Although there was some number of early riders who wanted to improve their speed and/or performance of their bikes by removing excess weight, it wasn’t until the 1930’s when it started to become more common. Up until 1934, the premier motorcycle AMA racing class, Class A, allowed for highly modified motorcycles and subsequently, very expensive racing motorcycles. This made it so only the manufacturers’ racing teams could compete in the AMA races.
As the Great Depression decreased sales, the amount of money the manufacturers had to spend on the size of their race teams was reduced. This started to reduce the number of races and these circumstances were a large part of the impetus for the AMA to create the C Class in 1934. This new class required the motorcycles to be 750cc (45 ci) side-valve or 500cc overhead-valve bikes made with stock components.
With these limitations, the way for C Class riders to increase their speed was to remove unnecessary parts, including to cutoff or “bob” their back fenders. What was called a “bob-job” motorcycle back then is now called a “bobber.”
During WWII, the AMA suspended races. After the war, veterans were looking for an adrenaline rush and with tens of thousands of military bikes coming back from the theaters of the war, there were plenty of affordable motorcycles. The motorcycles produced for the war were heavy, so many of the veterans followed the same path of the pre-war C Class racers by removing parts and bobbing the back fenders, thus creating bobbers.
As the economy improved and veterans came back with money to spend, bobbers started to have more decorative changes like custom paint jobs and other aftermarket improvements. Another post-war reason for the increase in the number of bobbers was the popularity of AMA dirt bike races. Additionally, throughout these years, hill climb riders also decreased the weight of their bikes in the same manner.
Fast forward to the mid-60’s, many of the early choppers followed the bobber style, but with the addition of chopped frames and raked front ends. Over the last decade the popularity of bobbers has increased greatly to where there are bobber style models coming off the assembly lines. With the relatively recent retro wave of older motorcycles being restored or partially restored, the bobbers are returning to some of their earlier roots.