AVANT-GARDE BIKE RACK - Pranksters are up to their old tricks, adding new touches to the sculpture known as 'Tree of Knowledge.' The hanging bike, however, is harmless compared with previous pranks. When the sculpture graced the traffic island at State and liberty streets five years ago, vandals repeatedly splashed it with paint. Later, it sat in a city storage yard, entangled in overgrown weeds. The 1,600-pound work, created by former Ann Arborite Ronald Bauer, was sandblasted and re-installed two weeks ago at Hanover square, a mini-park at the entrance of Packard Road and Division Street.