{
 "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": "rear-diffuser",
 "name": "Rear underbody diffuser",
 "kind": "standard",
 "category": "Car design",
 "group": "Car aero",
 "year": null,
 "question": "What does a rear diffuser do and what angle works best?",
 "tags": [
  "vehicle design",
  "car aero",
  "rc design",
  "aerodynamics"
 ],
 "aliases": [
  "rear diffuser",
  "diffuser angle",
  "underbody diffuser",
  "car diffuser",
  "diffuser design",
  "how does a diffuser work",
  "diffuser ramp angle",
  "flat floor diffuser",
  "rc car diffuser",
  "diffuser ride height",
  "diffuser downforce"
 ],
 "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/rear-diffuser/",
 "sheet_txt": "https://xemstudio.com/objects/rear-diffuser/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": [
   "A diffuser is an upswept rear floor section. Expanding the underbody flow lowers the pressure under the car, which adds downforce and can reduce drag if ride height is not too small (SAE 980030).",
   "Southampton tests of 5, 10, 15 and 17 deg diffusers found downforce rising as ride height falls until edge vortices break down, after which downforce drops; the 17 deg diffuser peaked at CL 1.77 (Mahon, Zhang & Gage).",
   "On a 35% DrivAer racing model, total downforce rose up to 10 deg then fell, and 10 deg gave the best efficiency (Cranfield / SAE 2018-01-0725).",
   "Used on: DrivAer research model (high-performance configuration).",
   "The file is a reference example built by Xem Studio from the numbers above (10 deg best-efficiency angle (Cranfield); length/width illustrative.). Sizes the sources do not fix are illustrative; scale the example to your own vehicle."
  ],
  "table": {
   "columns": [
    "parameter",
    "value or range",
    "source"
   ],
   "rows": [
    [
     "Angles tested (bluff body)",
     "5, 10, 15, 17 deg",
     "Southampton (Mahon et al.)"
    ],
    [
     "Max CL, 17 deg",
     "1.77 at hr/d 0.0573",
     "Southampton (Mahon et al.)"
    ],
    [
     "Max CL, 15 deg",
     "1.66 at hr/d 0.0446",
     "Southampton (Mahon et al.)"
    ],
    [
     "Max CL, 10 deg",
     "1.48 at hr/d 0.0446",
     "Southampton (Mahon et al.)"
    ],
    [
     "Angles tested (DrivAer 35%)",
     "0, 5, 10, 16 deg",
     "Cranfield / SAE 2018-01-0725"
    ],
    [
     "Best efficiency angle",
     "10 deg (AE 3.05 at h/h0 0.25)",
     "Cranfield / SAE 2018-01-0725"
    ],
    [
     "5 deg vs 0 deg",
     "max net downforce x2.15, drag reduced",
     "Cranfield / SAE 2018-01-0725"
    ]
   ]
  }
 },
 "sheet_text": "Rear underbody diffuser — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/rear-diffuser/  (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- A diffuser is an upswept rear floor section. Expanding the underbody flow lowers the pressure under the car, which adds downforce and can reduce drag if ride height is not too small (SAE 980030).\n- Southampton tests of 5, 10, 15 and 17 deg diffusers found downforce rising as ride height falls until edge vortices break down, after which downforce drops; the 17 deg diffuser peaked at CL 1.77 (Mahon, Zhang & Gage).\n- On a 35% DrivAer racing model, total downforce rose up to 10 deg then fell, and 10 deg gave the best efficiency (Cranfield / SAE 2018-01-0725).\n- Used on: DrivAer research model (high-performance configuration).\n- The file is a reference example built by Xem Studio from the numbers above (10 deg best-efficiency angle (Cranfield); length/width illustrative.). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.\n\nparameter  value or range  source\nAngles tested (bluff body)  5, 10, 15, 17 deg  Southampton (Mahon et al.)\nMax CL, 17 deg  1.77 at hr/d 0.0573  Southampton (Mahon et al.)\nMax CL, 15 deg  1.66 at hr/d 0.0446  Southampton (Mahon et al.)\nMax CL, 10 deg  1.48 at hr/d 0.0446  Southampton (Mahon et al.)\nAngles tested (DrivAer 35%)  0, 5, 10, 16 deg  Cranfield / SAE 2018-01-0725\nBest efficiency angle  10 deg (AE 3.05 at h/h0 0.25)  Cranfield / SAE 2018-01-0725\n5 deg vs 0 deg  max net downforce x2.15, drag reduced  Cranfield / SAE 2018-01-0725\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\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- 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- SAE International: The Aerodynamic Performance of Automotive Underbody Diffusers, SAE 980030 (Cooper et al.) — web — Publisher's page; dimensions are facts — https://saemobilus.sae.org/papers/aerodynamic-performance-automotive-underbody-diffusers-980030\n\nFree files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/rear-diffuser/index.json\nAPI: https://xemstudio.com/api/geometry?object=rear-diffuser\n",
 "sources": [
  {
   "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
  },
  {
   "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": "SAE International",
   "title": "The Aerodynamic Performance of Automotive Underbody Diffusers, SAE 980030 (Cooper et al.)",
   "url": "https://saemobilus.sae.org/papers/aerodynamic-performance-automotive-underbody-diffusers-980030",
   "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": "Why does downforce suddenly drop very low?",
   "a": "Below a critical ride height the diffuser edge vortices break down and downforce falls while drag rises (Mahon, Zhang & Gage)."
  }
 ],
 "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": "rear-diffuser-diffuser",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "step": "https://xemstudio.com/files/hardware/rear-diffuser/rear-diffuser-diffuser.step",
   "stl": "https://xemstudio.com/files/hardware/rear-diffuser/rear-diffuser-diffuser.stl",
   "note": "Diffuser: 200 mm ramp at 10° behind a flat floor, 150 mm wide, 3 fences"
  }
 ],
 "markdown": "https://xemstudio.com/objects/rear-diffuser/index.md"
}