CR21 spool differential.
See the Differential - Spool entry.
No part-specific design process is documented. The DriveTrain subsystem design process applies: 1. Set gearing from the tractive force diagram, top speed and the event speeds actually seen (autocross top speed 69 mph, endurance 72 mph, gear-limited top speed 82 mph). 2. Design the carriers non-adjustable with a fixed pin, integrated with the engine pins, to reduce assembly complexity and failure points and remove the need for weld-on carrier mounts on the frame. 3. Analyse the carriers at acceleration and deceleration load cases with a 2.0 shock loading factor. CR26I LH carrier at 39 lb-ft with shock: 32.949 ksi peak von Mises, minimum FoS 1.211, 0.013 in average axial bearing bore deflection. RH carrier: 9.432 ksi, FoS 4.229, 0.001 in. 4. Set the half shaft length with a custom measuring jig so the tripod travel range cannot be exceeded, then groove for retaining rings. 5. Validate on track and by inspection of the wear surfaces after testing.
CR22 static analysis: torque applied at the flower petal of 9,087.614 lb-in (757.3 lb-ft); bearing fixture applied at both bearing surfaces; left side tripod geometry fixed to simulate 0% wheel slip during a launch at maximum torque. Mesh quality over 90% of elements with aspect ratio under 5 and Jacobian under 10. Result: maximum stress 32.4 ksi, below the yield of 7075-T6, minimum factor of safety 2.258, maximum displacement 0.02274 in at the tip of the flower petal. A stress concentration appears inside the fixed tripod geometry but stays within the yield limit.
Five setups, all CAM in Fusion 360. Lathe, setup 1: turn the raw stock to create both bearing surfaces, the main body, the flower petal OD and one end. Use the tail stock and live centre for a rigid setup. Be patient dialling in the bearing surface OD, start big and work down, and measure with micrometers. Mill, setup 2: create the right-side tripod pocket, which is what holds the spool for the following operations. Fit the workholding for the next operation before removing the spool from the mill. A slight press fit is acceptable but makes removal harder; the engine pin removal tool works if hammering fails. Workholding is a steel brick with a central hole, clamping the flower petal flange to the table with toe clamps. Mill, setup 3: remove the extra material on the flower petal end left over from lathe chucking, and create the left tripod pocket and the flower petal. The flower petal finish is critical because it is a load bearing feature and is sensitive to poor surface finish. Workholding is a large aluminium plate with bolt holes and a tripod-shaped extrusion to locate the spool. Mill, setups 4 and 5: remove material from the main body and create the X pattern. This is the setup that needs the most attention because it works between two vises. Fit the two small tripod-shaped workholdings into the spool and bolt them down. Hold one side in an indicated vise with the other side sitting in a vise that is not yet bolted down, and do not use parallels here since the goal is to align the two vises. Tighten the loose vise to align it, then bolt it to the table.
7075-T6 aluminium. From the design reports and one-pagers: 7075-T6 aluminium round bar. The OD of the middle body is deliberately slightly smaller than the OD of the bearing surfaces to help the bearing pressing process. McMaster 98541A182 C-clips retain the bearings, with small spacers between the bearings and clips locating the spool laterally, which is what positions the rear sprocket in line with the front.
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.
CAD mass, CR26E master assembly: 2.521 lb (1.143 kg). CG at X 83.06 in, Y 0.63 in, Z 9.29 in in vehicle coordinates. (CAD component: CR26e Spool - rev002) 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: 1
Nothing in CrimsonStorage covers these yet.