I built and sold around 20 stir plates in search for a new product line to sell. This was discovered a bit by accident, but I greatly improved my performance by adding weight (inertia would be the correct term) to the motor/fan. I was using four pretty strong rare earth magnets, two stacked on each other, which were enough to counter the magnetic field of the motor. I glued a 1 inch tall by 1-1/4" chunk of wood dowel to the motor and glued the magnets to the dowel. This greatly slowed down the acceleration rate of the motor in both directions. The liquid will shear the stir bar from the magnetic coupling if it speeds up or slows down too fast. If you have room, stack a second set of magnets on the first. You'll get about 60% more pull on the stir bar.