If you have to take many things with you, it will be better to carry them in your car, rather than balancing them on your bicycle.

balancing 是动词 balance 的现在分词形式。balance 的意思是"保持平衡"，指使物体保持稳定不倾倒的状态。在这里 balancing 表示"在自行车上保持物品平衡"的动作。