What was Scott’s and Bobby’s relationship like in chapters 9-11?

Scott and Bobby's relationship ungoes a certain kind of distancing in these chapters, not because the brothers' feelings change.... but rather, they're simply growing up. Scott enjoys his studies, joins the school newspaper, and tries to engage his brother in reading.... something Bobby has no interest in. Bobby, on the other hand, gets a job and immerses himself in helping his father work on the Corvette.

In Chapters 9-11, I wouldn't say that Bobby is the best of role models, He sometimes gets into trouble, and sadly, he is very self-centered. There are, however, reasons for his attitude.... but you're not going to discover those reasons until you've read further into the novel.