The Stalker Frame is beefy, but not particularly efficient as far as rigidity goes. No one I have heard of has complained of chassis rigidity but there are some obvious simple improvements that can be made for minimal weight penalty. The easiest is to add two diagonals to the back rest frame. This will keep the rear frame from “parallelogramming” on a bump. Here are two views of the diagonals I added. Unfortunately there is no triangulation in the tunnel, nor is there room to add triangulation in the most important places.