This is part of the Chaos Rising DLC.
There are a total of 12 Customer Motorcycles within the city. They will be marked on your map as a blue side-mission indicator. Once you collect a bike, drive it back to Torque at Ironside Cycles. You need to drive the bikes into the showroom and place them in the blue circle marked on your mini map. Otherwise, they may blow up and cause you to miss the achievement, which will cause you to have to restart the DLC from scratch.