The Main Axle is from an S10 Chevy truck.  It's tapered shaft makes it easy to slide out of the bearings, the fork bolts-on just like a tire.  The bearings are surlpus, but have no defects.  The fork has 3/16" aluminum plating bottom and top to prevent Torsional Stress on the internal cross-box construction.  The main Axle Bearing Box has a hole in the bottom where the Axle protrudes for attachment of an optional Opto-Encoder.  The  Legs are attached to the Axle Box by 5/8" Steel Rods which go through the box; this makes for a very stable Axle mount, and comes apart by loosening 2 nuts.  The rear wheels are swivel mount with lever locks.