Brodie knob

A brodie knob (alternative spelling: brody knob) is a knob that attaches to the steering wheel of an automobile or other vehicle or equipment with a steering wheel. Other names for this knob include suicide, necker, granny, knuckle buster, and wheel spinner.
Design and use.
The device is a small, independently rotating knob (similar to a U.S. classic door knob) facing the driver that is securely mounted on the outside rim of a steering wheel. The protruding knob is an aftermarket accessory. The free rotation is intended to help make steering with one hand easier or faster.
Brodie knobs were popular on trucks and tractors before the advent of power steering. Their main use today is still in trucks, particularly semi trucks, where they allow simultaneous steering and operation of the radio or gearshift. They are also used on forklifts, riding lawnmowers, and ice resurfacers, where frequent sharp turning is required. The knob is also standard equipment on most modern farm and commercial tractors, its main purpose being to ease one-handed steering while the driver operates other controls with the other hand or is traveling in reverse.
Some boats are equipped with a helm featuring a stainless-steel wheel with a brodie knob.
Etymology and disadvantages.
The "Steering Wheel Spinner Knob" was invented by Joel R. Thorp of Wisconsin in 1936. The Brodie name is a reference to Steve Brodie and was meant to describe all manner of reckless stunts. The device is often called a "suicide knob" because of being notoriously useless for controlling the wheel during an emergency. It is also called a "knuckle buster" because of the disadvantage posed by the knob when letting go of the steering wheel after going around a corner, the wheel spins rapidly and the knob can hit the user's knuckle, forearm, or elbow. If the driver is wearing a long-sleeved shirt, the protruding accessory on the rim of the steering wheel can also become caught in the sleeve's open cut by the button. Other names include "granny knob", "suicide knob", and "wheel spinner."
Legality.
Brodie knobs are legal on private vehicles in most U.S. states.
U.S. Occupational Safety and Health Administration regulations restrict use of auxiliary devices for specific construction vehicles. OSHA prohibits modification of industrial equipment without the approval of the equipment manufacturer.