# Rear underbody diffuser

> What does a rear diffuser do and what angle works best?

- Category: Car design › Car aero
- Kind: Standard part
- Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
- Page: https://xemstudio.com/objects/rear-diffuser/ · JSON: https://xemstudio.com/api/geometry?object=rear-diffuser · Plain text: https://xemstudio.com/objects/rear-diffuser/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

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

## 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.

## Sizes

| 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

- University of Southampton: [The Evolution of Edge Vortices Underneath a Diffuser Equipped Bluff Body (Mahon, Zhang & Gage)](https://eprints.soton.ac.uk/22957/1/paper_35_2.pdf) — 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)](https://dspace.lib.cranfield.ac.uk/server/api/core/bitstreams/9b476386-44d0-4c0f-a49e-c3372274d351/content) — web — Publisher's page; dimensions are facts
- SAE International: [The Aerodynamic Performance of Automotive Underbody Diffusers, SAE 980030 (Cooper et al.)](https://saemobilus.sae.org/papers/aerodynamic-performance-automotive-underbody-diffusers-980030) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/rear-diffuser/index.json · license: https://xemstudio.com/files/hardware/rear-diffuser/LICENSE.md

- rear-diffuser-diffuser: [STEP](https://xemstudio.com/files/hardware/rear-diffuser/rear-diffuser-diffuser.step) · [STL](https://xemstudio.com/files/hardware/rear-diffuser/rear-diffuser-diffuser.stl) — Diffuser: 200 mm ramp at 10° behind a flat floor, 150 mm wide, 3 fences

## Questions

**Why does downforce suddenly drop very low?**

Below a critical ride height the diffuser edge vortices break down and downforce falls while drag rises (Mahon, Zhang & Gage).

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