The trolley rides on a I Beam , it is adjustable to fit a I bean that is from 3 1/4 inches wide to 8 inches wide measured on the flat part of the beam the bottom of the beam. This hoist and trolley are Industrial rated they are NOT cheap tiawanease junk. the trolley has ball bearings also , most only have brass sleev bushings. In the pictures the trolley is upside down ..if you look at the shape i couldnt get it to sit upright and it weighs about 30 lbs lol. Let me know if you would like to call my cell number is 231 392 5459.