Library › Car design › Car aero ·

Rear underbody diffuser

Name: Rear underbody diffuser dimension sheet
Creator: Xem Studio
License: https://creativecommons.org/licenses/by/4.0/

What does a rear diffuser do and what angle works best? · Standard part · basis: Design rules and figures from the cited sources; example geometry by Xem Studio

Also called: rear diffuser, diffuser angle, underbody diffuser, car diffuser, diffuser design, how does a diffuser work, diffuser ramp angle, flat floor diffuser, rc car diffuser, diffuser ride height, diffuser downforce

Exact dimensions. True size, with no printing tolerance added.

sizes: 7 in table

Notes

A diffuser is an upswept rear floor section. Expanding the underbody flow lowers the pressure under the car, which adds downforce and can reduce drag if ride height is not too small (SAE 980030).
Southampton tests of 5, 10, 15 and 17 deg diffusers found downforce rising as ride height falls until edge vortices break down, after which downforce drops; the 17 deg diffuser peaked at CL 1.77 (Mahon, Zhang & Gage).
On a 35% DrivAer racing model, total downforce rose up to 10 deg then fell, and 10 deg gave the best efficiency (Cranfield / SAE 2018-01-0725).
Used on: DrivAer research model (high-performance configuration).
The file is a reference example built by Xem Studio from the numbers above (10 deg best-efficiency angle (Cranfield); length/width illustrative.). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.

Rear underbody diffuser size chart

parameter	value or range	source
Angles tested (bluff body)	5, 10, 15, 17 deg	Southampton (Mahon et al.)
Max CL, 17 deg	1.77 at hr/d 0.0573	Southampton (Mahon et al.)
Max CL, 15 deg	1.66 at hr/d 0.0446	Southampton (Mahon et al.)
Max CL, 10 deg	1.48 at hr/d 0.0446	Southampton (Mahon et al.)
Angles tested (DrivAer 35%)	0, 5, 10, 16 deg	Cranfield / SAE 2018-01-0725
Best efficiency angle	10 deg (AE 3.05 at h/h0 0.25)	Cranfield / SAE 2018-01-0725
5 deg vs 0 deg	max net downforce x2.15, drag reduced	Cranfield / SAE 2018-01-0725

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

University of Southampton	The Evolution of Edge Vortices Underneath a Diffuser Equipped Bluff Body (Mahon, Zhang & Gage)	web	Publisher's page; dimensions are facts
Cranfield University / SAE	On the Aerodynamics of an Enclosed-Wheel Racing Car (Soares et al., SAE 2018-01-0725)	web	Publisher's page; dimensions are facts
SAE International	The Aerodynamic Performance of Automotive Underbody Diffusers, SAE 980030 (Cooper et al.)	web	Publisher's page; dimensions are facts

Free files

★ Free files 1 parts. Xem Studio design: free to use, print and sell things from; not to be republished as files. Xem Studio Design License. index.json

rear-diffuser-diffuser

Diffuser: 200 mm ramp at 10° behind a flat floor, 150 mm wide, 3 fences

STEP · STL

Let your AI pull this. Add https://xemstudio.com/mcp to Claude, ChatGPT, Cursor or any MCP client (free, no account, one minute) and it can look up exact sizes, mating fits and free STEP/STL from every chat.

Set it up →

Plain text: sheet.txt · JSON: /api/geometry?object=rear-diffuser