This problem has to do with the way weight is distributed. If most of the weight is in the front, the back of the vehicle will tend to flip over the front, like a bicycle going downhill. If the back is heavier, the front wheels will lift off the ground. If weight is not distributed equally on each side, the car will tip to the heavier side.
The first step is to rearrange the parts to balance the weight from side to side and from front to back.
Add weight to the sides that are too light, if rearranging the parts doesn’t solve theproblem. Almost anything metal will do as a weight: coins, large paper clips, nails, washers. However, be careful in adding weight, because too much additional weight may make the car too heavy to drive.