Provide the friction surface the front pads clamp, and absorb and shed the heat that braking generates.
A full-floating disc mounted on titanium bobbins so it can expand and centre itself rather than being constrained by the hat, which is what stops thermal stress concentrating and cracking the disc.
Material selection was driven by cracking and non-uniform pad wear, not by weight. Four candidates were compared on peak and minimum temperature and on minimum factor of safety: 6061 aluminium (573.86 C high, 325.38 C low, FoS 2.1651), AISI 420 stainless (462.25 C, 256.93 C, FoS 1.6875), Class 40 gray cast iron (490.5 C, 256.5 C, FoS 2.1622) and 4340 steel (741.2 C, 344.5 C, FoS 3.6438). 4340 was selected for its factor of safety, and it also came out cheaper.
ANSYS transient thermal for a single braking event: 2.4 s, 260 C initial temperature, 90 W/m^2-K film coefficient, 0.85-0.8 emissivity, 25,375 W heat flux front (28,000 W in the driver interface version of the analysis). Thermal result imported as a load into a static structural run at 1.84 g deceleration, 12 MPa pad pressure, 707 N-m moment (1262 N-m at 35 mph with 43,599 N clamp force in the worst-case run), with cylindrical and frictionless supports. Mesh convergence study run: peak nodes 45,456, peak stress 42.031 MPa, with element size limited to 0.001 m by the ANSYS Student Edition.
4340 steel (previously Class 40 gray cast iron). Cost $116.67 against $150 for cast iron. From the cost report drawings and CAD: From the cost report drawings: AISI 4340 Steel, annealed (Front Rotor).
Cracking on the previous cast iron rotors. Non-uniform pad wear. Brake fade, which may be residual heat rather than a rotor problem, pointing at cooling rather than the disc itself; recorded as unresolved.
Diameter 6.875 in, thickness 0.1875 in, mass 529.07 g. Front rotor assembly with Brembo P2.34 calipers totals $592.65 against CR22's $615.24, a $47.59 saving. From the cost report drawings and CAD: Measured mass: 506.0 g average across 2 weighed instances (FL Rotor 498.0 g, FR Rotor 514.0 g), uncertainty +-.5 g. Left-to-right deviation -3.11%. Weighed mass: 498.0 g (1.098 lb), 577.0 g (1.272 lb) (recorded as: FL Rotor). Weighed mass: 514.0 g (1.133 lb), 572.3 g (1.262 lb) (recorded as: FR Rotor). Measured mass, as built: CR25 577.0 g (1.272 lb) (weighed as: FL Rotor). Measured mass, as built: CR25 572.3 g (1.262 lb) (weighed as: FR Rotor). CAD mass, CR26E master assembly: 1.181 lb (0.536 kg), 2 instances, 2.362 lb total. CG at X 21.00 in, Y 0.00 in, Z 8.00 in in vehicle coordinates. (CAD component: CR26E_SUS_Front_RotorRevFINAL) 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. FRONT BRAKE ROTOR (in): 174.75, 4.75 CR22_SUS_F_Rotor_RevFinal (as drawn): 17.48, 0.64, 0.60, 1.11 Front Rotor (as drawn): 17.46, 0.64, 0.48 CR26I_FrontBrakeRotor (mm): 17.5, 0.6, 10.4, 0.5 CR26I_FrontRotorAssembly (mm): 17.5, 2.1, 0.5 Source: CR22 Suspension Component Weights.xlsx (weighed, not CAD) | Subteam weight books: CR22/Suspension/Validation/Full System Analysis/Component Weights.xlsx | CR26e Weight Tracker.xlsx, Final IRL Weights (weighed, not CAD) | SolidWorks mass model exports: CR22 (5 May 2024), CR25 (4 Jul 2025), CR26I (30 Jul 2025), CR26E (22 Mar 2026) | Cost report drawings: CR22/Document Submissions/Cost Report/Suspension/Cost Report Drawings/Brake System/PDFS/FRONT BRAKE ROTOR.pdf; CR22/Suspension/Design/01_Cost_Report/Cost Report Drawings/Brake System/CR22_SUS_F_Rotor_RevFinal.pdf; CR22/Suspension/Design/01_Cost_Report/Cost Report Drawings/Brake System/PDFS/FRONT BRAKE ROTOR.pdf; CR25/Suspension/Management-Comp/Cost Report/Drawings/Brakes - Rotor Assembly/Front Rotor.pdf and 2 more Quantity per car, as costed: 1, 2
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