If you go

What: Alley Cat Race

When: 2 p.m. Saturday

Where: Folsom Field, at the buffalo

More info: bactothefuture.blogspot.com

Every once in a while, University of Colorado student Kalin Puent rolls up to a bike rack on campus to park his fixie and someone takes note of the unique spoke card stuck in his wheel.

"Someone will read it and go, 'Yeah, I was supposed to go to that'" bike race, Puent said.

"That" bike race was the winter Alley Cat Race in Boulder. Puent earned the spoke card by being the fastest racer not using brakes on his bike for the race, which, like most alley cat races, mimicked a day in the life of a bike messenger.

The otherwise unidentifiable spoke card is part of the culture of these unsanctioned, urban races that are usually put on by fixed-gear bike enthusiasts (though fixies are not required) and usually carry a theme.

The next Boulder Alley Cat Race, themed Bac to the Future, starts at 2 p.m. Saturday at Folsom Field.

"Alley Cat's an underground bike race through the city," said Adam Mork, the organizer. "There's checkpoints you have to go to, and they might make you do something there, like carry something, a task."

"As far as the route, it's up to the rider. That's a big part of who wins -- who has the best route."

Mork started organizing alley cats in 2008, while he was a student at CU and working part-time delivering sandwiches by bike for Jimmy John's. (He started with a road bike but quickly traded it in for a fixed-gear.)

This Saturday's race will be the first since winter.

"It's summertime, and I haven't done one in a while," Mork said.

"I'm just doing it to bring people together."

Alley cat races tend to have different prizes than sanctioned road or mountain-bike races. Pabst Blue Ribbon was a prize at a recent race; it was part of the theme. At another race, Mork made the Tour de France a theme and gave out Tour-inspired spoke cards, like a climbing one for the first person to a checkpoint on a hill.

The spoke cards (Mork made them himself for this one) are a sort of badge of honor among fixed-gear riders.

"A lot of kids do it just as a joke," Puent said of the spoke cards. "I mean I've had Pokemon cards in mine before."

"But it's a way of representing street credit, almost."

The last race, in winter, was tough and had sparse turnout because of snowstorms the day before and day of. But Mork's expecting more riders for this race.

"I think there will be about 30 people racing, which will be one of my best turnouts."