Sirius Black is definitely the leader of a wholesome motorcycle gang.
Sirius habitually hangs out in the most run-down bars looking for interesting people to befriend.
He started going to those places after James died because he didn't want more friends, because losing James was too hard and he couldn't get over the idea that more friends was more loss just waiting to happen, and how much shit is one person supposed to handle, anyway? So he starts going to these shit, gutter-trash bars thinking he'd never connect with any of the people there, trying to be an outsider, hoping that everyone would just leave him alone.
Naturally, because Sirius can't help himself and despite everything that happened with Peter and James, he still seeks out the best in people and eventually he makes friends anyway.
From that point on, Sirius basically becomes a Hagrid for people. He finds the broken ones, the scary ones, and the fucked up ones that everyone prejudges and fears and he finds something great about them, because of course he's been prejudged his whole life because of his shit family and the love of his life has been put down for being a werewolf and he's just so goddamn sick of people writing off other people.
At first, Remus is a little concerned. Walking into one's own home and seeing Sirius Fucking Black at the table scooting pieces around on a chess board with what appears to be the large, tattooed, human incarnation of a windowless van is a terrifying prospect and he thinks that maybe, just maybe Sirius is trying to self-destruct. He sighs, puts on the tea, and resolves to talk it out with him later but by the time the game is over and the man leaves, Remus understands and tea-time with felons becomes a routine in the Black-Lupin household.
Inevitably, Sirius encounters the idea of a motorcycle gang from some kind of muggle media that Remus did his -goddamn best- not to let him get his hands on because he knew it would put ideas in head and of course he was right but what else is new? Sirius, documented lover of motorcycles, starts his own gang.
So he collects these broken souls into his little motorcycle group, (that's really more like a support group with motorcycles thrown in sometimes just for the look of it,) and everything is fine.
This is my hill and I will fucking die on it.