the mousetrap bar is to reduce the amount of torque applied to the wheels. If too much torque is applied to the wheels, the force between the wheels and the ground will exceed the maximum frictional force due to the coefficient of friction between the wheel and ground surfaces. When this happens, the wheels slip and energy stored in the spring is wasted. Using a long lever on the mousetrap bar reduces the tension in the string due to the spring's torque, and thus reduces the torque applied to the car's wheels. In addition to reducing the torque applied to the wheels, the coefficient of friction may be improved by using higher friction materials. Distance car Making this type of car revolves around having the most force from the spring transferred to the lever arm and making the drive axle rotate. In order to get the most distance out of your car, you must make the lever arm long. This allows the car to get more rotation out of the wheels because of the longer string that will come along with a longer lever arm. Then make the drive wheels larger because the higher the diameter of the wheel the more ground it covers. Along with this make sure to have the axles smaller than the wheel so there can be more pulling distance. Reduce the most friction you can because the more friction the more energy is lost in propelling the car. Mass should also be taken into account, the more the car weighs the more energy from the mousetrap spring will be used so it'll be best to make the car light weight. Speed car Making a speed mouse trap car involves extracting the most energy you can from the mousetrap spring in a short distance. The lever arm needs to be shorter than the distance car's because the shorter the arm is, the quicker the spring will snap, and thus more torque gets extracted from the spring. Most of the car's weight is distributed to the back, where the drive axle is located. Built this way, the car will not have much drag in the front, slowing it down. The wheels don't need to be too large but the axle should be larger in relation to the wheel. The wheels need good traction because without the right amount of friction the wheels attached to the drive axle