If you let the Bikers operate their business, meaning you don't attack their Empire Sites, after the Cholos are wiped out in the story missions and you don't purchase their sites, the Bikers will and gain in strength.
Then, once you reach the second island, attack a couple of the Sharks business in the northern part, around the North Point Mall, and either buy the empire site or leave it for the Bikers to purchase.
The idea is to place a foothold for the Bikers on the east island, connecting with their dominance in the north-western island.
Afterwards, in due time, The Biker gangs will wage multiple attacks against the Sharks and wipe them out of the game, and the Sharks won't bother your business since it's the Bikers attacking them and not you.
Indeed, at least once I've finished the game with only seven Empire sites, required to complete the story, and the twenty three other sites all belong to the Bikers!
It was marvelous.
But then you realize that two years later the Bikers are nothing but the Greasy Chopper's favorite customers.