In total there are twelve custom motorcycles around the city. Each cycle is marked on your map by a blue side mission indicator. You must collect all twelve bikes, and drive them back to Torque at Ironside Cycles. Make sure you drive the bike inside the showroom and place it in the blue circle marked by your minimal, otherwise they might blow up rendering the achievement missable. The custom motorcycle needed in story mode is not required for the achievement.
It appears there is a small glitch with this achievement but luckily it still unlocks. If you try to deliver any bikes after the captains have been killed, the game will launch into the story cutscene instead causing the bike to disappear. Thankfully this still counts as delivered as the bike is no longer in its original location. Sometimes the delivery marker at the garage will also disappear, but any bikes brought here will also still count.