[Andy M]

Thought I'd just make an official thread since I've PM'd/e-mailed/talked to several members about this. I've got enough info to know that it's feasible to post some info.


Background

So I've been looking around for rear diffuser options for my E90 LCI. So far, it's all m-tech bumper only and all the diffusers are cosmetic only. In fact a lot of automanufacturers are designing with "rear diffusers" when they're merely a cosmetic attachment to the rear bumper with no real functional purpose. What I'm really after is something that's function but will look nice too. Unfortunately I haven't found a product that really exists like that for BMW's. As an engineer, I used to do a lot of hydraulic flow calculations for bridges and rivers. So knowing a thing or two about flow and construction, I figured I'd be able to make my own.


How does a rear diffuser work? What is the difference between a functional rear diffuser and the ones available for the E9x's now?

If you ever watch F1 or Nascar, you'll notice that the cars have fins on the underside of the vehicle. If you see the Ferrari F430, that car has a functional rear diffuser as well.

A rear diffuser works by Bernoulli's principle. The air moving under the car is squeezed into a tight space and accelerated. When it comes out from under the car, it suddenly expands and drops in velocity suddenly creating a negative pressure. This negative pressure creates downforce which is applied to the rear of the car. Rear diffusers actually are more efficient at creating downforce than spoilers.

Most cars today tend to have a body shape which slopes up near the back. This is to create that negative pressure as desired.


So why the diffuser if our cars have the proper shape?

While the upper ceiling of the channel of air is sloped up, it's not really forcing the air to accelerate that much and expand since there aren't sidewalls to contain the airflow.

The diffuser will allow the air to be channelled to make the use of airflow more efficient and also provide a gradual flow transition to avoid causing turbulence (which results in drag).


What will it be made of?

At the moment I'm looking at anodized aluminum. I've considered many types of materials:

PVC, fiberglass, or some other type of polymer: this won't work because the muffler/exhaust system gets extremely hot during driving. Between the muffler and the car itself, there's a lot of heat shielding. Another disadvantage is that polymers experience something known as "plastic creep". Eventually, it'll start sagging.

Carbon fiber: the cost is extremely high for something that will be mostly under the car. While parts of the diffuser you'll see, the majority will be out of sight. Also, since this isn't an F1 car, the diffuser will be subjected to environmental hazards like potholes, rock chips, etc. Carbon fiber, while tough, shatters easily if chipped.

Aluminum: I've decided to use anodized aluminum because it's durable, lightweight, heat-resistent, and the price is alright. Another advantage of aluminum is if I design this right, I won't need custom forming, I'll just need bending and laser cutting.


So what's the design going to be?

So far, I'm designing for six fins under the vehicle. The fins will be mounted on an aluminum plate that will reuse the existing screw holes used for the rear bumper. The depth of the fins aren't that significant so it won't be very obvious. It'll be tasteful while still maintaining its purpose.

At the moment, I'm designing a rear termination so that it will extend past the rear bumper and bend upwards. There will be a gap between the rear bumper and the plate but I didn't want to make it flush due to issues with fitment. So instead of putting a gap which looks like an accident, I made a deliberate gap. It actually doesn't look bad at all.

The part that bends upwards will have a CF cap which will be done for looks and also function as part of the whole rear diffuser system. Again, it's all about reducing drag.

The fins will also be removable in the event that one of them gets damaged. This way the whole system doesn't need to be redone. Also, in the future, if I decide that I can make improvements to my fin system, all I have to do is design it, have the aluminum fins cut, and then mount them in.


So are you going to make this an actual product to sell?

Possibly. I may do a limited production run if my custom rear diffuser works and a lot of people like it. I would prefer to have a reseller if I were to do a continuous run: I just don't have the resources or time to handle stock, shipping, etc. I work during the day as an engineer so this will be just be done as a hobby in a way.

While I'm only doing this for the E90 LCI non-mtech, if there are enough requests, I may do other models later on. Again, it all depends on whether I find a business to act as the reseller and do all the ordering/shipping. I just like doing the engineering and design.

As for fitment, it's designed to my car now so it may or may not fit other vehicles. It looks as though it'll fit the pre-LCI's E90's as well but the bolt holes for the bumper may be different.


Updates:

No pictures yet. Well, none that I want to show. Currently drawn in CAD. I'll make a 3D render and Photoshop it onto a photo later (hopefully this week). I'm an engineer, not an architect!

I'll post updates on this thread and on my Twitter feed (andrewdmah). If anyone wants to see the prototype in person, I'll gladly meet up if you're in the GTA once it's installed.

[Me_Hi]

Hmmm... I'm looking for a diffuser...

[Malapropism]

Nice meeting you at today's meet. Subscribed.

[Rampage9]

disclaimer: I'm not trying to be an a$$, I'm genuinely interested and intrigued. I'm also an engineer (mech eng in the auto industry) who has a few questions so I'm playing the Devil's Advocate here

First, why? The reason most 'diffusers' on todays cars are cosmetic is because a real one isn't necessary for the target demographic or function of the car. If you are tracking your car, do you have a problem with high-speed grip that you require more downforce for stability? On track, are you even achieving speeds where it will operate? I get that it is cool, and we all want something functional, but do you need it and are you going to truly notice the improvement?

How will you know it works (besides the 'butt-dyno' ), do you have sim data (Fluent, StarCCM, CosmosWorks?) to evaluate and tune the design? The only true way is to plan a day at a wind tunnel to prove that it actually works (especially if you plan to sell it)

For full effectiveness, you shouldn't just add the diffuser at the rear. You should incorporate it with the whole aero design: beginning from the front of the car, then having an entirely smooth undertray to avoid stagnation, then entering the diffuser.

Will you even have enough space to build this in the rear? Are you planning to build it underneath the muffler cans? A potential idea, perhaps use the exhaust to 'feed' the diffuser?

Again, I'm genuinely interested! I think just a couple legitimate questions, please don't take this the wrong way!

[Andy M]

Originally Posted by Rampage9

disclaimer: I'm not trying to be an a$$, I'm genuinely interested and intrigued. I'm also an engineer (mech eng in the auto industry) who has a few questions so I'm playing the Devil's Advocate here

First, why? The reason most 'diffusers' on todays cars are cosmetic is because a real one isn't necessary for the target demographic or function of the car. If you are tracking your car, do you have a problem with high-speed grip that you require more downforce for stability? On track, are you even achieving speeds where it will operate? I get that it is cool, and we all want something functional, but do you need it and are you going to truly notice the improvement?

How will you know it works (besides the 'butt-dyno' ), do you have sim data (Fluent, StarCCM, CosmosWorks?) to evaluate and tune the design? The only true way is to plan a day at a wind tunnel to prove that it actually works (especially if you plan to sell it)

For full effectiveness, you shouldn't just add the diffuser at the rear. You should incorporate it with the whole aero design: beginning from the front of the car, then having an entirely smooth undertray to avoid stagnation, then entering the diffuser.

Will you even have enough space to build this in the rear? Are you planning to build it underneath the muffler cans? A potential idea, perhaps use the exhaust to 'feed' the diffuser?

Again, I'm genuinely interested! I think just a couple legitimate questions, please don't take this the wrong way!

Thanks for the questions. I don't mind the critique, I'm a bridge engineer by trade (hydraulics) so I'm applying what I know about that kind of flow for automotive applications. So I welcome as much critique as possible.

In terms of why, I plan on going Dinan Stage 3 on my car and tracking it eventually. I call it my "work in progress" (i.e. hobby). For normal driving, I doubt there will be anything noticible. At high speeds like 100+ kmph, there should be a noticible difference. Plus I'm more of a person who likes to buy things for functional values, not just cosmetics: the engineering spirit right?

I should have enough room. I already printed out some 1:1 paper templates from my CAD model to test for fitment and to figure out my bolt spacings. At the moment I'm putting two fins to the left and right of the muffler cans, and one fin in between. The one fin is shorter so it terminates right before the muffler can. (On the 335, I have two muffler cans so that's 6 fins total). I'm considering adding two more fins in just to make up for the shorter fin by the muffler cans. The fins towards the center will be the largest and deepest. I had trouble getting the fitment right but this will actually go right between the muffler cans and there will be room for it.

I was actually looking into doing a full smooth undertray. However, that might be a later stage (however, not that much later to be honest...it will help with corrosion issues of the underbody too since there will be no more exposed steel). For that it'll take much longer to do plus if I'm just using the bolts that are holding together the body pieces, it might not be strong enough (it'll be a cantelever structure essentially...technically it'll have to be bolted together from multiple panels but this will be tough to do). I'm already leaving room in my existing diffuser design for that and thinking about how I can do this and still make the car servicable by BMW (BMW Toronto techs would hate me if they had to do an oil change on that...will probably need some sort of removable panels for them to do oil changes with). The only issues I have is with the muffler. I might have to find some sort of venting method so that I don't create a void space of really hot air. Still figuring out that. It might not be something I can mass produce though since I might have to drill connection holes myself on the spot to make it fit.

In terms of testing various designs, there's not much theoretical calculation I can go with in terms of what design works best. Even with the best simulations, there are too many factors that will cause errors in the calculation. I simply don't have access to the detailed CAD drawings that BMW has. The best method is going out and testing it. By that I mean wind tunnel testing.

Of course...that's pretty tough to do right now. Fortunately, in my experience, we don't build full size models to test. With bridges, we like to build scale models and adjust various coefficients and factors to account for scale. In my case, I'm using a 1:18 E90 scale model. Comparing the scaled dimensions with the drawings from the BMW brochure (I used the PDF and imported them into CAD to do a 1:10 drawing for design and manufacturing), the scaled dimensions are pretty much dead on. And yes, I plan on finding a wind tunnel to test the full sized version just for fun even if I don't sell it. You might be able to help me here? Know of any wind tunnels in the GTA?

[Rampage9]

When I worked at Honda, we used to go to Detroit for wind tunnel testing as there wasn't anything around local. However, since then there is a brand new one that opened up at UOIT that I've read about.

But the cost is like $1000/hr !

[Andy M]

Originally Posted by Rampage9

When I worked at Honda, we used to go to Detroit for wind tunnel testing as there wasn't anything around local. However, since then there is a brand new one that opened up at UOIT that I've read about.

But the cost is like $1000/hr !

That's what I read too.

The option I'm considering is just getting the underside of the car laser-scanned (buying a laser scanning unit would be cheaper than the wind tunnel) and then doing a theoretical model based on that.

To be honest, I don't think there's much I can do other than track the car without the additions and then afterwards and see if I notice any difference with the handling. In all honesty, I think a lot of the companies that produce racing parts for Ferrari's, Camaro's, etc don't actually go and wind tunnel test their products either. It's just based off a theoretical design and then put in a simulator.

...then again if you think about it, every building you occupy and every bridge you drive over was designed on a computer in a theoretical setting. The only test whether it works or not is whether the structure stands or not...

[Havok_2011]

RWDI in Guelph has a full sized wind tunnel. They were the only ones with one before the University got one as well. My firm has dealt with them in the past, and I have been to their facility. Pretty cool stuff.

[Andy M]

Just an update, the first prototype is completed for the rear diffuser. The material and workmanship turned out quite well, however I need to do some design changes. Just had dental surgery so things are going a bit slow for me (note to all, don't get your wisdom teeth out when you're in your late 20's...it's not exactly fun. Also, try not to wake up during the surgery either...)

I realized that to make this work on other 3 series models, it's a very simple design change.

Renders to come soon once I get my design changes done, one of them is a major cosmetic design change. Photos to come soon once my second (and final) prototype is made and mounted!

Still trying to come up with ideas on testing the function of it...wind tunnel would be cool but might cost me way more $$$ than it's worth. Might just see if anyone is interested in buying it as a cosmetic mod to be honest (and yes admins, I'll get a sponsor account).

[gian335]

cant wait to see your design.......subscribed

[Andy M]

Well one thought I had was to make some sort of rear diffuser + cosmetic part. This will have a CF part onto the back so it gives non-M-tech owners like me something nicer on the back without having to break the bank for a new bumper. To be honest, that's the majority of this redesign, the others were very very minor.

(It's just a little hard to do CAD3D while on Tylenol 3...)

[Andy M]

As promised, a render superimposed on an 335 E90 LCI.

Finished the final design and drafting last night and sent for fabrication now.

Parts consist of a carbon fiber lip, 5052-H32 aluminum, and it uses the existing bolt holes under the car for attachment (CF piece will have to use existing bolt holes and double-sided 3M tape).

I'll probably post some more updates later. Updates usually go through my Twitter account first though (andrewdmah).

I'm considering also building some more aero parts since I've really gotten the hang of this 3D CAD. Everything's done in 3D first and then I use that to make my fabrication drawings. I'll post some screenshots of the work in progress as well as some photos of the first prototype later too.

[JunkStory]

Nice.. that's what a real diffuser should be.

[Andy M]

Thanks. It's been a while in the making and lots of ignoring my fiancee in the evenings (she's so understanding about it though! She's so awesome).

Btw, can one of the mods PM me about details on how to become a vendor? After going through the whole production of this, I'm really considering making more of these things or even doing custom aero design work as well.

[335isGuy]

Originally Posted by Andy M

Thanks. It's been a while in the making and lots of ignoring my fiancee in the evenings (she's so understanding about it though! She's so awesome).

Btw, can one of the mods PM me about details on how to become a vendor? After going through the whole production of this, I'm really considering making more of these things or even doing custom aero design work as well.

Anything for a E36 M3? Could possibly be a test car? I run the car in the casc and mosport bmw club events as a instructor.

[Andy M]

Originally Posted by 335isGuy

Anything for a E36 M3? Could possibly be a test car? I run the car in the casc and mosport bmw club events as a instructor.

I'll discuss this with you over PM. Don't want to look like I'm trying to push a sale or something until I can figure out how to get vendor status.

[dulak]

Andy, this was exactly what I was looking for. PM me with more details on your project, I would definitely be interested in this.

[gian335]

nice work, looks great and could be even better if it's truly functional. anything for e92 mtech rear?

[turugara]

Subscribed. LCI m-sport, very interested to see how this turns out. What's your guesstimate on cost?

[Havok_2011]

Nice work Andy! Like it so far

[triggz]

some e92 love would be nice

[Havok_2011]

Originally Posted by triggz

some e92 love would be nice

No worries, he is working on an E92 version as well.