I like to try and make lego rc cars (with motors and technic peices) but this time there is a problem with the front wheels, i dont know if it needs a differential or something but there is a problem, every time i put the motor in one of my 3 gears in the small makeshift gearbox comes out and leaves the one on the other side on its own, for example if the box is looking like this to start : |_| with one on each side once i try to go forward it ends up like this \_| and that gear on the left goes out of its place, because of this the car (which is meant to go forward) goes in the direction of the gear which is actually in place so in this case the car can only go right, but if press the backwards button it takes that right gar out a places the left one back in making it go left , so its like an unwanted turning system (that only turns). it does not happen when i lift the car of the ground and turn on the motor, but once the wheels touch it just moves a gear out of place. how do i fix this? some people say i need to build a strong frame around it, some say i need a differential, what do i really have to do?