A knob attached to the rim of a steering wheel that enables a driver to turn the wheel by moving the knob. Very common in the late 1940s and through the 1950s. Illegal in most places since then. Also called a nigger knob.
Ermal has a suicide knob on the steering wheel of his Case tractor. He decided to put one on the wheel of his Pontiac, too, so he could spoon with Ida Mae while he drives.