2G Rear Diffuser Design

[Atuca]

That is something I will design after the fact. The 5 point penalty for installing one in NASA sanctioned events is reason enough for me to create the underbelly tray seperately.

The mounting "sealing panels" I am making are taking into account that forward the rear axel a underbody panel would/could be utilized. That includes sealing the exhaust pipe up to the rear diffuser where the muffler and exit varies car to car. The underbody tray I design should universally fit all exhaust systems and could seal the bottom of the car completely.

I hadn't really got into the develoment of that but it has been taken into consideration. The cost factor alone though I thought would deter most people interested in a full underbody tray. Most would be looking just for the rear diffuser.

If there is more you would like to know about the underbody tray, or if it dramatically affects the use of the rear diffuser, I may have to design two different versions, something I am not against. I even considered making a diffuser that required cutting the spare tire well out of the trunk and moving the exhaust up north, that too has been considered.

For this first project though and the purpose of the thread, at this point, is to focus on a stock body bolt in rear diffuser for a 2g.

 

[eclipsegsx1736]

With the massively uneven undercarriage of our cars (just about any production car), a rear diffuser without a flat undertray isn't going to be nearly as effective. That said, in "street" class Time Attack, I'm binded by rules dictating the underbody of the car cannot be made flat between the axle centrelines, so a full undertray isn't allowed anyway.

I'd say the rear diffuser should be designed with the assumption that the exhaust will not be run to the back of the vehicle and will be either dumped under or out the side of the car. If one insists on running a rear exhaust, they can destroy this functional aero piece and disrupt the airflow with a large muffler if they so choose

More on that note, I've considered building a "blown" rear diffuser where the exhaust is blown over the strakes of the diffuser, but this would increase rear downforce and shift the aero balance rearward as torque is increased, which is counter-intuitive for our cars that tend to understeer more and more as torque is increased to the drive wheels.

I would DEFINITELY NOT have the strakes be a permanent part of the rear diffuser deck. They should be removable/replaceable. The deck is unlikely to get damaged, but the strakes are certainly so, and would be the first piece to suffer if you went off track backwards. Almost every race car I've seen with a rear diffuser has replaceable diffuser strakes. They're typically quite thin as they're not under very much aero load laterally. The part of the strake that mounts to the diffuser deck usually has a 90* lip of maybe 1/2" width to allow it to be riveted or otherwise attached to the deck. If you wanted to get really trick here, to reduce uneven edges along the deck and reduce turbulence, you could cut a slit in the diffuser deck and mount this lip to the top side of the diffuser, and along with using flush rivets, keep nothing protruding into the airstream.

Another advantage to having removable strakes is giving several options on strake height. Those not limited by strake height could probably benefit from a taller strake that's more likely to prevent air from spilling into the diffuser's channels.

As far as the angle of the end of the strakes as viewed from the side of the car, they're typically cut to be perpendicular with the ground when the car is resting on the ground. It would also be beneficial to extend the diffuser as far rearward as possible (even past the end of the bumper) to allow the longest distance possible, since we're bound by rules limiting how far forward the front of the diffuser deck can extend. The longer and deeper the diffuser channels, the more effective it will be.

Beau

 

[Atuca]

I was just sitting patiently waiting for some of you road racers to chime in

I never thought of the strakes as a damageable part that might need to be replaced, that almost single handedly swayed my opinion on wether I should do fiberflass fins as part of the diffuser plate or seperate. I will look into how I would physically manufacture the strakes now, but that might save me from vacuum bagging the diffuser to offset the initial cost for me.

As to the exhaust ruining the diffuser by running through it? That was part of me laying under my car for hours on end. I tried to think of ways to make it universal for all exhausts and in the end I decided I'll just create the flat diffuser panel and the installer can cut the diffuser to fit their exhaust. I'll have enough mounting points that both the left and right side of the diffuser panel will mount sturdily, but there wasn't a "bolt on" option that I liked that didn't require cutting. And as you stated Beau, I shouldn't assume people will be running their exhaust out the back anways.

I read about the exhaust being blown over the strakes, and they said the back end gets squirly when you let off the gas and that it can be a bad thing as well. I didn't get much more into it because I always assumed no one with our cars would do it, but if I leave it uncut as I am planning to, the option would always be there.

Thanks for your insight on some of the features and construction of the diffuser, I would never have thought of the things you brought up! And the fact you are thinking of the rules on the track is great! It is something I want to make sure I design for so that it is useable in alot of different senarios.

 

[arrowhead]

Look at the picture you sent me, right below the tag you can see where the center section seems to be a separate section, and held on to the pan by rivets.

There could be advantages to have this made separate as if they get damage they could be replaced.
I do not think there would be much advantage for autox since the speeds are not very high, but in road racing a diffuser will help and it could be made in one piece and if damaged. maybe a complete piece could be replaced faster as in the front sections are done now.
As far as the angles of the trays around 17 degrees seem to be correct, this angle would be from front to rear and in the boating industry this angle is called DEADRISE and it is used mostly when looking at the transom of the boat.
Why look at the bottom of the boats ,well air and water behave pretty similar under pressure and speed. The strakes or fins do have the same effect as chines in the bottom of boats.
The same effect they have on turning applies a flat bottom boat will slide over the water and the one with chines and V bottom will turn better as the chines catch the water

The fins on the diffuser we are now learning also helps in turning and side movement of the rear at higher speeds

I see maybe adding chines to the sides of the fins an improvement.
It would be very difficult to design the pan around the muffler, but for an open exhaust being dump in the center front of the diffuser, it may even reduce the decibels we are now tie to.
The creation of the lower wing is also to maximize the control of the flow of air coming up from the diffuser.

Want to make a cheap wind tunnel, get a fish tank, pump in front ,outlet in the back, so you can control circulation faster or slower, drop different shapes of objects hold them with a string and you can see it all happen simple but effective and you can see it from all angles.

arrowhead

 

[Atuca]

So are you suggesting I make a little model of a dsm with a miniture diffuser and test in a fish tank? That sounds just as difficult as coming up with the money for the real thing, but I like you thinking outside the box haha.

There's a plethera of ghetto home made wind tunnel "devices", I doubt any of which could produce dependable repeatable results to test and confirm any changes I make. But you got me thinking at least.

 

[talon2gbuyer]

hmm. I like where this thread is going. Cant wait to see some finished/installed products! keep up the ingenuity guys!

 

[AngelOfPassion]

I think he was just suggesting taking single items with the basic shapes of the designs you would like to make. Like take something flat and compare it to something with the curve upward to see the difference instead of speculating. Idk though that would only give you very basic results and I wouldnt think it would help much.

What I would do is tape about 50-80 long strings on the underside of the car and mount a camera to film at high speeds so you can see where the problem areas are yourself. Then you can do it again and compare it with the diffuser on. Who needs a wind tunnel when you can just drive on the freeway with a couple cameras mounted under the car. I have seen this done before just in a more controlled environment but I think it would be the best option on a budget. Not free but you could do it.

Or just do that but with a big ass fan in front of it.

Maybe just double the length than these...

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[Atuca]

If I have to do the tests above 100 mph though that will be a little tough for me, unless the testing can be done at lower speeds and yield the same airflow path results I am after. Testing down force I suppose could be done by taking a photo of the height of the vehicle at speed before and after? I'm a one man show and would need some volunteers to assist in aquiring that info/camera shot.

I could use a few of those "Go Pro" mini cameras anyways for race track recording, maybe I'll just budget that into "product development"

 

[eclipsegsx1736]

I was just sitting patiently waiting for some of you road racers to chime in

I never thought of the strakes as a damageable part that might need to be replaced, that almost single handedly swayed my opinion on wether I should do fiberflass fins as part of the diffuser plate or seperate. I will look into how I would physically manufacture the strakes now, but that might save me from vacuum bagging the diffuser to offset the initial cost for me.

As to the exhaust ruining the diffuser by running through it? That was part of me laying under my car for hours on end. I tried to think of ways to make it universal for all exhausts and in the end I decided I'll just create the flat diffuser panel and the installer can cut the diffuser to fit their exhaust. I'll have enough mounting points that both the left and right side of the diffuser panel will mount sturdily, but there wasn't a "bolt on" option that I liked that didn't require cutting. And as you stated Beau, I shouldn't assume people will be running their exhaust out the back anways.

I read about the exhaust being blown over the strakes, and they said the back end gets squirly when you let off the gas and that it can be a bad thing as well. I didn't get much more into it because I always assumed no one with our cars would do it, but if I leave it uncut as I am planning to, the option would always be there.

Thanks for your insight on some of the features and construction of the diffuser, I would never have thought of the things you brought up! And the fact you are thinking of the rules on the track is great! It is something I want to make sure I design for so that it is useable in alot of different senarios.

Glad I could help! Depending on how the design comes out I might be very interested in one of these, or helping out in some way if I can (testing?) It's definitely on the list to build a rear diffuser but I'm so busy with lots of other projects on the car and this would take a lot of time I don't have to develop effectively.

Beau

 

[Atuca]

Well I've already gone and got myself under contract with a really exciting thing I'll be able to share on tuners here next month hopefully. So with the time I have til then, and unfortunetly the effective end of the 2012 race year already, I am once again pushing forward and looking towards NEXT year to get to the track and can take the time to focus on this project.

This diffuser project and my "up and coming" project should keep me busy til 2013 race year. The track preparation stuff for my race car is little easy things that doesn't cost too much money. Hopefully between the time put into development of this, catching my journal up to today's date, and finishing the tech inspection check off list I'll be able to stay focused and keep on track to make the season happen. I've already got my eyes set on getting my HPDE stuff out of the way next year. Thats the goal anyways... If I manage more that's a plus

I'll keep you in mind for the testing Beau; I gotta see what all else I need to change in the design before I finish up my proto type. Havn't gotten much else about the physical demensions so if nothing comes up by Sunday morning, I am gonna try and see if I can get the proto type done and photos for all to see

Then the R&D can start

 

[Atuca]

Can someone explain to me if replacing/extending the mudflap/innfer fender wheel well plastic counts as adding "vertical panal" or "rear vertical panel". How many points are we adding for this diffuser, and how many points are we adding for the panels I want to encorportate for installation?

5) Add or fabricate flat bottom/belly tray (rearward of the centerline of the front axle) +5
6) 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
7) Replace or modify BTM rear diffuser, rear bumper cover, or rear "fascia" (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) +1
8) Add rear vertical panels in any location (note: see G.3), G.6), G.7), and G.10)) +2

How would the points work for that? Depending on how the points get added, it may influence how I design the integration of the diffuser and the attachment points. Are we looking at just +2 points, or are we looking at +4, or does the each fender well extender counts as +2 and total end up being +6? Are the strakes then limited to 5 inches from the bottom of the diffuser ceiling for no additional cost? It says no more then 5 inches in height, if the fender well panels I'm adding are only 5 inches high, and the strakes are 5 inches lower....Bah.. I'm just confused on the point totals here.

Again the goal is to extend the that area down and flat to the same level as panel under the cross member so that the diffuser can connect and completely seal the bottom rear of the car rear ward of the axel.


And how about SCCA rules, anyone got the scoop on that? From what I read in the 700 page rule book, it was not very specfic there..

E. A diffuser is permitted behind the front of the rear tires. The diffuser
may be divided internally into multiple sections. The radius of transitions
between the diffuser sides and adjacent horizontal structures
may be up to 25mm. The width of the diffuser, as measured
between its sides and above any radiused transitions, may not
exceed 95cm in any lateral section. Strakes within the diffuser are
allowed.

 

[Atuca]

I've spent the last few weeks working on the design of the diffuser and I believe I have come to a conclusion I thought inevitably was going to happen.

I am going to have to create two versions of the diffuser. To build one for race cars and to build one for street use. The rear spare tire sheet metal prevents the diffuser from achieving the ideal angle to produce down force, and the bumper would need to be cut nearly two inches, eliminating the stock mounting location of the bottom of the bumper cover.

I'd imagine the two types of people interested in this diffuser would either not be interested in cutting their spare tire sheet metal out and welding in flat stock, or they are interested in cutting it out and their race car already has the spare tire removed and likely has welded on their car plenty putting in a cage/ect (maybe already welded in flat sheet metal in the rear).

The bolt on option would decrease drag, and provide about a 3 degrees slope total. The attachment panel at the sub frame tucks up as to not create any drag by being tucked up more then the rest of the bottom of the car forward of it. While I could bring it lower to the ground to clear the trunk floor with the correct angle, it would extend 1.5 inches into the clean air stream under the car. I think any downforce benefits of the correct angle of a bolt on option would be negated by the drag it creates.

Therefore I am proposing a bolt on drag reducing street version, and a much more invasive downforce creating race version. Since I am counting on people welding their trunk sheet metal, I could also assume they would be able to weld mounting locations for adjustable arms to make the race version angle adjustable? Thinking out loud here but I need some feedback.



What are you guy's thoughts? Is there any interest in either of these "kits"? Should I bother making two versions? Or does having a "race version" completely take away from having a "street version" created? Is drag reduction alone worth having a diffuser? If no one wants a "street" bolt on version, I can save myself the time of not creating a mold for it as I need a "race" version for my personal use, but if enough people expressed interest in a "street" version I can put that together this weekend before I start cutting on my car.

Let me know, thanks.

 

[eclipsegsx1736]

I'm all in for the "race" version of course.

I could see the "street" version being very popular though if it looks good. At that point it would be more for looks and I'm sure tons of people would be all over it if it looked cool, i.e. JDM Evo rear bumper which isn't functional but definitely does improve the look of the rear end. It will be interesting to hear what others have to say; unfortunately, this subforum gets VERY little traffic so it may need to be reposted elsewhere.

Beau

 

[greengoblin]

You would sell more street that race units I would guess. Build both it you have the time and want to spend your days working on parts for others cars but make money.

Kevin

 

[snowborder714]

I'd agree with both of them in that if it looks good, you'll definitely sell more street units. People like things that they don't have to do any work to install except bolt on. I think there may be about a dozen or so people on here who would actually be interested in a fully functional unit.

That said, I'm still debating on whether to cut out my spare well or not...

 

[kamikazeredneck]

I would defiantly be interested in a race version but it would have to clear my fuel cell in the back. I could just move the fuel cell up and make a new firewall behind the seats...could be done if this unit was functional. I wouldn't really be interested for probably another year because my car is under the knife right now for a lot of other things and I'm not even sure if it will be out of the garage by spring as it is.

 

[Scott McIntyre]

My Sawzall is sitting in wait of your race version.

 

[Clutch450]

This thread is great, learned more in the past few minutes then class taught me all month!!

I would be all in for a fully functional race version!

 

[Atuca]

I'm seriously shocked people are expressing interest in a race version and not one street version comment. Maybe its the sub forum it's in? You all crazy ready to cut into your cars like me!

Thanks for your input guys, I promise pictures this weekend. They won't be exciting, but at least it will show you what has been keeping me busy the past month. I'd guess about 100 hours already invested into the design and execution of the diffuser panel. I don't have a car lift and measuring points on the car has required strings and rulers, taking the car off jack stands, and putting it back on over and over.

My weekend is sun-tue every week, so expect photos by Wednesday. I've been persuaded via pm to make a drag reducing street version bolt on version, so that will be the project this weekend. I will incorporate everything I've learned and can apply it to the street version, it just won't have the ideal angle to produce much down force. It will certainly be the easier route to take to install and be ideal for street use where speeds are not surpassing 100 mph anyways.

 

[ed1380]

I have a 2g on the lift right now. What measurements do you need?

 

[Atuca]

At this point? Nothing. I'm a very mathematical/logical person who loves documenting everything. I wanted to measure that angle from the rear mounting location of the bumper cover to the sub frame panel I made to cut it at the right height to then get the exact 7* angle. Hense the spending hours and hours fiddling with strings and weights to measure on the ground. Would have been nice to have a laser level too. I don't have every tool there is, but I have the enginuity to figure it out!

The problem was that trunk floor was in the way, but I tried to get creative. After hours of trying to think of different this and thats.. I came to the above stated solution of two versions.

What I do need though is:

A) Is the rs/gs/gst rear sub frame the same for all 95-99.

and if so is true ^^

B) I need to find a subframe or have access to a rs/gs/gst for 48 hours to create the sub frame panel for those cars to mount my diffuser to.

I want to spend this weekend working on the actual diffuser some more, but as it stands right now, my custom "air right" (ok not really..) fitting under the rear sub frame fits the GSX sub frame. For this to fit on the other models I need to make another version of that one mounting piece.

I don't think someone could get me what I am after with a cardboard cutout. Once I have a "finished" gsx panel to show, it may give you visual people an idea of what I am talking about.

 

[snowborder714]

What I do need though is:

A) Is the rs/gs/gst rear sub frame the same for all 95-99.

and if so is true ^^

B) I need to find a subframe or have access to a rs/gs/gst for 48 hours to create the sub frame panel for those cars to mount my diffuser to.

I believe I already mentioned via PM that they all are the same, including the 7th generation Galant.

I can get you a subframe, depending on how soon you want it. I'm actually picking up one for Paul to use to build a tubular unit from, so we all could share that if you're interested. I'm hoping to pick it up in the next month or so from the guy.

 

[Atuca]

A month? That sounds like about the time frame I may end up at after I finish with the diffuser panel. Shipping probably kill me though right? Meh, I'll PM you about it im sure later.

And about PM conversations? They call them private messages for a reason! I'd figure I just make it public that I was going to have to develop a second sub frame panel but jeeze Brian, just take all the credit for knowing about the differences!

 

[ed1380]

At this point? Nothing. I'm a very mathematical/logical person who loves documenting everything. I wanted to measure that angle from the rear mounting location of the bumper cover to the sub frame panel I made to cut it at the right height to then get the exact 7* angle. Hense the spending hours and hours fiddling with strings and weights to measure on the ground. Would have been nice to have a laser level too. I don't have every tool there is, but I have the enginuity to figure it out!

The problem was that trunk floor was in the way, but I tried to get creative. After hours of trying to think of different this and thats.. I came to the above stated solution of two versions.

What I do need though is:

A) Is the rs/gs/gst rear sub frame the same for all 95-99.

and if so is true ^^

B) I need to find a subframe or have access to a rs/gs/gst for 48 hours to create the sub frame panel for those cars to mount my diffuser to.

I want to spend this weekend working on the actual diffuser some more, but as it stands right now, my custom "air right" (ok not really..) fitting under the rear sub frame fits the GSX sub frame. For this to fit on the other models I need to make another version of that one mounting piece.

I don't think someone could get me what I am after with a cardboard cutout. Once I have a "finished" gsx panel to show, it may give you visual people an idea of what I am talking about.

Well if you ever need to demo test anything let me know
Depending on how friendly the staff is at college, I can 3d print/machine stuff at cost.

 

[Atuca]

Well I have good news and I have bad news.

Bad news: I created another subframe mounting panel. This was my third shot at it, but I am finally happy with it. I just gave it its last two coats of fiberglass a few mins ago. Once that cures, I'll then sand and bondo it to a perfect smooth finish, it will then be ready for the molding process.

Good news: You will not have to wait for a race version to get the benefits of the downforce. This third mounting panel couldn't be any more perfect in my opinion. It is within nearly an 1/8" of becoming drag, but as is, it is tucked up higher then any other part of the car's stock floorpan. I'm getting really good with kite string, weights, levels, and measuring tapes

I set up a straight edge perpendicular to the ground against the back of the bumper. Using a metal tape measure, I measure from the mounting location at the sub frame panel to the back of the bottom of the bumper. Then with a magnetic level attached to the measuring tape, I drop the tape down until its paralell with the Earth and take the reading at the straight edge. With the two known measurement lengths, and the calculated 90* angle, I plug it into a triangle calculator and get the measured angle of 7.8*

That is the "ideal" angle we all talked about in this thread to have the diffuser sloped at. Really the only other variable here would be the ground the car is set at, but it appears to be as even as I can get it. The difference in 6.8* and 8.8* is only 1/4 an inch on the measured "parallel to Earth" line in either direction.

I hope that helps clear up why this project has taken so long. I am trying to measure things as perfect as I can and when 1/4 can change the angle so much, it can take a while to get the numbers I am after.

This is also a hurdly I am having to overcome. I think a "BFH" and removing the rubber seal will solve this problem, but these two things are the last thing in the way of this diffuser and the mounting flush against the bumper cover. I took a picture of one (at an angle to prove the mounting panel was currently installed) and you can see it mounts flush from the bottom of the bumper, to the bottom of the trunk, to the mounting panel I made. The angle the shot was taken (about 7-8* + jack stand angle ) The only thing in the way are as pictured. You guys agree a BFHammer to that and no need for a race version now?

Here is a shot of the panel (before it got its last 2 layers of fiberglass and before sanding.. and you know what, don't say nothing about my plug.. I'm very self concious about it 3rd time around It mounts in 7 stock holes (3 unseen but same as the right except there is no sub frame "cross member? w/e that thing is), and seals up around the sub frame. Hopefully this gives oyu an idea of all the talk I've been doing. The thing is sturdy, and even after people cut into it to make room for their exhaust, there will be plenty of mounting spots left for the diffuser to this panel. You'll have to use your imagination for now. But that is why there is so much material on the front part of the top of the panel that otherwise serves no purpose and takes up weight.

Its really the panel and not the diffuser thats taken all this time. The Diffuser panel itself is pretty basic in design. Its going to be a flat piece of fiberglass thats shaped to fit the bottom of a 2G DSM. The strake/fins placement will take a little bit of designing, but I plan on straight fins on each inside of the rear wheels and a pair on the inside of the muffler and the mirrored side. The only "style" I have planned is 4 inches sticking out past the bumper between the muffler and mirrored side.

Anyways.. that's where I am at now. I did get to start some work on the diffuser itself finally this weekend. I still have alot of measuring and cutting to do to get a perfect fit, and keeping it symetrical along the vertical center line. Assuming I get another weekend like this one next weekend, I may have some pictures to share of it.

Anyone have any input on the 4 inchest sticking back past the bumper? Think it will look good? Since I couldn't have it climb vertically real sharp since I wanted to retain the stock lower bumper mounting location, that was my solution to get a little length on the strakes.

Is the angle the strake at the back look right? I am supposed to have the bottom parrallel with the ground and the "rake" is supposed to be perpindicular to the ground correct? Or should the strake be same length as the diffuser which would angle the strake back a little bit?

Would using exterior mounting pins be enough to hold the diffuser to my panel and the bumper? Or maybe do I need the metal clips with screws ont he side be better? Or do I need to divise a way to mount to the frame somehow? I know from what I've read the diffuser helps other parts of the car create down force, but does that mean the diffuser itself will get sucked down twards the ground and eventually come off the car with just the pins?

You guys have til next Sunday to get me input on the actual diffuser design and fin placement. I know I have to do something about the fin height on the outside fins, I'm all ears! Sorry I don't have any photos of IRL stuff to share, hopefully these will do!

What do you think!?!? Opinions folks!