Convert the driver's foot force into force on the balance bar and therefore into brake line pressure, at the designed pedal ratio.
A lever pivoting on a clevis pin in the brake pedal tray, driving the balance bar between the two master cylinders. Pedal ratio sets the relationship between driver force and line pressure, and therefore the pedal force required to reach wheel lock.
Geometry chosen from the master cylinder sizing and the required pedal force. CR26I requires 137 lbf for the 1.84 g wheel lock case, with brake sensitivity of 77 lb/g. Driver force targets from the NASA Anthropometry, Biomechanics and Strength handbook. From the design reports and one-pagers: From the CRE25 Brake Pedal Arm Design Report. The previous setup had presented problems with serviceability and machinability. The brake arm had historically been a CNC'd part, which limited the design; the CR25 designer instead designed a part to be cut on the waterjet, to reduce machining time and allow faster iteration. The final design uses two waterjet plates as arms instead of one CNC'd arm, which greatly reduces machining time and reduces total pedal weight. The hardest part of that decision was containing the bias bar trunnion, solved with a middle plate sandwiched between the two arms and held by bolts, with the holes on the arms slightly undersized to stop the trunnion shifting laterally. Two brackets are permanently bonded to the pedal face and bolted to the arms.
Mount to the new rail design, withstand braking force, and validate pedal ratio. Rules: T.3.1.3 two independent hydraulic circuits, with a leak or failure at any point maintaining effective brake power on at least two wheels. T.3.1.4 each circuit needs its own fluid reserve. T.3.1.6 brake by wire is prohibited. T.3.1.7 unarmored plastic brake lines are prohibited. T.3.1.8 the brake system must be protected with scatter shields from drivetrain failure or minor collisions. T.3.1.10 fasteners in the brake system are Critical Fasteners. T.3.2.1 the brake pedal must be fabricated from steel or aluminium, or machined from steel, aluminium or titanium. T.3.2.2 the pedal and associated components must withstand a minimum 2000 N without failure of the brake system, pedal box, chassis mounting or pedal adjustment, and may be tested by an official pressing the pedal with maximum force while seated normally. T.3.2.3 failure of non-loadbearing components must not interfere with pedal operation.
SolidWorks FEA at 225 lbf applied load, fixed at the hinge, minimum factor of safety 1.23, with a mesh convergence study. From the design reports and one-pagers: SolidWorks static study on one arm, assuming the load is evenly distributed between the two arms. Setup: evenly distributed load where the pedal face contacts the arm; fixed joint at the pedal pivot; bearing/roller joint at the pivot where the spacer contacts the arm; bearing loads where the master cylinder forces attach. The master cylinders are assumed static, which the report identifies as the worst case: in reality a large frictional force binds the arms to the trunnion retainer and reduces the force at the retainer mounting points, and that force was deliberately not simulated. The result is a maximum stress significantly below what the pedal would ever be required to take. The two-plate design also means side loading is resisted by the plates in tension and compression rather than by resisting bending moments.
Waterjet from 6061-T6 plate.
6061-T6 aluminium.
None recorded. This part has no entry in Knowledge_Transfer/Issue Logging/Fault Vault.xlsx, in either Issue Log.xlsx, in CR22/Issue Tracking/Shakedown 7-15-23.xlsx, or in any design review issue list found in CrimsonStorage or Box.
Open items recorded in the report. Topology optimisation was attempted and stalled on three problems: defining how force passes through the member and how to constrain it, the simulation wanting to remove the whole bottom portion of the arm when supported higher up, and very long iteration times. The proposed way forward is hand calculations treating it as a 2D system to find how much force goes through the arm against the master cylinders, then re-running the optimisation. A true-bar system is identified as the better balance bar architecture: more accurate balance adjustment, less brake bias migration, easier to visualise what the bias is set at, and physically measurable from pushrod distance from centre. That recommendation predates, and is confirmed by, the CR26I Fault Vault entry about the master cylinder and bias bar having an extra degree of freedom.
CAD mass, CR22 master assembly: 0.269 lb (0.122 kg). CG at X 6.74 in, Y -2.99 in, Z 9.86 in in vehicle coordinates. Source: SolidWorks mass model exports: CR22 (5 May 2024), CR25 (4 Jul 2025), CR26I (30 Jul 2025), CR26E (22 Mar 2026) Quantity per car, as costed: 2