Rigid Harley Davidson
In the United States servicemen returning from World War II were looking for a thrill. Many veterans had been trained to work on automobiles and motorcycles and were looking to add a little excitement to their post-war lives with their newly acquired mechanical skills. Motorcycles and Hot Rods were the perfect hobby for them.
Motorcyclists bought up surplus military bikes and removed all the unnecessary parts like windshields and saddlebags to minimise weight. Rear fenders were "bobbed" or shortened just enough to handle a passenger and keep the rain and mud coming off the rear, and sometimes removed the mirrors, or replaced them with tiny ones, such as the type used by dentists in their work.
This type of home customization led to the rise of the "bobber". Then in the 60s, motorcyclists found that a longer front end allowed the bike to run smoother at faster speeds. The degree of neck rake and length of front end was modified on these bikes with this in mind.
Source: wikipedia.org