"It's total chaos -- everyone's screaming and laughing, people are spinning out, going down backward," said Alon Zaslavsky, a first-time participant. "It's great. The whole point is to crash."
The derby attracted more than 2,000 celebrants to Sunday's event taking place on Vermont between 20th and 22nd, one of the curviest, steepest streets of San Francisco.
There is a 14.3 percent grade down Potrero Hill on which the street lies and racers riding their Big Wheels without brakes, helmets or elbow pads had to sign waivers before embarking on their runs, the San Francisco Chronicle said.
Bring Your Own Big Wheel started in 2001 when Jon Brumit and his friends rode Big Wheels down Lombard Street, dodging traffic down the famously windy tourist attraction.
Lombard residents started complaining about the crowds, which grew over several years, so Brumit and his buddies moved the event to Potrero Hill where they have permits, security and portable bathrooms, the Chronicle said.
Anyone with a plastic, three-wheeled kids' vehicle was welcome to participate in the free, downhill challenge.
Some got thrashed, but they took it in good humor.
"I ended up backward, upside down," gorilla-costumed Jeff Boenisch said. "But it's amazing, the intensity when you hear the Big Wheels coming behind you. It's really fun."