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Venturi tunnels (ground-effect floor)

How do venturi tunnels under a car create downforce? · Standard part · basis: Design rules and figures from the cited sources; example geometry by Xem Studio

Also called: venturi tunnels, ground effect floor, ground effect tunnels, underfloor tunnels, venturi floor car, porpoising, F1 ground effect, wing car, underbody downforce, lotus 79 venturi

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

sizes
4 in table

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.

Venturi tunnels size chart

parametervalue or rangesource
Lotus 78 sidepod forminverted aerofoils sealed with skirtsWikipedia
Fan car (Brabham BT46B)raced once, 1978 Swedish GPWikipedia
Ground-effect banflat floors from 1983 until 2022Wikipedia
Diffuser force-reduction onset (17 deg)below hr/d 0.0573Southampton (Mahon et al.)

Where the files are

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

WikipediaGround effect (cars)webPublisher's page; dimensions are facts
University of SouthamptonThe Evolution of Edge Vortices Underneath a Diffuser Equipped Bluff Body (Mahon, Zhang & Gage)webPublisher'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

venturi-tunnel-floor-diffuserDiffuser: 1500 mm ramp at 10° behind a flat floor, 400 mm wide, 3 fencesSTEP · STL
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Plain text: sheet.txt · JSON: /api/geometry?object=venturi-tunnel-floor

Venturi tunnels (ground-effect floor), isometric render of a generated part
Render of venturi-tunnel-floor-diffuser from the free STEP; not to scale.

Fits with

Single-element rear wing (race car), isometric render of a generated part
Single-element rear wing (race car)7 sizes★ 1 file
Dual-element (two-element) rear wing, isometric render of a generated part
Dual-element (two-element) rear wing9 sizes★ 1 file
Swan-neck rear wing mount, isometric render of a generated part
Swan-neck rear wing mount5 sizes★ 1 file
Gurney flap / wickerbill, isometric render of a generated part
Gurney flap / wickerbill7 sizes★ 1 file
Wing endplates / side dams, isometric render of a generated part
Wing endplates / side dams6 sizes★ 1 file
Front splitter, isometric render of a generated part
Front splitter8 sizes★ 1 file

Questions

What is porpoising?

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