We recently discussed the birth and history of the bobber. Bobbers, while an important factor in the evolution of motorcycles themselves, did differ from their chopper offspring.
As the word implies, choppers were often chopped and welded to elongate the frame, and, obviously, out of these new customized chopper frames came the need for custom chopper parts. Though the spirit of the modifications remained the same, choppers took the trend to a whole new level.
Of course, the bobber wasn't necessarily developed with aesthetics in mind, though this often was a byproduct of their modifications. At the time, it was about speed and performance. Since the late 1930's, people were using dried up lakebeds as makeshift racetracks in various isolated regions across the country, most notably in southern California. These dirt and mud tracks obviously demanded performance that you just didn't get from bikes designed for the pavement; different handling and suspension specifications were required. The formation of the Southern California Timing Association helped to sanction such events and served to validate the hot rod culture in a way. Bikers wanted in, and needed to find a way to keep up (literally!) with the ever-evolving hot rod scene.
Many of the earliest customized bikes were chopped up from Harley's, and at the time you could get quite a steal on Harley Davidson bikes at military or police auctions. The Sportster was quite a popular model for modding, being affordable and easily customizable. Before long, people were making their own custom Sportster parts, adding chrome any which way they could, chopping up frames, and customizing every imaginable detail of their rides.
By the late 1950's, hot rods and choppers were competing in the same events, and the phenomena had grown from geographically isolated subculture to full on national craze. As it grew in popularity, aesthetics became more and more prominent a reason to customize your ride (this focus on appearance would later inspire the hardtail frame look of Peter Fonda's bike in Easy Rider, which would take the custom chopper parts scene to an entirely new level of popularity)
It is practically impossible to overstate the importance of World War II's effect on the chopper's evolution. There were over 7 million US soldiers stationed abroad at the end of the war, many of whom were trained in mechanics, welding, and/or engineering. The knowledge of these soldiers would drive the evolution of engines. Our entire economy had come to a halt and focused on winning the war. This left a surplus of all sorts of alloys, factories, and engines, with no immediately obvious or practical use. These surplus engines would make great engines for your custom chopper.
No one would think to thank President Dwight D. Eisenhower for his contributions to the world of custom chopper parts, but after signing the legislation to lend the allies $50 billion in aid to fight the axis, many European countries found themselves in a huge amount of financial debt to the United States. Britain had borrowed over $30 billion themselves. Now, us chopper enthusiasts like to think of that as an investment in our hobby, as popular manufacturer Triumph would send most of their bikes to the United States to help satisfy that debt. Triumph had already set the tone for inline twins, debuting the Triumph Speed Twin in 1937. These twin cylinder engines were lightweight, compact, and perhaps most importantly easy to maintain and modify. Custom Triumph chopper parts were being hacked and welded in garages across America.
Fast-forward to the present day, and the chopper scene is thriving more than ever. After a revitalization in the chopper culture in the 1990's, really anyone could customize their bike anyway they wanted to. Advancements in engineering and manufacturing techniques, as well as the founding of many more aftermarket parts suppliers brought custom chopper parts to anyone. Now we have a large number of websites, magazines, and television shows dedicated the chopper, and the culture seems to grow every day.