Previously I'd observed that wheel colliders were occasionally preventing the modular car rigidbody from sleeping, even with a forced sleep call and having motorTorque and brakeTorque set to zero. To counter that, I was simply disabling them entirely when the rigidbody went to sleep. However, that caused a surprising new bug: If you shot a sleeping car with a rocket, it would correctly wake the rigidbody, but the act of the wheels waking up as well would completely cancel the rocket explosion's force [because Unity], meaning the car didn't move. I've now determined that it was the steering being non-zero that was preventing sleep [because Unity], so I've removed the disable/enable for wheelcolliders and set steerAngle to zero on sleep as well, which lets sleep work correctly, and explosion forces on sleeping cars now work again.