I have the driver rest. It does not have height adjustment. The passenger backrest has the height adjustment(2 position I think) but do not think it has the froward rearward adjust-ability. Mine does not flex at all when I sit against it. These just push in to the seat and "snap" in to the fixed height. Therefore, a substantial force upward could pull it out. If you are depending on this to prevent the pack from rolling forward/rearward you should be OK. If you need this to tie down to, I would say not. I would think if you secure the pack to the rack and just use the backrest for lateral support you should be fine.