{
 "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": "diffuser-fences-strakes",
 "name": "Diffuser fences / longitudinal separators",
 "kind": "standard",
 "category": "Car design",
 "group": "Car aero",
 "year": null,
 "question": "Why do diffusers have vertical fences (strakes)?",
 "tags": [
  "vehicle design",
  "car aero",
  "rc design",
  "aerodynamics"
 ],
 "aliases": [
  "diffuser fences",
  "diffuser strakes",
  "diffuser separators",
  "diffuser fins",
  "underbody strakes",
  "diffuser vanes",
  "diffuser dividers",
  "rear diffuser fins",
  "diffuser vortex",
  "strakes under car"
 ],
 "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/diffuser-fences-strakes/",
 "sheet_txt": "https://xemstudio.com/objects/diffuser-fences-strakes/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": [
   "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."
  ],
  "table": {
   "columns": [
    "parameter",
    "value or range",
    "source"
   ],
   "rows": [
    [
     "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.)"
    ]
   ]
  }
 },
 "sheet_text": "Diffuser fences / longitudinal separators — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/diffuser-fences-strakes/  (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- 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.\n- 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).\n- 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.\n\nparameter  value or range  source\nSeparator angle / spacing tested  5 deg, 100 mm spacing in 10 deg diffuser  Cranfield / SAE 2018-01-0725\nFront downforce change  dCL about 0.03 lower  Cranfield / SAE 2018-01-0725\nDrag penalty  3, 5, 4 counts at h/h0 0.25, 0.50, 0.75  Cranfield / SAE 2018-01-0725\nEdge vortex origin  near diffuser inlet  Southampton (Mahon et al.)\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Cranfield University / SAE: On the Aerodynamics of an Enclosed-Wheel Racing Car (Soares et al., SAE 2018-01-0725) — web — Publisher's page; dimensions are facts — https://dspace.lib.cranfield.ac.uk/server/api/core/bitstreams/9b476386-44d0-4c0f-a49e-c3372274d351/content\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/diffuser-fences-strakes/index.json\nAPI: https://xemstudio.com/api/geometry?object=diffuser-fences-strakes\n",
 "sources": [
  {
   "publisher": "Cranfield University / SAE",
   "title": "On the Aerodynamics of an Enclosed-Wheel Racing Car (Soares et al., SAE 2018-01-0725)",
   "url": "https://dspace.lib.cranfield.ac.uk/server/api/core/bitstreams/9b476386-44d0-4c0f-a49e-c3372274d351/content",
   "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": "Do fences add downforce?",
   "a": "In the Cranfield test they mainly shifted balance (less front downforce) at a small drag cost."
  }
 ],
 "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": "diffuser-fences-strakes-diffuser",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "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",
   "note": "Diffuser: 200 mm ramp at 10° behind a flat floor, 300 mm wide, 3 fences"
  }
 ],
 "markdown": "https://xemstudio.com/objects/diffuser-fences-strakes/index.md"
}