Problem in adding new car models in my unity car racing game

Hello every one please help me this is very major problem right now for me ... as i am developing a car racing game in unity the game is perfect every thing is awesome the A.I car .. But when i am trying to insert new car models (these models have no issues they are perfect) the car is not stable i mean on turn car lift its right side or left side and not stable seems something wrong with its center of mass i set it also via script i take the new model of car and put all required colliders on the car and wheels .. but car seems not stable ... some time car speeds goes slow it look's like due to some friction i adjust the position of wheels collider and the problem has been solved but on turns the car lift it left side up or right side up .... why my new cars lost balance i am unity unity car tutorial script and some car models from asset stores please help me by telling what is is the right way to import new car in your game so they also remain stable like dummy car model some of cars are all right now by my effort but do not know the exact problem i want some one to help me so i can easily import new cars into my game I want someone to tell me how to take a new model of car and make it work perfect like dummy car by importing it to unity and putting colliders on it and rest of the thing or do any one can guide me to a tutorial ... ?? i am not asking for making a new car i am just asking for inserting new car;s in my game that remain stable .. :)