{
 "version": "2026-10-20",
 "license": "Dimension sheets: CC BY 4.0. Standard and published-size files: CC BY 4.0. Xem Studio's own designs (files_license = Xem Studio Design License): free to use and sell things from, not to redistribute as files — https://xemstudio.com/license/. Other publishers' files: their own license, linked beside them.",
 "slug": "venturi-tunnel-floor",
 "name": "Venturi tunnels (ground-effect floor)",
 "kind": "standard",
 "category": "Car design",
 "group": "Car aero",
 "year": null,
 "question": "How do venturi tunnels under a car create downforce?",
 "tags": [
  "vehicle design",
  "car aero",
  "rc design",
  "aerodynamics"
 ],
 "aliases": [
  "venturi tunnels",
  "ground effect floor",
  "ground effect tunnels",
  "underfloor tunnels",
  "venturi floor car",
  "porpoising",
  "F1 ground effect",
  "wing car",
  "underbody downforce",
  "lotus 79 venturi"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": false,
 "free_files": 1,
 "url": "https://xemstudio.com/objects/venturi-tunnel-floor/",
 "sheet_txt": "https://xemstudio.com/objects/venturi-tunnel-floor/sheet.txt",
 "sheet": {
  "basis": "Design rules and figures from the cited sources; example geometry by Xem Studio",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "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."
  ],
  "table": {
   "columns": [
    "parameter",
    "value or range",
    "source"
   ],
   "rows": [
    [
     "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.)"
    ]
   ]
  }
 },
 "sheet_text": "Venturi tunnels (ground-effect floor) — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/venturi-tunnel-floor/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)\nBasis: Design rules and figures from the cited sources; example geometry by Xem Studio\nCatalog version: 2026-10-20\n\nSizes: see table below\n- 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.\n- 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.).\n- Used on: Lotus 78, Lotus 79, Brabham BT46B.\n- 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.\n\nparameter  value or range  source\nLotus 78 sidepod form  inverted aerofoils sealed with skirts  Wikipedia\nFan car (Brabham BT46B)  raced once, 1978 Swedish GP  Wikipedia\nGround-effect ban  flat floors from 1983 until 2022  Wikipedia\nDiffuser force-reduction onset (17 deg)  below hr/d 0.0573  Southampton (Mahon et al.)\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Wikipedia: Ground effect (cars) — web — Publisher's page; dimensions are facts — https://en.wikipedia.org/wiki/Ground_effect_(cars)\n- 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\n\nFree files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/venturi-tunnel-floor/index.json\nAPI: https://xemstudio.com/api/geometry?object=venturi-tunnel-floor\n",
 "sources": [
  {
   "publisher": "Wikipedia",
   "title": "Ground effect (cars)",
   "url": "https://en.wikipedia.org/wiki/Ground_effect_(cars)",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "University of Southampton",
   "title": "The Evolution of Edge Vortices Underneath a Diffuser Equipped Bluff Body (Mahon, Zhang & Gage)",
   "url": "https://eprints.soton.ac.uk/22957/1/paper_35_2.pdf",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "rear-wing-single-element",
  "rear-wing-dual-element",
  "swan-neck-wing-mount",
  "gurney-flap-wickerbill",
  "wing-endplates",
  "front-splitter"
 ],
 "fits": [],
 "dimensions": "exact (nominal, no printing tolerance)",
 "dimension_rule": "All geometry on Xem Studio is exact (nominal, true size) with no printing tolerance. Where a part meets another part, the object lists its mating features (feature, exact size, what it mates with, fit class: press, slide, run, socket, clamp or designed). Apply your process's allowance to those features only; 'designed' gaps are part of the design and stay as drawn.",
 "mating": [],
 "faq": [
  {
   "q": "What is porpoising?",
   "a": "A bouncing oscillation when the aero centre of pressure shifts with ride height and interacts with the suspension (Wikipedia)."
  }
 ],
 "files_license": {
  "name": "Xem Studio Design License",
  "holder": "Xem Studio",
  "url": "https://xemstudio.com/license/",
  "note": "Xem Studio's own design: free to use, print, modify and sell things made from it; not to be republished or redistributed as files."
 },
 "offers": [
  {
   "file": "venturi-tunnel-floor-diffuser",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "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",
   "note": "Diffuser: 1500 mm ramp at 10° behind a flat floor, 400 mm wide, 3 fences"
  }
 ],
 "markdown": "https://xemstudio.com/objects/venturi-tunnel-floor/index.md"
}