Equation is not yielding the expected output

The equation you've posted appears to be an analytical hand calc of rollover as a function of half track-width and CG height. With a multibody model, you don't need to code this anywhere. Instead you would use your vehicle model to run a steady state cornering event like Constant Radius or swept steer. In these events, the lateral acceleration slowly ramps up (either by increasing speed around a constant radius corner, or by increasing steering wheel angle at a constant speed around a reducing radius corner), until the vehicle is no longer stable in roll.