Originally Posted by aley

Bikes turn by rotating about their center of mass; to optimize quick handling, a bicycle must centralize its mass. The rider is by far the most massive thing on the bike, and it's pretty easy to see that the center of mass of a bike with a rider is relatively close to the rider's center of mass. Adding mass closely to the rider's center of mass will yield both the quickest rotation because it will result in a smaller radius of gyration of the system as it rotates around the CM. It will also cause the least impact on the handling of the bike because it will require about the same amount of effort and the same degree of movement to move the wheels out from under the CM, where steering geometry causes the front end to pivot to track the corner and camber thrust steering makes a not-insignificant contribution.