How much do you want to set it to? If there is a limit, it's beyond the 5300 kg required for the FIA race trucks (This is the anatomy of a 1,100bhp racing truck | Top Gear).
its is set at Mass=630.0 but when i go above Mass=830 and load it in developer mode the game just crashes, i wonted to go to about Mass=1200
It could be, that, if your Car uses a flex Chassis ini, the Mass that you use in the hdv, exceeds the added Masses from the Chassis.ini. Normally, you should have an Error for that, in the Trace File.
As I recall, that's usually a problem with something going negative in the totals, like sprung mass vs unsprung mass. [redapg beat me to it!]
Well….if You increase the mass in the HDV file, You have to increase also the inertia values of mass ( this is possible , as calculation, with RF2 spreadsheet ). Then, in mod dev, You have to start the game with the car you updated in mass and inertia values. Close the game. Go in Userdata, and You will find a new chassis, autogenerated from RF2, in which there are updated values of mass and inertia for front and rear subbodies. Take this new chassis, renaming it like your original one. Copy and past it in the Mod Dev vehicle folder of your car, overwriting the previous chassis. In this way, no errors or crashes will happen for an HDV updated in its mass value.
