Spaces are filling up fast for Levi Leipheimer's King Ridge GranFondo next October, with nearly half of the 6,000 spots reserved on Tuesday, the first day of registration.

The second annual cycling event, the largest group ride in Sonoma County, raises money to help Santa Rosa host the annual Amgen Tour of California.

It also benefits Forget Me Not Farm of Sonoma County, a nonprofit that provides therapy for troubled children.

Last year, the GranFondo attracted 3,500 cyclists from 39 states.

Online registration opened Tuesday at 9 a.m. for the cycling event, which features short, medium and long-distance rides in western Sonoma County. All the routes start in Santa Rosa.

The event's Web site couldn't handle all the traffic when registration opened, said Greg Fisher of Bike Monkey, the event producer.

"It was crazy," he said. "Our site really slowed down."

There are 3,000 spaces for the 103-mile GranFondo, 2,000 spots for the 65-mile MedioFondo and 1,000 spaces for the 30-mile PiccoloFondo.

The Oct. 9 event also features a post-ride festival. A tradition in Italian cycling, GranFondo is aimed at cyclists ranging from recreational riders to professional racers.

The first GranFondo was held in October. It raised nearly $100,000, with $60,000 set aside for Santa Rosa to bring a leg of the Amgen Tour to the city. Santa Rosa must raise $175,000 to be a part of the tour.

Registration for the 2010 GranFondo is open at the event Web site,

Leipheimer, a professional bike racer who lives in Santa Rosa, is an Olympic medalist and three-time winner of the Amgen Tour.