The hardest question to answer here is how outlaw culture came to be so Republican. During the 10-day extravaganza, the last remnants of the Hells Angels mingle with Christian bikers, stockbrokers riding expensive Harley-Davidsons, old men on Triumphs, and young gearheads on light Suzukis. Together they cruise the Black Hills, enjoy concerts by George Thorogood and Keith Urban, drink at the bars, and buy lots and lots of Trump memorabilia. 

Cornell “Tuffy” Nicholas, a circus owner from Florida, opened five “loud and proud” Trump shops in anticipation of the rally. But nearly every pop-up store in Sturgis sells Trump merchandise alongside rally souvenirs. There’s nothing comparable for the Democrats—no Beto mugs or Kamala Harris cigarette lighters. I watched a mother holding a Trump shirt up to her 10-year-old daughter, gauging the size. “We sell 30 a day,” one vendor said of the shirts, and the rally is definitely “one-way political.” After only a single day, “we’re already on our second box,” another vendor confided. 

Asked what brings him to the Sturgis Rally, Mike Ford, a 55-year-old biker from Oklahoma City, answered, “I come for the crazy people.” The crazies are certainly around. A man in a beer barrel waddles through the crowd. Bikers perch dogs, some wearing doggy goggles, on their handlebars. A self-proclaimed “professional hobo” named “Tinker” offers a business card and fancy knives for sale. Todd Gilmore from Colorado, dressed up as Batman, captures Goblin, his mock-criminal companion in a green camo morphsuit. “It makes everyone so happy when I’m here,” he boasted when I interviewed him. Meanwhile, a biker gang of California Sikhs, all in turbans, rolls past the sales tents and the strolling police, who are careful not to notice the public nudity and minor misdemeanors.

Even in a weak year the half a million people who come to the Sturgis Rally drop around $800 million. South Dakota’s total state budget is only $4.6 billion. Concert headliners have ranged from Bob Dylan to Snoop Dogg to “Weird Al” Yankovic. Sarah Palin and the 78-year-old drag racer Don Prudhomme are among the visiting celebrities this year. 

“We’re all on the same side,” a motorcycle-shop owner named David MacDonald explained about the politics of the riders. “It does seem a lot of bikers are on the Trump side,” a young man told me, while asking not to be named for fear he would lose his job in California. “I saw bikers wearing Trump gear like the gear I’m wearing”—because under Trump “unemployment has gone down and the economy is doing better.”

And yet the Sturgis gathering isn’t overwhelmingly political. Almost everyone there is for Trump, but the rally doesn’t feel like a Trump campaign event. The vibe is different. “We have good jobs, we’re educated,” insisted David Breunig, president of the Highway Riders, a California motorcycle club. “Just because we ride a Harley and look like this, we’re [still] good people.” The point of the Sturgis Rally, he said, is to be “a pilgrimage,” an “expression of freedom” and deeply “patriotic, [with] everyone flying American flags.”

And maybe something like that explains how we got from outlaw biker culture to support for Donald Trump. The answer is partly sociological: There’s an old American tale of freedom, and the open road became a conservative cause when the left embraced the coastal cities over Middle America’s countryside and turned against fossil fuels. The answer is also partly economic: Motorcycles were always associated with the working class, and the blue-collar Rust Belt now falls in the Trump column. Vietnam vets and their MIA movement added the American flag back into the mix.

In the 1953 biker movie “The Wild One,” a waitress asks Marlon Brando’s character what he is rebelling against. “What do you got?” he answers. Bikers are still rebels, only now they are rebelling against a dominant liberal culture that seems dull and prissy, and never liked them anyway.

Ms. Bottum is a civil-engineering student at the South Dakota School of Mines.