The "convex" front wheel is to increase the distance between the two parallel front wheels so that the front of the car can not deviate too much from the center due to centrifugal force when turning. Too much deviation can easily cause the car to overturn.

The reason why the rear wheel is "concave" is that it must be able to drag heavy objects as much as possible without deforming the axle due to pressure.

Generally speaking, it is all about stabilizing the car's shape and preventing safety accidents.