About biker gangs i genarol, bikes are useful tools for theft and fast escape they can be smal withs is very use ful for robing someones bag or purse (scooter are a good exsemple of smal bikis).
They can also be very fast and that useful for escape the cops or how is foloing you.
And they can also be pretty big and usefull for lang distences withs is perfect for smukling stuf, in the 80,s and 90,s organasations such as the Columbion cartel often used bikes to spread drugs. (a good exemple of a big bike is a Harley Davidson)
In most country there are biker clubs, and biker clubs also have people how comit crime and hang out on the streets that gives the moter club a bad name and then there asociated whit biker gangs.
The Hells Angels are a biker club that have spred out true the world, and there active in many country,s so i wouldn't be surpised if there is also a Japanese section of the Hells Angels.