Biker gang believable?

I really wonder if the gang of bikers seems authentic for somebody who lived in the USA in the 70s? I was rather shocked to see the nasty thing they've done -- well, the Hell's Angels probably would do something like this if you offended them at a service station, but this four had no black leather, no chains, only soft colorful clothes and 50% girls. Such a group would have been rather harmless here in Europe.