The Stock Polaris RZR 4 frame has a bolt together joint. This includes a rear sub frame and a front sub frame, with the joint being directly under the front passengers. This joint is a week point that can bend and leave the Polaris RZR 4 frame being totaled, as well as injuries to you or your passengers. The ABF Polaris RZR 4 Frame Gusset Kit, creates a weld-in sleeve that spans the entire passenger compartment. It sandwiches the bolt-together joint and eliminates the weak point. The Frame Gusset Kit is constructed of 11ga steel for strength.