Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Physics won’t be denied. Think of this or any engine as a rotating mass that twists, such that it tries to take the surrounding mass with it. As the speed of the rotation is rapidly altered, the directing of the twisting force momentarily reverses, and the mass tries to hop. If that mass has a long enough lever arm on a short enough base, it will try to twist instantaneously, before relaxing just as fast, and you get a hop. 

My M91 isn’t the slowest or most steady runner, but it no longer hops. The longer, lower, most importantly longer, and heavier base defeats the torque.