For years the Key West Poker Run that draws thousands of bikers to the Southernmost City happened "automagically," as its founder was fond of saying.

"That's what dad always said every year," said Drew Peterson, son of the bike rally's founding organizer, Phil Peterson, owner of Peterson's Harley-Davidson of Miami/South. "In in the early days, it just sort of happened and dad would say at the last minute, 'Hey, it's poker run time again.'"

This is the 40th anniversary of one of South Florida's most popular biker runs that began with 42 riders in 1972. Just three years later, the run attracted 300 riders, Peterson said.

Now it typically lures up to 10,000 road warriors -- depending on weather -- to Key West's hotels, bars and restaurants. The run now raises about $80,000 a year for the Key West Sunrise Rotary and diabetes research.

It has become a tradition every third week of September for riders across Florida as well as motorcyclists out of state and continues to put much-needed cash into business coffers during the slowest month of the year for Key West.

Peterson's business partner and Key West resident Mike Horn changed the date of the event 21 years ago from August to September after hearing that the event wasn't impacting local businesses much, he said.

"They've been booked solid ever since," Horn said.

The event has evolved over the years into more of a Duval Street bike show that draws offbeat and custom creations from mostly professional or "weekend warriors," organizers said.

In 1971, Peterson and Horn fired up their friends in the Mustang Motorcycle Club and sought to boost interest in both the Miami motorcycle shop and the (now-closed) Truman Avenue Key West store, the men said. Neither had any idea the run would become the "biggest party in South Florida" as advertised on Peterson's website.

Horn affectionately recalled former Key West Mayor Sheila Mullins' attempt to halt the annual gathering.

"We canceled it, but everyone came anyway," Horn said, laughing.

Early events included skill challenges in which riders would have to dribble a basketball or maneuver their bikes in close quarters on closed tracks. There also was a popular dinner and dance event at which couples would dress in matching outfits that also matched the paint schemes on their megacycles.

"Now, it's more about camaraderie," Peterson said.

Organizers charge $5 for wristbands that allow a certain amount of riders to park on Duval Street, the artery that has become party central as well as a "reunion" site, Peterson said.

"It's become a place to run into friends and find those people who have moved away as well as just show off your bike," Peterson said.

In the past 40 years, the event has proven its worth to the city, said Tourist Development Council Director Harold Wheeler.

"For most of the part, these are doctors, lawyers and professionals and I think the bikers themselves have done a very good job of policing themselves in terms of working with the Police Department," Wheeler said.

"It's a great event that has made the cash registers ring when we really need it."

The city is working with Poker Run organizers this year to help with traffic given the work on North Roosevelt Boulevard and informing them of alternate routes out of town via South Roosevelt Boulevard, said city spokeswoman Alyson Crean.

This year's run coincides with the Navy UNITAS Atlantic 2012 event, a multinational maritime exercise drawing 14 frigates, destroyers and amphibious assault ships from the U.S. Navy and other allied nations, some of which began arriving Friday.

An influx of thousands of sailors also is expected to bring money to Key West, said 4th Fleet spokesman Corey Barker.

"It's going to be a fun weekend," said Assistant City Manager David Fernandez.

alinhardt@keysnews.com