ive rrecently done some minimal adjustments to the tyre javas. its important you change individual values when testing so you can make notes on the behavior.
also, I know for a fact that the grip levels on the tyres doesn't need reducing, its already accurate. keep notr of how the ai drive after your modifications, if they understeer into a wall, you're doing it wrong.
as for unlocking the full 20 variables, ill look into it, simulation style tyre physics should be possible
