The world of classic motorcycles is almost as diverse as the world of classic cars, but a handful of makes and genres of bikes make up the majority of what enthusiasts find appealing. Larger cruisers and more nimble sport bikes make up most of the bikes on this graph, and volume motorcycle manufacturers from the U.S., Great Britain and Japan make up nine of the 10 most popular makes.
Harley-Davidson's spot on top makes sense given that it was the only large-scale American motorcycle manufacturer for decades after Indian stopped building bikes in 1953. More performance-oriented riders in the 1950s-70s, when most of the bikes that make up this graph were built, had a wider range of choices. British bikes from companies like Triumph had a huge share of the market until the Japanese started arriving in the 1960s with more modern and reliable bikes, and by the end of the 1970s had more or less taken over the market that the British once enjoyed.
People shopping for a classic bike are drawn to these bikes in large part thanks to affordability, relatively easy maintenance and parts availability, and an abundance of choice (Honda made over half a million CB750s). Especially with Harley-Davidson and Honda, as the graph shows, it's the high-volume, easy to own classic bikes that attract enthusiasts.