The Pro models allows for up to 18lbs of flywheel that yeilds a very smooth road like experience on the trainer. The resistance unit is Kinetic's top-of-the-line fluid model from the Road Machine.  The most versatile trainer you can find, featuring two separate flywheel options: an attached 6 lb. flywheel designed for interval training, plus an additional 12 lb. flywheel (18 lbs. total) for hard-core endurance workouts, like tempo and Long Slow Distance rides. Plus, the incredibly stable 540 square inch base keep each pedal stroke stable and focused. Variety has never felt more road-like