How to Set a Minimum Length for the Camera Spring Arm

I did some experimenting with adjusting the probe size and disabling Do Collision Test, but couldn’t get exactly what I’m wanting.

I want to have an RTS style camera, which allows for full rotation around an invisible “player”, but once the camera collides with the ground plane it will not rotate down any more; hence it won’t zoom in, or clip through the ground. Any thoughts on how to accomplish that?