car.. you guys think its worth the extra money? I dunno..

From a racing perspective, I'm all for whatever is cheaper, yet strong and not overly heavy (yes I know, pick two, right?). I think thin fibreglass makes sense for most, if not all, the parts of the diffuser. A CF option could be available for those who want to dish out the extra cash, but a thin fibreglass unit would still be quite light, and plenty strong in this application (no particular part of a diffuser is under any unusually high load in any direction). It's a pretty low-stressed part.

The most important thing would be the quality and "grade" of the resin. I'm no expert, but it will need to be able to handle high heat if an exhaust is run anywhere near it. I worked for a Formula 3 chassis builder in my former job, and we had fibreglass rear diffuser decks and strakes. We sent one to a reputable motorsport carbon fibre manufacturer to have reproduced in carbon. The sample came back, lighter and stronger, and seemed great until we took it to the track for testing. We had a spare sitting in the sun for a few hours and I picked it up. It literally bowed down on both ends like a sad face. In our cold shop, it was stiff as a board. Unacceptable on a race car. We sent them back.

Beau

 

[greengoblin]

I would just do fiberglass on your prototype test it. Then either stick with fiberglass or look into having them laser cut and CNC press formed out of alumium and powdercoated. I have found that if you only build parts on the side the less you have to do the better.

 

[H@xtGSX]

I agree. Just gel coat it, vacuum bag it if you can (stronger for the same amount of material), and just use a cloth/mat mix. Cloth for the exposed exterior, mat for bulking up the interior. If you have a good mold, carbon fiber, Kevlar, even Zylon, Tegris, and nano fiber is easily do-able. Prototyping is always done on the cheap in case something is wrong.

 

[eclipsh]

A given mass of air that transitions to a larger area will slow down, not speed up. This is what creates a low pressure zone. The diffuser should be designed to keep this slower, lower pressure air from mixing with other airstreams on the sides of the car.

Beau

Been following this thread for a bit and this comment threw me. I just took fluid mechanics last semester and I'm dealing with convection heat transfer right now this semester. Isn't slower moving air at higher pressure, not lower? Isn't that how a front splitter works? I thought that caused a low speed, high pressure buildup on top of the splitter for down force. Conversely, doesn't the diffuser make the air move faster and cause a low pressure zone to help suck the car down?

On a side note, this thread is very bad for me to follow. I keep wanting to hack my bumper and build a diffuser every time I read it. Glad to see you sticking with it.

 

[eclipsegsx1736]

Been following this thread for a bit and this comment threw me. I just took fluid mechanics last semester and I'm dealing with convection heat transfer right now this semester. Isn't slower moving air at higher pressure, not lower? Isn't that how a front splitter works? I thought that caused a low speed, high pressure buildup on top of the splitter for down force. Conversely, doesn't the diffuser make the air move faster and cause a low pressure zone to help suck the car down?

On a side note, this thread is very bad for me to follow. I keep wanting to hack my bumper and build a diffuser every time I read it. Glad to see you sticking with it.

I didn't really do a thorough enough job explaining it, I think. I understand the confusion.

From what I understand there are 2 things happening here. The expansion of the underbody at the back of the car does create a negative pressure, as the air mass now begins to occupy a larger area, slowing it down. However, this causes air to accelerate underneath the car as it tries to occupy this negative pressure zone, and yes, due to Bernoulli's Principle this creates a low pressure underneath the car. This is why an effective underbody and rear diffuser can also increase front downforce, not just rear.

Beau

 

[Getritch]

I didn't really do a thorough enough job explaining it, I think. I understand the confusion.

From what I understand there are 2 things happening here. The expansion of the underbody at the back of the car does create a negative pressure, as the air mass now begins to occupy a larger area, slowing it down. However, this causes air to accelerate underneath the car as it tries to occupy this negative pressure zone, and yes, due to Bernoulli's Principle this creates a low pressure underneath the car. This is why an effective underbody and rear diffuser can also increase front downforce, not just rear.

Beau

^^this

was just going to reply to your first comment until i read this. this is exactly what i was saying. sorry if i was unclear. only thing you're missing is that the air will slow, but only after it rushes out of the high pressure zone and settles back toward S.T.P.

with a fast moving car, this happens well behind it, meaning the air will accelerate FROM the high pressure zone for the entire duration that it is underneath the car so long as turbulence does not ensue.

its great how many intelligent people there are on this forum.

not at all like the chicago facebook group....

 

[Dstratt01]

This has been a huge help on trying to figure out design possibilities for a diffuser I am going to be building sometime in the future (hopefully before next spring.) Are you going to be building it all the way up to the mounting points you showed earlier or shortening/ lengthening it to a new location? Also what material are you thinking of using? (I may have missed this earlier) I am probably going with a carving foam for the design and then most likely fiberglass. Im sure abs would be another quality material for this.

 

[Scott McIntyre]

Hmmm.... Do I see a "bolt-on" rear diffuser there?

You must be logged in to view this image or video.

Might be worth talking to them about their design: BB Racing

 

[Atuca]

Still probably have another day worth of sanding to do. Thought I had it perfectly smooth with the body filler, but after a few light coats of sandable primer I have found a few more small imperfections. Once those are all out, its time to begin the molding process.

At this point though, I am officially calling the AWD rear subframe mounting panel "DONE"!

I'll have to drill the mounting holes out again on each piece, but I think there is no avoiding that if I want a perfect release from the mold.

I did not finish the diffuser itself this weekend, I'll keep trying next weekend. Anyone who thinks they can whip together a diffuser overnight or during a weekend is out of their mind; no wonder there aren't any being mass produced! I thought this was going to be couple weekends, not a couple months! Oh well..

Hope you enjoy the pictures!

 

[Atuca]

Man, roughly 2 months of work on the rear subframe panel plug and not one comment on a "finished" pic? Tough crowd here!

Looking for a suggestion on counter sinking the fins. This is a two part question really.

A) How thick of metal (and what type of metal) should be used to create the fins? I was thinking some 1/8" (11 gauge) or maybe thicker, depending on strength needed. Then depending on that, assuming a 90 degree bend all along the mounting side, I would counter sink it on the diffuser panel.

B) Would a Square Router Bit work for creating the counter sunk indentation the length of the diffuser panel for each fin? Do I need to buy a router (suggestions?) or is there a cheaper alternative to make these indentation? I don't do any wood work and could hardly justify buying an expensive power tool just to make a few 1/8" grooves haha. I am using 1/4" hardboard for the diffuser panel plug, hence the idea for wood tools here.

So looking for some advice here. Thanks.

 

[ed1380]

I honestly have no idea how the plug fits. Looks very complex. You could use a dremel instead of a router.

 

[Atuca]

Hmm.. I'll see if I can get more installed pictures of it.

Right now this is all I got.

You must be logged in to view this image or video.

Without drilling holes in the plug now though, I couldn't really get an installed pic as it it now.

Red shows where holes would be drilled (would assume less holes on the diffuser mounting surface, but you get the idea)

This is upside down, but hopefully you can orientate it to make sense?

 

[greengoblin]

What you have done looks good. I think most just don't know where is goes, what it does or how it fits.
The pics help.

On the stakes I would build a few and see how thinkyou think they need to be. My gut says 16ga would be plenty once the are bent and mounted but I would play with a few and then make up your mind.

On mounting them I would either just cut a slit and slide them through the the top and rivet them in place.

Or just rivet them to the bottom side like most do.

http://www.difflow.com/5elementrailerjr/pic2.jpg

For cutting The dremel will work. However if your going to make lots of cuts a router or an angle grinder would work better. I usually only use my dremel for making very small cuts.