Venturi tunnels (ground-effect floor) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/venturi-tunnel-floor/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
Catalog version: 2026-10-20

Sizes: see table below
- 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.

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.)

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Wikipedia: Ground effect (cars) — web — Publisher's page; dimensions are facts — https://en.wikipedia.org/wiki/Ground_effect_(cars)
- University of Southampton: The Evolution of Edge Vortices Underneath a Diffuser Equipped Bluff Body (Mahon, Zhang & Gage) — web — Publisher's page; dimensions are facts — https://eprints.soton.ac.uk/22957/1/paper_35_2.pdf

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/venturi-tunnel-floor/index.json
API: https://xemstudio.com/api/geometry?object=venturi-tunnel-floor