It makes my f**%ing blood boil when I read its inertia only about Dynojets.. I had an early model 248 , and it was loading as they all were. It had proportional air brakes that I could load the car against and hold it at tire speed, engine speed or % load.. I don't think the people I bought it from even knew it was capable of load holding. The new dynojets have the same load hold, or you can order eddy brake, that gives you real time torque output. The roller is 2 feet in diameter, its weighted, but serves several purposes: first it allows the the dyno to have a fixed calibration for cross dyno repeatability, that can't be monkeyed with, second is for contact patch, and it doesn't destroy sidewalls of the tires with excessive loading, really an important aspect when it comes to road racing.