As someone who was around at the time of the big boom in sales, and who actually participated, I'd like to make some comments.
There were two major influences to the dramatic growth in sales neither of which receives a mention in the article as far as I can see.
Prior to the late 60s most young motorcyclists were "enthusiasts", although there were also more mature riders motivated by the advantages of relatively inexpensive transport. The bikes were unreliable, dirty, smelly, burned holes in your clothes, and required a degree of physical strength to master the arcane starting procedure. The riders were also therefore, virtually exclusively male.
What motorcycle advertising there was had previously been restricted to specialist publications. Following the massively influential adoption by the Japanese of the electric start, which meant anybody could start a motorcycle, Honda mounted a huge advertising campaign "You meet the nicest people on a Honda." Full page ads appeared in all sorts of publications that had never previously seen a motorcycle ad: Playboy, Time, Rolling Stone etc etc.
That campaign brought motorcycling into the mainstream of American, and subsequently New Zealand life. All sorts of people who would (could?) never previously have contemplated riding a motorcycle took to two wheels.
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