Heavy-Duti is a model, like Typhoon or Sting ray. When speaking of heavyweights vs. middleweights, heavyweight generally means older multi-bar frame w/plenty of room for big balloons, middleweights are narrower w/usually just enough room for 2.125 tires, but come w/narrower sheet metal and 1.75"'s. The difference in the weight of the bike is minimal, although heavyweights can be viewed as having a greater load capacity. Lightweights are skinny tired bikes without cantilever, or lower bars(tube). I believe, but am not absolutely certain, that a Heavy-Duti model is classified as a middleweight.