Explanation
A. Do you always come to the office on a bike? B. Do you always come to the office by bike? In sentence a, the word "riding" is implied. He goes to work by "riding on a bike". In sentence b, " by bike " is a method of travel. He could go there "by bike" or "by bus" or "by train" or "by car" or "by taxi" . Those are all different methods of travel.
