Rear underbody diffuser — dimension sheet Source: Xem Studio, https://xemstudio.com/objects/rear-diffuser/ (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 - 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. parameter value or range source 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 Files elsewhere (not hosted by Xem Studio; license as stated by the publisher): - 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 - 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 - 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 Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/rear-diffuser/index.json API: https://xemstudio.com/api/geometry?object=rear-diffuser