Documenting the dark corners of the Unity Engine.

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  • HingeJoint does not constrain the absolute rotation or localRotation of the Transform that it is attached to. It constrains the relative rotation between its own RigidBody and the Rigidbody in the ConnectedBody field.
  • HingeJoint.angle and HingJoint.axis do not represent the rotation of the Transform that it is attached to. Ex:
    Quaternion.AngleAxis(hinge.angle, hinge.axis) 
    // is *not* equivalent to either of

Rest Angles

To quote the official docs:

The rest angle between the bodies is always zero at the beginning of the simulation.

The consequence of this is that HingeJoint.angle does not depend on the starting orientation of the joint's RigidBody or the connected RigidBody. And because HingeJoint.limits represents limits on HingeJoint.angle, the limits are also independent of any starting orientation of the bodies.

An example: Consider a GameObject with rotation (0, 0, 0) and a HingeJoint on the x-axis with limits of [-90, 90]. When played as-is, this GameObject will be free to rotate between (-90, 0, 0) and (90, 0, 0). However, if you rotate this GameObject in the editor such that it starts with rotation (90, 0, 0) and play it, the GameObject will then be free to rotate between (0, 0, 0) and (180, 0, 0) - even though the angle limits have not changed.

HingeJoints with moving parents:

In most cases, HingeJoint should have a ConnectedBody assigned. If left empty it causes your HingeJoint to be anchored to the global physics coordinate system. If your HingeJoint is the child of an object that moves, the joint will apply its limits within the global physics coordinate system - not in the joint's local coordinate system.

To ensure that your HingeJoint behaves nicely, create a RigidBody on the parent object and set that as the ConnectedBody.1) In this configuration, the axis and angle of the HingeJoint will correspond to the localRotation of the Transform 2). This makes the joint behavior much easier to reason about!

The parent body can have IsKinematic enabled if you don't want it to act like a physics object.
though it will still be relative to any initial localRotation
hingejoint.txt · Last modified: 2021/05/01 16:59 by