# Venturi tunnels (ground-effect floor)

> How do venturi tunnels under a car create downforce?

- 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/venturi-tunnel-floor/ · JSON: https://xemstudio.com/api/geometry?object=venturi-tunnel-floor · Plain text: https://xemstudio.com/objects/venturi-tunnel-floor/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

- Shaping the underside into channels that narrow and then expand speeds up the air beneath the car and lowers its pressure, pulling the car down (Wikipedia). Skirts or close-fitting edges keep side air out.
- The downside is sensitivity: as the centre of pressure moved with speed, attitude and ride height, early ground-effect cars began to rock or 'porpoise' (Wikipedia). Diffuser studies show the same thing: below a critical ride height the vortex flow breaks down and downforce drops (Mahon et al.).
- Used on: Lotus 78, Lotus 79, Brabham BT46B.
- The file is a reference example built by Xem Studio from the numbers above (Tunnel modelled as long diffuser; 10 deg per Cranfield best-efficiency; size illustrative.). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.

## Sizes

| parameter | value or range | source |
|---|---|---|
| Lotus 78 sidepod form | inverted aerofoils sealed with skirts | Wikipedia |
| Fan car (Brabham BT46B) | raced once, 1978 Swedish GP | Wikipedia |
| Ground-effect ban | flat floors from 1983 until 2022 | Wikipedia |
| Diffuser force-reduction onset (17 deg) | below hr/d 0.0573 | Southampton (Mahon et al.) |

## Where the files are

- Wikipedia: [Ground effect (cars)](https://en.wikipedia.org/wiki/Ground_effect_(cars)) — 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/venturi-tunnel-floor/index.json · license: https://xemstudio.com/files/hardware/venturi-tunnel-floor/LICENSE.md

- venturi-tunnel-floor-diffuser: [STEP](https://xemstudio.com/files/hardware/venturi-tunnel-floor/venturi-tunnel-floor-diffuser.step) · [STL](https://xemstudio.com/files/hardware/venturi-tunnel-floor/venturi-tunnel-floor-diffuser.stl) — Diffuser: 1500 mm ramp at 10° behind a flat floor, 400 mm wide, 3 fences

## Questions

**What is porpoising?**

A bouncing oscillation when the aero centre of pressure shifts with ride height and interacts with the suspension (Wikipedia).

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- [Single-element rear wing (race car)](https://xemstudio.com/objects/rear-wing-single-element/)
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