I've been working on a simulation of a Lorenz Waterwheel, and friction was killing it - I couldn't use realistic coefficients of friction. The problem, I discovered was that I was not using the correct equation to calculate the force of friction that counters the wheel's rotation. Having misplaced my physics reference book, I thought I could easily find this equation on the web - not so! After devoting hours of internet search, I cannot for the life of me find this simple equation! Can anyone help me? To clarify what I need, I've provided a basic model below: View attachment wheel_and_axle_friction.bmp A wheel of radius rw is centered on an axle of radius ra. The wheel has mass, m. There exists a coefficient of static friction between the wheel and axle, us. What is the equation to calculate the force of friction that opposes rotation of the wheel? Many thanks!