knob knocker
 n.— «“Imagine a razor shaving your face, but on a much bigger scale,” says Shawn McKinney, president of the Springfield Area Mountain Bike Association about the knob knocker, a device created to shave the bumps off a mountain bike trail.» —“Smooth moves” by Chris Young State Journal-Register (Springfield, Illinois) June 26, 2004. (source: Double-Tongued Dictionary)