I Can't Believe They Invented It!

S03E09 - Saturdays of Thunder

Homer takes a fatherhood quiz and discovers he knows next to nothing about his son. He signs up for therapy at the National Fatherhood Institute and after a confidence-building pep talk, offers to help Bart build his own soapbox racer for the upcoming Soap Box Derby. At the qualifying race, Bart and Martin form an alliance vowing that either must somehow beat bully Nelson and his very intimidating racer, the Roadkill 2000, As the race starts, Bart can barely reach any kind of noticeable speed with Homer's racer and it eventually falls apart, along with Nelson using every dirty trick to beat Bart. Martin wins the race, but his aerodynamically advanced racer causes him to lose control at high speed and crashes. Martin finds out that due to his injuries he can no longer race and opts for Bart and himself to combine forces by having Bart race Martin's racer over the racer he and Homer built.

Bart gladly accepts Martin's offer, as there is no other way he could defeat Nelson. Homer, however, feels betrayed by Bart's choice to drive Martin's racer instead of the one he helped Bart to make. When Bart tries to apologize to Homer, he selfishly denounces both Bart and Martin, telling Bart to do whatever he wants. Marge steps in and tells Homer that she has defended him through a lot of things over the years, but his latest actions prove that he is a bad father. As Bart gets ready to race in the final match with Martin's newly tuned racer, Homer thinks to himself about how selfish he has been, realizing that Bart needs his support regardless of whose racer he is using, and he rushes off to the race. At the starting line, Homer wishes Bart good luck and tells him that no matter how the race ends he will still be proud of him. The race is tough as Nelson pulls every dirty trick in his arsenal, but eventually through his skill Bart finishes first and the team enjoys their victory.

Source: Wikipedia