The "retractable fifth wheel" in an automotive context refers to a parking assistance invention from the mid-20th century that was never mass-produced or widely adopted, and is therefore discontinued.

Description of the Invention: Invented by Brooks Walker and patented in the 1950s (after a 1930s prototype), the system was designed to simplify parallel parking.

Mechanism: A spare tire would lower from the trunk onto the ground behind the rear axle of the car.

Function: Once deployed, this extra, or "fifth," wheel was powered to slightly lift the rear drive wheels off the ground, allowing the vehicle to pivot or rotate sideways into a tight parking space.

Purpose: The intent was to eliminate the difficult back-and-forth maneuvering typically required for parallel parking.

Reason for Discontinuation: Despite public demonstrations that garnered interest, car manufacturers ultimately decided the retractable fifth wheel was too complex and expensive for mass production. The invention never caught on commercially and was discontinued before it truly hit the market.