"When Swann goes miles off course he stumbles across a time travel experiment that utilizes two "cannons" that fire temporal energy to send each other back in time. Swann gets sent back to 1877 but rides off before the time cannons can return him to the future. Unsure of what has happened to him, Swann rides across the wild west in a red motorcycle riding suit and a red dirt bike, scaring local Mexicans who think he is the devil.
Swann meets a beautiful woman, Claire Cygne, and sleeps with her, but she is later kidnapped by a ruthless criminal, Porter Reese (portrayed by Peter Coyote) and his gang of rapists, thieves, and murderers. They also manage to capture Lyle Swann's dirt bike, leading to a series of hijinx, while Swann gets help from a posse trying to capture or kill the gang of criminals.
In a final showdown, Reese and Swann face each other atop a plateau. Just as a helicopter sent by the builders of the time machine shows up to take Swann home, his dirt bike falls off the side of the plateau, distracting Reese. He gets mangled by the copter's tail rotor, leaving only a pair of bloody boots behind¹. Just as the helicopter pulls away, Claire snatches from Swann's neck a pendant handed down from his great-grandfather... and he realizes that he is his own great-grandfather."
Bit of trivia: The writer of the film's screenplay and soundtrack was Michael Nesmith, who was formerly a member of band The Monkees. He appears briefly as one of the Baja 1000 officials in the beginning of the film.
You can buy the Timerider DVD through Aerostich.