If the car tips forward or backwards when driven, the car might not be not balanced well.  If the front of the car pops up when it is driven forward it is too heavy in the back or too light in the front. This is shown in the pictures below.
       
       Bad:

 
                  Stationary                                            Moving

To fix this problem, you need to adjust the weight of the car. You can either add more weight to the front or reduce weight from the back. Also, the wheels can be spaced farther apart so that more force is needed to knock the car off balance.  In the picture below, this problem was fixed by moving the RCX more in the middle of the car.

      Good:
  
                  Stationary                                            Moving





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