Strap On Your Helmet and Start Pedaling. Don't Worry About Cars.

Lube the chain. Inflate the tires. Pad the seat (if you feel the need). In the immortal words of Billy Corgan (cue the feedback): I wanna go for a

ride. . .

The Washington Area Bicyclist Association has arranged with Mayor Adrian M. Fenty's bike-friendly administration to close some of the city's most striking downtown streets Saturday morning for a car-free community ride.

Vicky and I thought it would be a perfect opportunity for a trail challenge. (Actually it was the idea of our ever-opportunistic Health section editor, Frances Stead Sellers, but at this point in the newspaper industry you need to claim what authority you can.)

The Bike D.C. Trail Challenge will be a bit different from our previous walks.

You'll need a bike (and a mandatory helmet, including one for kids in trailers . . . no dogs, skates or skateboards allowed). You'll need to commit the morning; the streets will be closed from 8 a.m. to noon. And all riders need to register and pay a fee to WABA: $35 for adults and $15 for kids 12 and younger if you sign up in advance online ( http://www.bikedc.net), or $10 more on the day of the event. You can also coordinate a bike rental through the Web site.

Apologies in advance to anyone who can't make it at that time for one reason or another, but this felt like too much fun to pass up. We aim to get back to walking and other free stuff later in the fall.

This a chance for those of you who are already bikers to tour Embassy Row and Capitol Hill unimpeded by traffic. For others, it's an opportunity to spend a morning outdoors with the family or get reacquainted with an activity that is great for general fitness. After the event, WABA volunteers will lead other rides around town, as an introduction to city biking in traffic.

The Bike D.C. Ride: It will start at the plaza of the Ronald Reagan Building & International Trade Center, 1300 Pennsylvania Ave., with check-in about 7 a.m. The 17-mile route will go past the White House, up Embassy Row to the Washington National Cathedral and pause for a rest stop next door on the grounds of the Twin Oaks Estate, opened for the day by its owners, the government of Taiwan. After looping back to the Capitol, the ride goes through Capitol Hill to RFK Stadium, along a piece of the newly opened Anacostia River Trail, then back to the start.

To get there, remember that you can take your bike on Metro. If you drive, WABA recommends staying away from the Mall (the National Book Festival will be underway) and parking north of Pennsylvania Avenue.

The Trail Challenge Riddle: Since time on the road is limited and we don't want to distract from the ride, we will only have one riddle -- but to solve it, you'll have to show up for the ride. (We hope.)

Here it is: What's the magic number? (Okay . . . It will be at the registration table. Or ask me and Vicky if you see us.)