By Mike Nowak

IMAGINE YOU'RE HAPPILY pedaling down Valencia Street when some obscure part of your bike gives out with a sudden, sickening clunk! If you're like most people, your immediate reaction (after brushing the dirt off your Lycra) is to pedal or push your not-so-trusty steed down to the nearest bike shop and feebly wave your credit card in the air until the mechanic agrees to fix whatever is wrong.

Of course, it's not just bicyclists who suffer from acute bouts of mechanical helplessness – the mere mention of the words "timing belt," "calipers," or "slave cylinder" is enough to cause most car owners to reach just as dumb-facedly for their wallets.

But thanks to a couple of community-oriented San Francisco businesses, it's easy to maintain your wheels on your own – and save a good deal of money in the process.

At the San Francisco Auto Repair Center (611 Florida, S.F. 415-285-8588), owner Jerry Lewis has welcomed do-it-yourselfers since founding the shop in 1978. After purchasing a membership ($5 for a day, $10 for a month, or $25 for a year), solo tinkerers can reserve a small slice of garage space to work on their own vehicles. Doing so costs $1 an hour for those with their own basic tool kit, with a 50¢-an-hour surcharge to rent a tool set from the shop.

According to Lewis, amateur wrenches are not as common a sight as they once were. "It used to be two or three people a day," he said. "Now, in any given week, we'll have two or three people.... I think it's mostly [due to] the complexity [of today's cars], and that everybody seems to have no time in their lives."

But while Lewis and his cohorts probably can't clear your calendar, they just might be able to make that daunting mess of metal and rubber under your hood less intimidating. Starting in May and again in September, the center will offer six-week beginner-friendly auto repair courses; for $120 a person, staff mechanics lead participants through all types of basic maintenance. Members can always consult with the shop's mechanics while wrestling with their wheels, but assistance is billed at the standard rate of $104 an hour. You'd be wise to pay close attention in class.

If muscle-powered two-wheelers are more your speed, consider paying a visit to Pedal Revolution (3085 21st St., S.F. 415-641-1264), where a $35 annual membership buys unlimited access to the store's community workbench, along with the use of tools, staff know-how, and a dog-eared copy of Bicycling Magazine's Comprehensive Guide to Bicycle Maintenance and Repair.

But even if you don't know the difference between a down tube and a derailleur, you can stoke your inner bike-shop know-how by attending one of the store's free maintenance classes. Held every other Sunday at 5:30 p.m., the hands-on workshops are offered in a series over four months, starting with tire changes and moving on to advanced topics like headset adjustments. (Classes aren't listed on the Web site, so call ahead to make sure that the next class is appropriate to your skill level.)

Best of all, any cash you spend at the shop directly supports Pedal Revolution's outreach to disadvantaged youths. As a subsidiary of social enterprise Golden Gate Community, the store is staffed largely by a corps of at-risk young people assigned to six-month internships that provide valuable employment and vocational skills.