# Diffuser fences / longitudinal separators

> Why do diffusers have vertical fences (strakes)?

- 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/diffuser-fences-strakes/ · JSON: https://xemstudio.com/api/geometry?object=diffuser-fences-strakes · Plain text: https://xemstudio.com/objects/diffuser-fences-strakes/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

- Diffuser fences are vertical plates running fore-aft in the diffuser. Diffuser downforce depends on vortices formed at its edges near the inlet, which move inboard and upward downstream (Mahon, Zhang & Gage); fences organise that flow.
- On the Cranfield DrivAer model, separators at 5 deg and 100 mm spacing in a 10 deg diffuser trimmed front downforce by ~0.03 CL, improved balance at low ride height and cost only 3-5 drag counts (SAE 2018-01-0725).
- The file is a reference example built by Xem Studio from the numbers above (Fence spacing 100 mm per Cranfield test (35% model).). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.

## Sizes

| parameter | value or range | source |
|---|---|---|
| Separator angle / spacing tested | 5 deg, 100 mm spacing in 10 deg diffuser | Cranfield / SAE 2018-01-0725 |
| Front downforce change | dCL about 0.03 lower | Cranfield / SAE 2018-01-0725 |
| Drag penalty | 3, 5, 4 counts at h/h0 0.25, 0.50, 0.75 | Cranfield / SAE 2018-01-0725 |
| Edge vortex origin | near diffuser inlet | Southampton (Mahon et al.) |

## Where the files are

- 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
- 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

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

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

- diffuser-fences-strakes-diffuser: [STEP](https://xemstudio.com/files/hardware/diffuser-fences-strakes/diffuser-fences-strakes-diffuser.step) · [STL](https://xemstudio.com/files/hardware/diffuser-fences-strakes/diffuser-fences-strakes-diffuser.stl) — Diffuser: 200 mm ramp at 10° behind a flat floor, 300 mm wide, 3 fences

## Questions

**Do fences add downforce?**

In the Cranfield test they mainly shifted balance (less front downforce) at a small drag cost.

## Fits with

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