This image costs buyers of a new Harley-Davidson $8000 to $25,000 - far more than other brands - and as much as $150,000 for rare, early models in Japan. The latter has become a major market for Americans.
"Many motorbikes look like Harleys, but a Harley sounds so different," said Lin Todd, a former helicopter pilot who, like other Harley fans, revels in the "heavy-throated, muscular sound" that other makers try unsuccessfully to imitate.
"Harleys hold their value better than any other brand," added Mark Andrew, who works for a trucking company. He estimated that his fifth Harley, for which he paid $15,000 in 1998 and accessorized, could sell for up to $22,000 - unless he trades it in at book value at a dealership.
The loud roar of the engine - which riders amplify by adjusting or removing the mufflers - is just one aspect of the black leather, bad-boy image that Harley-Davidson tried to shake for most of its life. The company - founded in 1903 by William Harley and Arthur Davidson, and joined later by Davidson's brothers Walter and William - finally embraced that image in recent decades as "bad" became marketably "cool" and rock n' roll became mainstream.
"Riders talk about motorcycling as a peaceful experience, a frontier activity. There's a tribal nature to it"
Senior archivist at Harley-Davidson Motorcycle Co.
"It's one of those images associated with freedom, individuality," said Bill Jack, senior archivist at Harley-Davidson Motorcycle Co in Milwaukee, Wisconsin. "Riders talk about motorcycling as a peaceful experience, a frontier activity. There's a tribal nature to it. Information and history are passed on orally."
That motorcycling aura has been popularised by movies such as Marlon Brando's The Wild One (1953), Steve McQueen's The Great Escape (1963) and, recently, Bad Boys.
But this image goes back to the 1880s, when inventors such as Gottlieb Daimler in Germany motorised a bicycle with a gas engine. By the early 1900s, hundreds of companies were making these new loud and fast machines with no brakes or clutches that posed a danger to pedestrians and horses on the road.
Harleys were made especially rugged and heavy for the clay-rich mud of Milwaukee in the winter. The daredevil image stuck when in 1936, Harley-Davidson focused on the sports market to survive the plentiful cheap automobiles rolling off Henry Ford's assembly line on to the roads.
Motorcycles from that period and earlier are the most valuable, because most were junked in a country that favoured the new.
Models from 1905 to 1909 are now worth $80,000 to $150,000, according to Manabu Okada, owner of Semba, Japan's largest dealership (www.semba.co.jp). Taxes, freight and other charges push prices up in Japan, said the collector, whose grandfather started the business by buying up 200 military Harleys from the US Army after World War Two. These models now sell for $13,000 to $18,000 on both sides of the Pacific.
In 1971, when cheap, reliable Japanese motorcycles ruled the market, broken-down Harleys from the 1930s and 1940s sold for just $75 each, said Herbert Wagner, author of At the Creation: Myth, Reality, and the Origin of the Harley-Davidson Motorcycle, 1901-1909 (www.atthecreation.com).
"You could get a runner for $300 to $400, he added.
Today, a restored 1936 Knucklehead - named after the shape of its engine - is worth $50,000 to $100,000, double the price in the mid-1980s. One completely restored model sold for $95,000 just six months ago in the United States, Okada said.
The Knucklehead, also called the Blockhead by Harley enthusiasts, was a milestone in design, with styled square blocks that housed the valves. It begat other innovative engines such as the Panhead (1948), Shovelhead (1966), Evolution (1983) and Twin Cam 88 (1999) that collectors pay a premium for in landmark models.
In 1984 Harley-Davidson went public, and antique motorcycles sold for record prices at an auction after Steve McQueen's death, setting the tone for the secondary Harley market until now.
To celebrate its 100th Anniversary, Harley-Davidson has released 100 Years of Harley-Davidson (Bulfinch Press), by Willie Davidson, grandson of company co-founder William Davidson.