Reference Beta

Universal Rear Diffuser

A rear diffuser from numbers you can measure off your own car: how wide the floor is where the diffuser starts and where it ends, how far it runs, and how high its two ends sit above the floor line. Strakes are laid out from a nominal channel width and rounded to a whole count, the outer pair can be full side walls, and the trailing edge can carry a lip. The preview shows it as it sits on the car, strakes down and tail up; switch the orientation to flat before you export and it lies body-side down on the bed with nothing overhanging. Like the splitter it is cut into tiles both ways, with dovetails, and the cuts across the car always land in the middle of a channel rather than through a strake.

aero
diffuser
downforce
rear
universal
tiled

This part is in beta. It renders and exports, but the geometry is still moving: dimensions may change between versions, and it has not been through a printed fitment check. Worth knowing before nine hours and a kilogram of filament.

Printing it

Material: PETG / ASA
Minimum wall: 3 mm
Bed splitting: Cut into tiles across and along the part, so a board wider than any printer bed still prints. Joints carry dovetails, so tiles cannot pull apart sideways.

Set Orientation to 'Flat for printing' before you export. That puts the flat body-side face on the bed with the strakes standing up, and nothing overhangs: the countersinks open upward at 45 degrees and every strake, wall and lip is a vertical wall. SIZING THE TILES: a tile across the car is a whole number of channels plus a 15 mm dovetail, and fore-and-aft it is the plate length along the ramp divided by the row count plus another 15 mm. The default 1200 x 600 mm plate in 5 channels: 5 tiles across and 3 rows gives tiles of roughly 255 x 217 mm. Solid infill through the fastener rows and the dovetails. BE HONEST ABOUT MATERIAL: a diffuser this size is over three litres of plastic and it sits in the hottest, dirtiest air on the car; print it to check fit, then consider cutting the plate from 6 mm ABS or aluminium composite and printing only the strakes.

What it works out for you

Recomputed from your numbers as you type, and shown beside the viewer.

Ramp angle (°)
Strakes
Actual channel width (mm)
Channel length : width
Plate length along the ramp (mm)
Plastic, roughly (L)

Parameters

Every number this part is generated from. The geometry is built from these rather than scaled from a fixed model, so a value you can measure off your own car is a value you can type in.

Outline

Parameter	Range	Default	What it does
Width, leading edge	300–1800 mm	900 mm	Across the car where the diffuser starts, usually just behind the rear axle line or wherever your floor ends. Cannot be wider than the trailing edge.
Width, trailing edge	300–2000 mm	1200 mm	Across the car at the trailing edge. The strakes divide this width, so it is also what sets how many channels you get from the pitch below.
Length	200–1200 mm	600 mm	Along the car from the leading edge to the trailing edge, measured on the floor, not up the ramp. The plate itself comes out a little longer than this because it runs up the slope.
Corner radius	0–200 mm	30 mm	Rounds the two rear outer corners. The side walls follow it round; the lip stops where it starts.

Ramp

Parameter	Range	Default	What it does
Leading edge height	0–80 mm	0 mm	How far the leading edge sits above the floor line. 0 is flush with the floor, which is what you want if the diffuser continues a flat floor. This only shifts the part; the ramp angle comes from the difference between the two heights.
Trailing edge height	0–400 mm	80 mm	How far the trailing edge sits above the floor line. With the length this sets the ramp angle, which is shown beside the viewer. Flow stays attached on a simple diffuser up to roughly 10-12 degrees; steeper mostly makes drag.

Strakes

Parameter	Range	Default	What it does
Channel width	60–1500 mm	240 mm	Nominal. The trailing-edge width is divided into a whole number of channels of about this size and the strakes go on the boundaries, so the count and the real pitch are shown beside the viewer. Set it wider than the plate for no strakes at all.
Strake height, front	5–200 mm	40 mm	How far the strakes stand off the plate at the leading edge. They taper straight to the rear height.
Strake height, rear	5–300 mm	90 mm	... and at the trailing edge. Roughly the trailing edge height is the usual choice, so the strakes reach down to about the floor line all the way along.
Side walls	On or off	On	Makes the outer edges full-height walls with the same taper as the strakes. They stiffen the plate enormously, and they are what stops air spilling in from the sides.
Trailing-edge lip	0–60 mm	15 mm	A small flap along the trailing edge on the strake side, the same thickness as the plate. 0 for none. Adds downforce for its size at the cost of drag; 10-20 mm is the usual range.

Plate

Parameter	Range	Default	What it does
Plate thickness	4–20 mm	8 mm	The lip is the same thickness. 6-8 mm prints well and stays flat; thicker mostly buys weight.
Strake thickness	3–15 mm	6 mm	Strakes and side walls. They are vertical walls when printed, so this is exactly the wall you get.

Mounting

Parameter	Range	Default	What it does
Fastener size	M4 · M5 · M6 · M8	M6	For the rows that fix the plate up to the floor or the rear bumper structure. Holes are ISO 273 normal-fit clearance.
Countersink heads	On or off	On	Sinks every head flush into the aero face -- the strake side, which is the side you can reach with the diffuser under the car. Off for plain clearance holes.
Mounting row span	100–1800 mm	700 mm	Lateral distance from the first hole to the last on the leading-edge row. Keep it inside the leading-edge width.
Nominal pitch	40–400 mm	150 mm	Spacing along the row, nudged so it divides the span exactly. A hole that would land under a strake, on a tile joint or off the plate is dropped rather than made useless.
Row setback	10–300 mm	30 mm	How far the leading-edge row sits aft of the leading edge.
Trailing-edge row	On or off	Off	A second row 30 mm forward of the trailing edge, same size, pitch and span, for hanging the tail off the bumper beam. Holes that would land in the lip are dropped.

Tiling

Parameter	Range	Default	What it does
Fit to my printer	On or off	Off	Work the tile grid out from the bed size below instead of setting the two counts by hand. The resulting grid and the largest tile are shown beside the viewer.
Printer bed	100–600 mm	220 mm	The largest square that fits on your bed. 220 for an Ender 3 or a Bambu A1 mini, 256 for an X1 or P1, 350 for an X1E or a large Prusa. Tiles come out this size less the 15 mm a dovetail reaches into its neighbour.
Tiles across the car	1–10	1	The cuts land in the middle of a channel, never through a strake, so a tile is a whole number of channels rather than an equal share of the width. Leave at 1 while you dial the shape in.
Which tile	1–10	1	1 is the leftmost. Render and download each in turn with everything else left alone.
Tiles fore-and-aft	1–6	1	Cuts the plate into equal rows along the ramp. The strakes are cut with it and butt back together; the dovetail is in the plate.
Which row	1–6	1	1 is the frontmost. A front row can be empty out at the corners of a plate that narrows a lot towards its leading edge, and says so.
Joint clearance	0.1–0.6 mm	0.25 mm	Gap between a dovetail and its socket, per side. 0.25 mm is a firm slide fit on a well-tuned printer; go up if your first pair will not go together.

Export

Parameter	Range	Default	What it does
Orientation	As fitted to the car · Flat for printing	As fitted to the car	'As fitted' shows the diffuser the way it sits under the car: strakes down, trailing edge up at the ramp angle. 'Flat' lays it body-side down for the bed. Export in flat, or your slicer will have to guess.

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