Driveshafts, halfshafts & prop shaft (AWD)

The spinning driveline — the parts nobody usually animates. One role, Driveline, covers all of it:

• a halfshaft (diff → hub) that aims at the wheel, plunges as it travels, and spins;
• a centre prop shaft (down the tunnel) that just spins in place.

For AWD, you add one shaft per driven output — a rear prop shaft and a front one, plus front halfshafts — each reading its own driven wheel. It’s a visual layer: it spins off a real wheel’s roll but adds no physics.

Parts → roles

Hierarchy

Rear
├── PropShaft            Driveline  (no target — spins · Source = RL)
├── RL
│   ├── RL_Hub               Hub · SteerCamber
│   │   └── RL_ShaftEnd      (no component — the shaft's hub end)
│   ├── RL_DiffJoint         (no component — fixed inner joint)
│   └── RL_Halfshaft         Driveline  (Pivot = RL_DiffJoint → Target = RL_ShaftEnd · Source = RL)
└── RR                       ... mirror of RL

A prop shaft has no hub to aim at, so “Build Rig Hierarchy” parks it on Front/Rear (like an axle). A halfshaft aims at a hub, so it lives in its corner like an arm.

What makes it different

• One role, two behaviours. Set an Outer Target and it’s a halfshaft: it aims at the hub and telescopes to stay attached (CV plunge), and spins. Leave the target empty and it’s a prop shaft: it keeps its pose and only spins.
• Spin comes off a real wheel. Pick a Spin Source — a driven wheel (RL/RR on a RWD car). The shaft turns as fast as that wheel rolls. Spin Scale is a purely visual multiplier.
• Spin Axis is the shaft’s own long axis. In local space, which way does the mesh point? Usually +Z (0,0,1) or +X. Get this wrong and it wobbles instead of spinning cleanly.
• AWD is just more shafts. There’s nothing special to switch on — add a second prop shaft and a front halfshaft pair, each pointing at its own front Spin Source.

Setup, step by step

1. Add an empty under each hub where the shaft plugs in: RL_ShaftEnd, RR_ShaftEnd.
2. Add an empty at each diff-side joint (RL_DiffJoint, …) — on the chassis, it doesn’t move much.
3. Put a KSuspBone on each halfshaft mesh, Role = Driveline. Inner Pivot = the diff joint, Outer Target = the hub-side marker, Telescopes ON, Spin Source = that side’s wheel.
4. Set Spin Axis to the shaft’s local long axis (aim it down the shaft — e.g. 0,0,1).
5. For the prop shaft: KSuspBone on the mesh, Role = Driveline, leave Outer Target empty, pick a driven Spin Source, set Spin Axis down the shaft.
6. (AWD) repeat step 5 for the front prop shaft and add front halfshafts with FL/FR sources.
7. Build Rig Hierarchy → Bake .ksusp. The report should list … and N driveline(s).

Type gotchas

• Spin looks frozen? The spin is read from the game’s wheel-mount transform. If a particular car spins its wheel on a child mesh instead, the mount doesn’t rotate and the shaft won’t either — that’s a known limitation (a car-speed fallback is on the list). Everything else (aim, plunge) still works.
• Wobbling instead of spinning = wrong Spin Axis. It must be the shaft’s local long axis, not a world direction. Rotate the view to the shaft and check which local axis runs down it.
• Use a driven wheel as the source. A front prop shaft on a RWD car reading a front wheel will still spin (wheels roll from rolling), but for the right look tie each shaft to the wheels it feeds.
• The prop shaft shouldn’t aim. If you accidentally set an Outer Target on it, it’ll try to point at that and pivot — leave it empty so it only spins.
• No physics. This never drives the car, adds grip, or changes anything the wheels do — it’s paint.