the problem is the freedom of the ballon; if the ballon starts moving the car tends to move in zigzags. To fix this problem ...
a ) insert the balloon in net and attach net onto car
b ) create a fence around ballon to prevent it from moving
We tried both, but havent decided on which one is better ... * still in discussion
concusion - we did not do any of the ideas, we just taped the balloon securely to the car.