that number is only precompression adjustment and only affects how high/low the truck spawns in. Your not tuning the actual suspension at all and most of the time when you go to low, the actual suspension beams break, causing the axle to go until the chassis.
I suggest checking ou the ROR wiki page on how truck files work, there is alot to tinker but you can edit yourself into an undriveable mess rather quickly.
http://www.rigsofrods.com/wiki/pages/Truck_Description_File