To transmit engine torque to the rear wheels reliably for the length of an endurance event, without losing power to friction or losing the event to a failure.
Chain drive. An 11-tooth steel front sprocket on the gearbox output drives a DID 520VX3 X-ring chain to a 40-tooth Vortex aluminium rear sprocket on a sprocket mount, at 6.30 in centre-to-centre and a 3.636:1 final drive. The chain runs inside a guard. Torque passes into a Drexler FSAE adjustable differential held in left and right 6061 carriers that locate off the engine pins. From the differential, Taylor Race 4130 half shafts with RCV FSAE tripods at each end drive the rear spindles. Outboard tripod housings carry 440C stainless steel tripod inserts, which take the torque from the tripod without wearing the aluminium spindle. On CR26E the same architecture carries a spool differential rather than a Drexler, driven from the motor output shaft.
The differential is a live trade. The Drexler's advantages are tunable preload through clutch packs, adjustable acceleration and deceleration lockup curves, better reliability than custom implementations, and much better drivability than a spool, which lets a driver keep a consistent style over a long stint. Its costs are stated equally plainly: roughly 9 lb heavier than the spool, $4,204 against $362, and more failure points (housing seals, drain plugs, stub axle seals). The CR26I goal is to validate the selection on track and confirm the performance benefit actually offsets the weight. Eliminate the tensioner by controlling manufacturing. Precise centre-to-centre distances through the differential carriers, combined with a sealed chain, removed the need for a chain tensioning system entirely. Over 500 miles on a single chain with no loss of tension. Match hardness at wear interfaces. The tripod insert failure history was a hardness mismatch problem, not a strength problem. 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.
- Validate the differential selection on track and by analysis. - Minimise wear on drivetrain components to improve reliability without adding mass. - Serviceability: the differential must be easy to remove so it can be tuned. - Weight: further loading and fatigue analysis is needed before a reduction target can be set, so the standing goal is no weight increase. - Reduce parasitic drivetrain losses through bearing selection and tolerance study.
- Tripod insert wear. 304 stainless inserts (70 HRB, 35 HRC max) were far softer than the hardened 4340 tripod rollers at roughly 60 HRC. Thickening them eliminated ultimate failures but they still pocketed and wore. Fixed on CR25 by moving to 440C stainless heat treated to an average measured 60.0 HRC, verified on three inserts from the batch with three samples each. - Tripod ejections. Moving from 18 in to 16 in tires on CR25 raised driveshaft angles, but the tripod housings were not deepened to match, which caused multiple ejections and limited rear ride height and camber adjustment. Depth was increased from 1.25 in to 2.00 in. - Differential carriers being remade seven times in one year. Fixed by the non-adjustable fixed-pin design and by asking for a chain tensioner, which the improved manufacturing then made unnecessary. - Tripod grease. Past failures were partly attributed to using CV-2 grease; the system now runs GKN tripod grease as the manufacturer recommends. - Differential carrier simulations that did not converge. Called out in knowledge transfer as an unresolved analysis problem.
SolidWorks FEA for carrier and sprocket mount analysis (0.045 in global mesh, 0.002 in tolerance, cylindrical face constraints on engine pin bores, roller/slider on engine contact surfaces, bearing load on half the bearing surface with a 2.0 shock factor). Hardness testing for heat treatment validation. Custom measuring jig for half shaft length. Strain gauges on the differential carriers are identified as the next validation step.