Dumbbells can be classified from casting into two categories: fixed weight and adjustable weight. Fixed weight dumbbells. Cast with cast iron, the middle is an iron bar, the two ends of a solid ball. 2. Adjustable dumbbells. Similar to the reduced barbell, the short iron bar on both ends of the weight of the round cast iron pieces, about 40 ~ 45 cm long, lifting or exercise can increase or decrease the weight.