American Flyers (1985)

When Dr. Marcus Sommers (Kevin Costner) realizes that he and his troubled, estranged brother David (David Grant) may be prone a fatal brain disease that runs in their family, he decides to make peace with his sibling, and invites him on a trip to the Rockies. There, the brothers bond over their shared enthusiasm for cycling and decide to enter a grueling bike race through the mountains. However, Marcus' health soon begins to fail, and David must compete without his brother at his side.