MINI Cooper S rear diffuser w/ underpanel
 

From the rear valance, my design extends back 26" to the rear subframe and is nearly 4ft wide. From the subframe, the underpanel begins a gentle rise at the same angle another 12" back. So we are talking about nearly 12 square ft of area acting on the air.

http://img72.imageshack.us/img72/4629/venturism8.jpg

A and B of diagram above, the best angle for drag reduction (CD) is from 2 and 4 degrees.

A (like mine), the best angle for reducing lift (CLR) is 6 degrees.
B, the best angle for reducing lift (CLR) is 11 degrees (at the expense of increased drag)

Made out of black coroplast, total weight with strakes, just over 4lbs.

http://lh6.ggpht.com/_tPeOcA9yGbA/TM...Y/IMG_4341.JPG
http://lh6.ggpht.com/_tPeOcA9yGbA/TM...M/IMG_4342.JPG

Visual indication of how far back the 6deg rake goes back to combine with the flat underpanel:

http://lh3.ggpht.com/_tPeOcA9yGbA/TM...mNBY/angle.jpg

Now for the shocking results!

I drive the same commute to work everyday and usually average about 26-27mpg. After logging 106 miles with the diffuser, I filled it back up with 3.538 gallons....that's now an avg of 29.9mpg!!!:eek:

Joe

Awesome! Beautiful diffuser. How did you close out the coroplast edges, and how did you mount the strakes?

Thanks!
Just used painters caulk to close the trailing edge...I may revisit to clean the edge a bit more. For the strakes I had a brainstorm...they are coroplast pieces folded in half so bottom edge is perfectly flat...which also made them much more rigid. They reside in aluminum C channel adhered with Gorilla Glue.

Here is a view of the top side:
http://lh5.ggpht.com/_tPeOcA9yGbA/TN...I/IMG_4390.JPG

I'm jealous! I'm certain my Fit would benefit from a rear diffuser, and it would be relatively easy to do. Please let us know if your improved results hold up over time.

Another 302.1 miles, this time 26.9mpg (some of which was stuck in traffic, so i'm happy).

Nice work! It looks great and I think you will continue to see good results. I did a similar thing from just behind the catalytic converter and I am amazed what it did for my mpg. I may have to add some strakes like yours. I used some thin aluminum roof flashing to fill in between the fender liner and the diffuser. I'm in snowy Colorado and I didn't want that filling up with slush and snow.

The area where you put your diffuser was just open chassis before? Presumably it meets your belly pan on the leading edge...?

It looks like a really nice job, I hope I can make one just as nice for my Beetle.

Really nice work! Very clean, it looks life factory car!

After more logging, on average, my freeway mpg has increased 1mpg to 27mpg. I'm very pleased with the results!

How did you construct your diffuser?

Black Coroplast with aluminum C channels for the strakes and 2 perpendicular C channels on the top side for further support. Top side also is covered with aluminum tape to reflect exhuast heat. The strakes are also Coroplast pieces folded in half and glued into the C channels with Gorilla Glue.

Hope this helps...the folded strakes was a great revelation on my part!

Joe

I have since lowered the car .75"-1" (KW v2 coilovers) which resulted in an improved average of 29mpg!!!

Diffuser and underpanel is now more efficient closer to the ground.

sorry, im a visual learner. If it's not too much trouble, could you post more pictures of your diffuser? (ie. different angles and such).

This should help ya, these are pics of the 1st prototype made from wood...same basic shape, but easier to see because its white:
https://lh5.googleusercontent.com/_t...0/IMG_0738.JPG
https://lh3.googleusercontent.com/_t...0/IMG_0742.JPG
https://lh4.googleusercontent.com/_t...0/IMG_0743.JPG
https://lh3.googleusercontent.com/_t...0/IMG_0744.JPG

This is by FAR one of the most well constructed aeromods i've seen on this site.

This thing looks FACTORY. I give you a HUGE thumbs up man, that's great.

Now if I had a car instead of a truck.... I don't think a pickup would do much with something like that. My wife's Santa Fe though...

But yeah, that's sick dude.

I agree, it looks factory. I used to own a 2003 Mini.

Couldn't agree more. Really nicely integrated design. I have a 2009 Mini with the high performance JCW engine. Direct injected. Helps the mpg game.

What guided you on how many strakes and where to put them (or spacing them, or something :) )

Looks very factory, I agree. Well done!

very sweet and very clean!

The width of the Exhaust cutout lended itself to placing a strake on either side to channel airflow, the others are equidistant apart for asthetics.

The strakes probably do several things, but one of the most obvious is that they prevent low-speed, high-pressure air getting in along the sides of the car/tires from disturbing the flow in the central area of the diffuser.

this looks professional. How'd you make the strakes. They don't look like coroplast???

...they are coroplast pieces folded in half so bottom edge is perfectly flat...which also made them much more rigid. :thumbup:

Great tips for working with coroplast

Close up detail of the strakes

https://lh3.googleusercontent.com/_t...Y/IMG_4337.JPG

And from the side

https://lh3.googleusercontent.com/_t...w/IMG_4336.JPG

Really appreciate your posting the pictures. That makes it clear as glass how to whip one of these up. I'm going to make one for my 09 Mini.

How are the vertical strake attach to the coroplast. Just glue. Isn't the coroplast too flexible? Does it buffet around?

Thanks so much for the coroplast tips!!

Are you planning any treatments to the rear inner fender area? There's a glaring hole in the last pic. lol

The thought had crossed my mind, but it's not really necessary. The inner fender hole allows adequate ventilation for the rear exhaust and rear brakes. The strakes prevent most of the turbulent air from disrupting the flow of clean air through the diffuser.

The folded strakes (surprisingly rigid!, no bufeting whatsoever) reside in aluminium C channels glued in with gorilla glue. Gorilla glue makes for an extremely rigid bond.

The C channels are attached to the diffuser with nuts & bolts. Compared to the wooden prototype, the coroplast version weighs half as much, 4lbs!!!

Nice work dude,

Good to see that you have made something in keeping with the aethetics of your car. I've always like the look of the new Mini (despite having an 'old' one as a track car).

You've defo set a standard for me to aim for when I translate my plastic undertray into sheet metal.

Looks great thanks for sharing, how did you anchor the front? Have you seen the retro fit GP body pan??

Next time i'm under the car I'll have to take a pic of the front attachment point.

Yes on the GP underpanels. More common though, people retro the r56 panels for the r53 MINI, very similar to the GP panels but less expensive. Still, these are much more expensinve than my $20 4x8 sheet of black coroplast ;) Plus, there is no gap between the underpanal and the diffuser...the two actually attach to each other.

Cool that would be great, yes very true on the $$$$. My side skirts are made out of plastic rain gutters and the front air dam out of a rubber which I run year round.

Hey, John. How about a picture of your side skirts out of the plastic rain gutters? Thanks.

Sure I will have to do it tonight when I get home

side skirts

[ATTACH]http://ecomodder.com/forum/newreply....treply&t=15189[/ATTACH]

side skirts
http://ecomodder.com/forum/attachmen...1&d=1306136181

Very nice job there. Looks factory. I almost expected to see a BMW part number on those! :)