Rear Diffuser Build

I've been saying for a long time that I'd make posts about the stuff I'm doing... maybe it's time I actually do that lol

Today's post: Functional rear diffuser build.

Parts used at this point:
2'x4', 1/8 ABS sheet ($20)
2'x4', 1/4 ABS sheet ($27)
16"x36", 3/4 Laminated Pine Shelf X 2 ($28)
1.25"x1.25" Angle Iron - Two 5" lengths (one side has a 1 inch section removed creating a tab, which is bent up to match the required angle (7 deg.) ($scrap)
Triple corner bracket X 2 (sandwiched by angle iron between front and rear assemblies) ($4)
3" x 7" Mending Plate X 2 ($6)
#6x32 Machine Screw X 8 ($2?)
#6 Washer X 8 ($1?)
#6-32 Nylock Nut X 8 ($1.50)
1/4-20 Bolt @ 0.75" X 24 (Box of like 50 for $14)
1/4-20 Bolt @ 1.25" x 12 ($3?)
1/4-20 Bolt @ 1.50" x 4 ($2?)
1/4 Washer X 54 (Box of 100 for $12)
1/4-20 Nut X sum of ^^that (Box for $8)
1.5" double wide corner bracket X 4 (for strakes) ($6)
1.5"x2.5" corner brace X 4 (for wings) ($5)

Parts being installed next:
1" wide steel strip - from wing tip back to main body, bent at 90 and up to the ABS, fastened with one 1/4-20 bolt near the wings insertion point. ($9)
1/4-20 RivNut near the end of the wing ($scrap)
2" long 1/4" eye bolt mounted to the RivNut ($3)
3" long 1/4" eye bolt mounted above the 1/4-20 bolt back at the main body through the upper end of the bent steel strip ($3)
3/16" 7x19 coated aircraft cable attached through thimbles and cable clamps -> the 3" long eye bolt can then be tightened to pre-load the wing. ($6)

(total cost... very very roughly = ~$160)

The design:
Attachment 4045

The design... again:
www.youtube.com/watch?v=EDy-UcvuLCM



The ABS sheets
Attachment 4043

The Pine which was cut using printed templates
Attachment 4044

the two sections and wings laid in place + front and rear assemblies separately
Attachment 4046

Assembled
Attachment 4047

Underside view
Attachment 4048

Once the cutting out of the parts was complete I shaped the two short sections for the front using a sander (sloped the entrance inwards) -> the mid wing (large black thing - shown in red on the design) was hand planed in to an aerofoil using another template and its angle of attack set to 4.5 degrees.

The rear pine pieces each diverge from mid-line at 7 degrees - the insertion angle of the rear and front assemblies is tangential, with the ABS sheet on the rear on a smooth curve (25' diameter) - mid point of the curve is 6.5 degree's from horizontal, ending angle is 11 degrees from horizontal. The idea being that, the mid-wing will assist in energising the boundry layer, helping air ride the upper surface - the 6-32 screws are also all offset from each other in this area and left at 0.5" in to the air stream so that their tiny bodies may provide a uniformly spaced set of vorticies.

Questions / comments / etc. are welcomed - I also have a tonne of pictures regarding the removal of the interior components and carpet to put down dynamat, and I've got flares going on soon... and I'm having custom side skirts, somewhat similar to PCI Racings but "better" being priced right now.

I like you. ����
Can't wait to see it on the car. Looks great so far. Any plans to make more?

A friend of mine has an audi A4 and wants a decorative one from 1 piece of ABS (or 2... specifically no wood) to mimic a rocket bunny, so come october I'll be taking measurements and stuff. I think if I do this again I'll remove the wood from the equation - having never worked with thick ABS before I didn't realise just how strong and resistant it is. The aerofoil could've just been a curved groove to bend a panel in to shape and mounting points on the side could all have been 1/4 - I'd just reinforce some points with extra wide washers. The cost would likely come out to that same.

Being a first run though, and retaining wood templates if I wanted to build it again, time wise I'm sitting at ~3H cutting ~4H sanding and planing 2 nights wasted to painting - (reason alone to replace the wood with ABS, and I still have to give the wood a final coat of penetrol and a a thick coat of plastidip just to seal all the tiny gaps), ~5H bending / assembly... and like 7H of shopping for parts lol side note - only the really beefy metal brackets from homedepot will accept 1/4-20 bolts... found that out the hard way and drilling out holes can take a while with so many of them.

To do it again but solid ABS... it'd likely drop from the ~19 hour first run, to about 6 if I preemptively had all the mounting hardware and bolts figured out

Quote:

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Originally Posted by Azmodon

A friend of mine has an audi A4 and wants a decorative one from 1 piece of ABS (or 2... specifically no wood) to mimic a rocket bunny, so come october I'll be taking measurements and stuff. I think if I do this again I'll remove the wood from the equation - having never worked with thick ABS before I didn't realise just how strong and resistant it is. The aerofoil could've just been a curved groove to bend a panel in to shape and mounting points on the side could all have been 1/4 - I'd just reinforce some points with extra wide washers. The cost would likely come out to that same.

Being a first run though, and retaining wood templates if I wanted to build it again, time wise I'm sitting at ~3H cutting ~4H sanding and planing 2 nights wasted to painting - (reason alone to replace the wood with ABS, and I still have to give the wood a final coat of penetrol and a a thick coat of plastidip just to seal all the tiny gaps), ~5H bending / assembly... and like 7H of shopping for parts lol side note - only the really beefy metal brackets from homedepot will accept 1/4-20 bolts... found that out the hard way and drilling out holes can take a while with so many of them.

To do it again but solid ABS... it'd likely drop from the ~19 hour first run, to about 6 if I preemptively had all the mounting hardware and bolts figured out

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Sweet bro

:P once I get the mounting figured out, we'll see. It will likely require drilling holes in to where the jack is located / modifying the foam block to accept the rods - I'm thinking I'll square off the end of threaded rods and use long couplers, that way 2 wrenches can be used to adjust the height and angle at the rear - the front end will basically be touching the rear axles triangle box... thing.

or... reinforced stainless corner brackets attached to the rear plate behind the bumper. Use 2 more chunks of angle iron, bolt them on at an angle, bracket to that so the rods are slightly tipped, preload it laterally so it can't swing... I'll have to draw this up to figure out the angles/length of rod/mounting locations... I'll be back some time next week with stuff

Very cool! Subscribed.

I love it when people make stuff for their cars.

Looking forward to seeing this installed.

I'm trying my damnedest to have it on for for the weekend... but the car's had to go back in to the exhaust shop this morning as it was melting my gas tank -> the flares should be mounted this evening as they're priority 1, but if everything looks fixed and there's daylight left I hope to get it on...

Mounting is going to be "fun" - the only option is supporting the rear section through the floor - there's a control box and the muffler on the other side are blocking any chance of using exterior accessible positions -> 7/16ths rod and some nylocks will do the trick...

The front though, the triangular tunnel that is the rear "axle" I guess, the nose of this thing extends just past it, and will basically be touching it at full extension (while jacked up)... should give me ~3 inches of ground clearance, which doesn't sound like a lot but I only have about 1.5 inches of travel before the body makes contact with the rubber of my tires... so it should be good :) but "mounting it" may involve metal strapping wrapped around the metal rods that hold the spare tire in place. Hence it "may not" be on before Quebec lol

mini update - wing braces complete, steel on steel riv-nuts, cable's a little thicker than is stylish but at the extremely acute suspension angle, a 160lbs downward force can create a tension in excess of 4K... and I've figured out how I'm going to mount it finally so, aluminum tape to fill the gaps and a coat of plasti-dip to unify / last line of defence against water... and a pair of holes in the trunk... and she's ready to go on.

Attachment 4173
Attachment 4174
Attachment 4175
Attachment 4176

Currently I'm redesigning the redesigned splitter, because Infinity Wings are a thing and I'm not a huge fan of canards. The fun part now is getting the splitters inboard tire wake controls to feed the diffuser, while the outboard infinity wings interact with the skirts... yay CFD

So much want.

can we get some pictures of it mounted up!! looks great!

it's on a very large list of things I have planned to complete before the first race, basically once the snows gone I'll be back with an update.

Can we have some more of these thingys ?!?!lol holy look at em all

Might be overkill with an enclosed wheel :P but I'll be starting to print and mould the infinity wings for the splitter this weekend.

Only six months late...

I don't know how many clips are along the bottom, I don't care to count them since they're no longer on the car lol. The only thing holding the bumper on (as it's been for months) is the 2 bolts under the tail lights, the clips along the edge, and the flares. (also removed the bolt and nut at the termination point in the wheel well.

So - removed lights, bumper, and foam thing:
Attachment 4973

If you don't have an impact wrench, get to twisting... spare tire needs to come off and the cage removed - once removed you can unbolt the latching bracket for it near the rear of the car - there's also the hub mount for the spare hanging down in the middle:
Attachment 4974

Used a jack to mock up the location and measure for support rods - homedepot had a sale on 3/4" thick wall aluminum tubing... so summon your inner smith and get to banging
Attachment 4975

There's a dimple in the middle of the rear end, being stiffer than the surroundings, that's the centre of the truss assembly - 1/4"-20 steel riv nut insterted
Attachment 4976

For the front end, another dimple a few inches off centre was picked and drilled through in to the trunk and sealed with silicone - and the hub pad for the spare had its central hole enlarged to receive a 'PlusNut' (giant rivnut that can take ~2500 pounds) - 1/4-20 stainless steel rod and front mounting tube attached giving the front 2 points of contact - a third that's not pictured is from the remaining cage bracket right above the rear axle, gives the front two separate supports at 45 degrees to one another, and the threaded rod applies tension to both.
Attachment 4977

The cage's front latch is in the shot below for clarity, it has a slot so it's more forgiving for positioning, you can also see the hole pre-enlargement:
Attachment 4980


Moving back to the rear, a washer stack was used behind the second tube to bring it off the body panel, and 2 strips of neoprene (chemical / petroleum etc resistant) rubber were zip tied to the central pin and around each tube so movement doesn't cause tinking:
Attachment 4979
Attachment 4978

The next step was balancing the unit on my chest while doing the worm to get under there. Had a friend install the two bolts at the truss while I mounted the front end - once it was up, it was still wobbling in the rear (truss only acts on a plane, need the third datum)

A long time ago I had ordered two sets of splitter support rods from asia, they impressed me because the threaded ends are cut from the main rod, it's not a threaded rod inside a decorative casing, and the thing's some type of steel given they're ferritic. While I didn't end up needing them in the front, to my pleasant surprise they just so happen to be exactly the right length to reach from pre-drilled holes I made "just in case", and the windows in the crash bar.

Attachment 4981

So the smaller of the two made the rear end completely rigid. But after some leaf blower testing my cable support system wasn't stable - the main body prevents rotation on the Z, the cable prevents up and down flapping (rotation on the Y), but the winglette could still feather from 7 degrees to flat near the tips, speed dependent, as it rotated on the X... so the long rods run from 1.5" from the leading edge, to the middle mounting hole for the vents I printed... and tada 'dat booty doh'

Attachment 4982

Kicking it feels stiffer than most body panels

Hot damn! Will look good on the track at Tremblant!

its unique and a feat of engineering....