A part-time four-wheel-drive truck is brutally direct. In 2H, the transfer case splines the transmission to the rear driveshaft and lets the front output coast. In 4H, a dog clutch locks the chain sprocket to the mainshaft, so the front and rear driveshafts turn together at a rigid one-to-one ratio. There is no centre differential, which is why this kind of system belongs on loose ground — or on the service-bay bench rig shown here.
This exhibit exposes the two axle differentials as working bevel epicyclics. Each carrier is driven through the final-drive ratio, while the side gears must satisfy the open-diff identity wL + wR = 2wC. Drag the friction sliders and the free side speeds up as the braked side slows; close a Tru-Lok-style collar and the constraint switches to wL = wR = wC, with the spider gears orbiting but no longer spinning.
The drivetrain kernel is deterministic and fixed-step: transfer-case collar travel, front driveshaft coast-down, final-drive gearing, spider-pinion speed, wheel-hub brake drag and the live invariant checks are all computed, not keyframed. It is a mechanical version of the same lesson DigiSim teaches with gates: behaviour falls out of constraints, ratios and state.
Specifications
- Layout
- Part-time 4WD transfer case, high range, front chain drive
- Differentials
- Open bevel diffs with dog-clutch axle locks
- Gearing
- 41/11 final drive; 14-tooth side gears, 10-tooth spiders
- Simulated
- Fixed-step kinematics, collar engagement, hub brake friction
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