Hi everyone. A door closer is that thing above your office door that makes it close on it's own, but not super fast. It's basically a piston to slow it down and spring to pull it shut. It has two arms, one hinged to the middle, horizontally speaking, of the top of the door. The other connects to the 1st arm and rotates attached the main housing of the door closer where it interacts with the spring and the piston. Okay, hopefully everyone knows what I'm talking about. What I'm really curious about is how the length of the various arms and the placement of the closer and the arm that attaches to the door affect how it works, how strongly or quickly it closes the door and so on. My real interest is building an automatic door opener/closer for my chicken coop. My idea is to build a door closer but instead of a piston and spring, use a stepper motor. Unfortunately, I really don't understand how all the variables affect how it works. I need to make it open the door all the way and then close it tightly without putting too much strain on the stepper motor. I've figured out that positioning where the arm attaches to the door has a pretty straightforward affect, in that closer to the hinge opens the door faster and requires more force, and positioning it closer to the edge makes it open slower but it requires less force. I built a model out of cardboard, and it really surprised me. I was able to get the door to close better by positioning the stepper-motor further from the door perpindicular to the door frame. The stepper had to swing a shorter distance as the door was further open, which I don't understand at all. The arm attached to the door in my model is a bit longer than the one attached to the stepper. I have almost zero engineering education, but I like to tinker, so if there's any good resources that will help me understand this, links would be great. Even the names of the principals or theories would be great, because then I could just go read about them. Thank you!!