Decreasing distance for a Configurable Joint at run-time | Unity

You need to adjust the anchor(s). Note, that they are defined in the local coordinates, relative to the owner object. There are two anchors: ConfigurableJoint.anchor and ConfigurableJoint.connectedAnchor. You can adjust either one, but if you create the bodies in autoConfigureConnectedAnchor mode, then it may be challenging to figure out how to adjust which anchor.

I'd suggest to no rely on the automatic behavior and set both anchors manually when creating the joint. This way you'll know exactly how and what to adjust.

Setup the joint like this:

 // Given: GameObject hingeObject; GameObject ballObject; // Then: var joint = hingeObject.AddComponent<ConfigurableJoint>(); joint.autoConfigureConnectedAnchor = false; ballObject.transform.LookAt(hingeObject.transform); // Optional. joint.connectedBody = ballObject.GetComponent<Rigidbody>(); joint.anchor = Vector3.zero; var initDistance = Vector3.Distance(hindgeObject.transform.position, ballObject.transform.position); // ATTENTION! It's important to know what axis your joint uses! // If doesn't work, try to move initDistance to X or Y. joint.connectedAnchor = new Vector3(0, 0, initDistance); 

Now changing the distance is trivial:

 // Don't forget about axis. joint.connectedAnchor = new Vector3(0, 0, newDistance);