The Shell Cycle (literally: "Shell Motorcycle") is a red and black motorbike, mostly used by Raphael. It is first seen in the episode 'Meet Casey Jones' in the 2003 TV Series and continues to be used throughout the series. It is used for making quick getaways to catching villains on the loose. Raphael is shown to be a skilled motorist. It is also used in the 2003 GBA video game.
The Shell Cycle is Raphael's personal vehicle; apart from using the Battle Shell, Raphael uses his cycle for going on missions on his own. Its first appearance was in the episode Meet Casey Jones, shortly after the debut of the Battle Shell, which the Turtles were taking to Central park to confront the vigilante. Donatello reveals that he had "whipped up something in the back," and Raphael discovered that his brother had built a motorbike with accompanying helmet.
During subsequent conflicts, the Shell Cycle was kept in the Battle Shell in case it needed to be used, and continued to be extremely useful to Raphael.
Raphael's motorbike is basically nothing more than a heavy motorcycle that Donatello specifically designed for Raphael's use. In order to construct it specially to Raphael's needs, it was assembled from various parts of high-performance vehicles, and was painted bright red, presumably to match Raph's bandana. It has a turbo boost to increase its speed, and because of Raphael's reckless driving it has been specially designed for stability and could therefore easily endure a heavier rider, such as Hun. It can perform wheelies which make the motorbike go further and faster, and by necessity it also has more powerful brakes than a typical motorbike. It also seems to have a remote control mode, seen in Meet Casey Jones when Raphael bailed off of the vehicle and it stopped automatically.
A brief description of the vehicle is in Simon & Schuster's - Turtle Tales: An Insider Guide by Leonardo.
The Shell Cycle occurs in the 2003 GBA video game based on the 2003 series.