Harry Potter Q: Why did Sirius Black say he no longer needs his motorcycle?

Because of the fact that everyone believed he was the Potters' "Secret Keeper". when they were killed he understood that the suspicion would fall on him and that he would probably be forced into hiding to maintain his freedom unless he could prove his innocence. He gave Hagrid the motorcycle because he knew it was too conspicuous to allow him to hide effectively while he tracked Pettigrew down.