The primary structure of the vehicle. Carries every suspension, powertrain, aerodynamic and electrical load, protects the driver, and provides the torsional stiffness the suspension needs to control lateral load transfer.
A TIG-welded 4130 chromoly space frame. Suspension loads enter through welded tabs at each pickup point and are reacted as a truss. Torsional stiffness acts as a spring in series with the front and rear roll stiffnesses, so raising it moves the platform toward infinitely-stiff-frame behaviour and pins the lateral load transfer distribution where kinematics wants it.
Kinematics supplies the pickup points and an acceptable LLTD range; the intersection of the frame-stiffness curve with that range sets the torsional stiffness target. CR26E: LLTD target 41.6% within a 39.5-42.5% range gave a design target of 1651 lb-ft/deg after applying the historical model error factor. Tube-by-tube stiffness against weight is then traded in an iteration log.
The rules-defined primary structure (main hoop, front hoop, side impact structure) is fixed. Cockpit template must pass. Every other subteam mounts to it, so packaging is a constraint on the frame as much as the frame is a constraint on them. Front roll hoop position is set by the DDU-11 dash and minimum steering distance while keeping visibility.
SolidWorks 1D beam element FEA: representative engine tubes, couple moment applied across the front axle, rear upright points fixed, actuation geometry modelled as trusses, displacement measured at the front upper pickup points, 0.5 in mesh spacing from a convergence study. Physical correlation on the torsional stiffness jig with solid dampers installed, rear spindles fixed, front spindles on a pivot beam and weights hung at known distance. Historical error: CR22 2212 simulated against 2279 measured (2.94%), CR25 2359 against 2140 (10.23%), CR26E 1668 against 1995 (19.60%). From the design reports and one-pagers: Validation as planned in the CR25 one-pager: weight before tabs, with paint and without paint; torsional stiffness testing across several conditions; and weld and normalising testing before frame completion to establish best practice, including material hardness and tensile testing. The one-pager is candid that "based on Ainsley's research we are doing most of this wrong", which is what led to the CR26I heat treatment characterisation.
TIG welded with ER70S-2 filler wire in CNC-milled 6061 aluminium jigs with locating dowel pins. Jigs are simple, modular, rigid, heat resistant and durable, and were reused from CR25 on CR26I. Roughly 295 hours of frame manufacturing. Suspension nodes are heat treated at 1300 F; a 30 second dwell was selected after hardness testing. Finished in single stage urethane paint plus automotive clearcoat. From the design reports and one-pagers: CR22 logged manufacturing time, from the CR25 frame one-pager: 74 hours of jigging machining, 283 hours of frame welding and 6.5 hours of frame painting. CR26I came in at roughly 295 hours of frame manufacturing with the jigs reused from CR25. CR22 costs: tube coping and bending $4,640, jigging material $1,600. The one-pager budgets for both rising, noting VR3 has become more expensive each year.
4130 chromoly steel tube (yield 66.7 ksi, 29,000 ksi modulus, 20.5% elongation), ER70S-2 filler (77 ksi yield, 31% elongation). ER80S-2 was compared (94 ksi, 24%) but ER70S-2 was retained.
Fault Vault, CR26I, Design, Complete Failure. A rule change from 2025 to 2026 outlawed engine mounting that connects to the span of the main hoop brace. This was not caught in the CR26I frame edits or in the Structural Equivalency Spreadsheet, and the car failed SES tech inspection at competition. The trackside fix was to cut the existing 1/2 in support tubes and hand cope and weld 3/4 in tubes back to the main hoop / main hoop brace node, at competition. Fault Vault, CR26I, Design, Potential Failure. Pushing the car with the push bar at competition began to deform the rear frame tubes. The jack bar was used for the rest of competition, and future frame designs will consider push bar accessibility in the design rather than after it. Fault Vault, CR25, Design, Complete Failure. The steering rack cover was not designed to extend far enough to the side to cover the new internal damper package, which required a second cover to be laid up, cut and bonded to the first. Two CR26I suspension node outliers measured with the FARO arm (rear right upper fore off 0.431 in, rear left upper aft off 0.286 in) moved roll centre height, caster and camber curves away from design.
Future frame designs will consider push bar accessibility in the design, after pushing the car with the push bar at competition began to deform the rear frame tubes. The SES review process needs to catch rule changes affecting engine mounting; missing the 2026 main hoop brace change cost an SES tech failure at competition.
CR26I torsional stiffness 2140 lb-ft/deg measured against a 1736 lb-ft/deg target. SES/functional frame weight 60.43 lb on CR26I against 59.03 lb on CR22. Net tubing change -1.16 lb, with cockpit bracing alone worth -3.22 lb and the new pedal rail costing +1.61 lb. Average manufactured node deviation from CAD 0.101 in total. From the cost report drawings and CAD: Chassis weight breakdown: CR25 70.580 lb, CR26E CAD 77.639 lb. Frame section mass, Front: CR22 24.750 lb, CR25 27.140 lb. Frame section mass, Cockpit: CR22 26.082 lb, CR25 22.170 lb. Frame section mass, Rear: CR22 16.814 lb, CR25 15.410 lb. Frame section mass, Total: CR22 67.646 lb, CR25 64.720 lb. Note: Weights do not account for any tabs, even sus tabs Note: FBH and FRH are included with front Note: Main hoop is included with cockpit Tube stock reference, mass per foot: Round 0.5" OD x .028" wall: 0.14 lb/ft Round 0.75" OD x .028" wall: 0.22 lb/ft Round 0.75" OD x .035" wall: 0.27 lb/ft Round 1" OD x .035" wall: 0.36 lb/ft Round 1" OD x .049" wall: 0.50 lb/ft Round 1" OD x .065" wall: 0.65 lb/ft Round 1" OD x .095" wall: 0.92 lb/ft Square 1" OD x .035" wall: 0.45 lb/ft Square 1" OD x .049" wall: 0.61 lb/ft Square 1" OD x .065" wall: 0.79 lb/ft From the Structural Equivalency Spreadsheet as submitted: FH to Steering Wheel gap: 6.32 in Fastener shear stregnth: 10833.7086 lbs Harness Test Load: 3371.0 lbs Lap belt fastener diameter: 0.375 in Lap belt fastener rated UTS: 170000.0 psi Minimum radius::diameter ratio: 3.25 Minimum tube centerline radius: 3.25 in Tab shear strength: 3388.4325 lbs Tab thickness: 0.125 in Top of MH to top of MHB tube: 6.026 in Top surface of Lower SIS to Highest UpperSIS Point: 12.062 in Turned Steering Wheel minimum below FH top: 0.46 in Tube sizes declared in the SES: 1.0 in OD x 0.047 in wall (round); 1.0 in OD x 0.065 in wall (round); 1.0 in OD x 0.095 in wall (round); 25.4 mm OD x 1.2 mm wall (round). SES material properties, Steel (4130): E 29000000 psi, Sy 44200 psi, Su 52900 psi, Sy-weld 26000 psi, Su-weld 43500 psi. SES material properties, 6061-T6 aluminium: E 10000000 psi, Sy 34800 psi, Su 42100 psi, Sy-weld 16700 psi, Su-weld 25400 psi. Weighed mass: 32840.1 g (72.400 lb), 47169.1 g (103.990 lb). CAD mass, CR26E master assembly: 75.376 lb (34.190 kg), 2 instances, 150.751 lb total. CG at X 48.67 in, Y 0.00 in, Z 12.11 in in vehicle coordinates. (CAD component: CR26E Final Frame) CAD mass, CR26I master assembly: 70.443 lb (31.952 kg). CG at X 43.59 in, Y 0.01 in, Z 12.47 in in vehicle coordinates. (CAD component: CR25 Final Frame) CAD mass, CR25 master assembly: 70.443 lb (31.952 kg). CG at X 43.59 in, Y 0.01 in, Z 12.47 in in vehicle coordinates. (CAD component: CR25 Final Frame) CAD mass, CR22 master assembly: 69.379 lb (31.470 kg). CG at X 43.73 in, Y -0.01 in, Z 12.96 in in vehicle coordinates. (CAD component: CR22 Final Frame) Dimensions called out on the cost report drawings. These are the figures printed on the drawing, not a labelled specification; open the drawing itself for what each one refers to. CR26I Frame (mm): 960.9, 711.4 2255.8 Source: CR26E/Systems/03 - Mass Model/ChassisWeightBreakdown.xlsx | CR26E Frame Section Weights.xlsx | CR26E Tube Weights Reference.xlsx | SES: CR22/Document Submissions/SES/2024/Univ_of_Alabama_-_Tuscaloosa-SES-2024-10-11-076.xlsx | Subteam weight books: CR22/Chassis/Design/2 Full System Analysis/Weight Book.xlsx | SolidWorks mass model exports: CR22 (5 May 2024), CR25 (4 Jul 2025), CR26I (30 Jul 2025), CR26E (22 Mar 2026) | Cost report drawings: CR25/Chassis/Design/_Frame/Cost Report/CR26I Frame.pdf Quantity per car, as costed: 1