ZATARA NEPOMUCENO
20XT Transfer Case
I designed, optimized, and manufactured a fully custom transfer case for Blue Jay Racing's 20th Baja Car.
Project Overview
In addition to my responsibilities as the drivetrain lead, I designed the transfer case for the 2024 Hopkins Baja vehicle, the JH20XT.
The system architecture features a custom two-stage spur gear reduction to transfer power from the CVT to rear axles. Off the middle stage, a pair of custom spiral bevel gears integrated with a dog-clutch shifter drive the propshaft. On the output shaft, a floating brake caliper provides braking to the rear wheels.
My primary focus was to reduce the number of high-risk failure points while reducing the weight and rotational inertia as much as possible. This entailed thorough analysis on all mission-critial components based on load cases determined by vehicle testing and data collection. I also focused heavily on serviceability and minimal dependencies between the 2WD/4WD components.
20XT Transfer Case with Internals
20XT Transfer Case with Externals
Custom Spur and Bevel Gears
To meet my primary design goals, the transfer case gears were designed to an aggressive 5-hour lifetime, with a new set of gears being replaced for each Baja competition. The overall drive ratio was selected to take the best advantage of our continuously variable transmission (CVT) while the stage ratios were tuned to reduce mass. The tooth geometry analysis was performed primarily in KISSsoft, where a fatigue load spectra was used based on loading conditions from an endurance race, to produce gears tuned for a high-performance Baja vehicle.
Completed spur gear C2
Processed data used to generate fatigue load spectra
Load summary of forces on gear stage shafts
Shaft and Case Analysis
Final tooth geometry and fatigue load spectra were used to derive the loads on shafts and case bearings. These loads, in conjunction with CVT belt tension and inboard braking forces, were used to conduct a comprehensive 24-scenario assembly-level study of the case and its mounting using Ansys. The primary goal of this analysis was to prevent yielding of the case or tabs in addition to reducing deflection of the CVT assembly in order to avoid efficiency losses and wear.
Stress plot of shaft with probing at stress concentrations
Stress plot of case components and frame-mounting tabs
Check out this slideshow documenting the static structural analysis that went into designing the stiffening ribs of the sealing case!
Click the above image to open FEA documenatation!
Completed sealing case components
Manufacturing
With the recent addition of CNC capabilities being made available to our workspace, we relied heavily on the CNC to make most of our parts in-house. Check out some photos of the transfer case being manufactured below!
Spur gear pocketing
Bevel gear and dog clutch profiling
Completed spur gears
Sealing case (left half) profiling
To Be Continued...
As of early February, my aluminum parts are currently being anodized, the vehicle frame is being powder-coated, and I am waiting on sponsors to deliver the titanium shafts featured in the transfer case. Check back in a few weeks to see the car running!
Completed sealing case with shifter installed (rear)
Completed sealing case with shifter installed (front)
Pictured: Zatara Nepomuceno holding Blue Jay Racing's smallest transfer case!