One problem is is your “Wheel” prefabs are colliding with your “Car Body”. I think this is causing the jitter you described.
The capsule collider isn’t scaled by the wheel transform scale.
One way to fix this is by assigning the Wheel prefab child object’s CapsuleCollider to, e.g., a “Wheel” layer, and assigning the “Car Body” to a “Body” layer, and turning off collisions between those layers
Ah, I think you can do this per-object actually with LayerOverrides in Unity 2022 (I mostly use 2021 rn). You can also do it in code, and I thought there was a “don’t collide with parents” thing, but maybe that’s only RigidBody.
I don’t think you need a “Suspension_Base” ArticulationBody or GameObject.
Your Car Body weighs 100kg, your wheels weigh 5kg and your suspension spring weighs 1kg. I’d maybe go with ~real world values (no idea what to do for suspension spring, I’m not sure how sensitive the physics maths are to small values), I’m not sure how sensitive articulations are to a 1:100 mass ratio between joints.
You have a 90 degree Anchor Rotation in your prismatic joint, which I don’t think you’ll need, but I found all the anchor stuff pretty confusing:)
I wasn’t brave enough to try actually spinning actual meshes to make friction to drive my vehicles. My feeling is that because the wheels and terrain won’t have any squish in them, you’ll lose contact quite a bit, perhaps even on smooth surfaces, for sure on bumpy terrain. I had the same problem tbh.
I found I had to crank up the fixedTime step (120Hz), Solver Iterations, Solver Velocity Iterations to get my suspension to behave nicely, but this was some time ago, & I’ve not revisited it, maybe I can ease off a bit.
Depending on your requirements, I’d really recommend Edy Vehicle Physics or Vehicle Physics Pro. Neither will really work as-is for large protruding wheels like your example project (like, e.g., a monster truck), but for normal cars on normal roads they’re great if you’re careful with your colliders, I think.
