Yes, this time Philadelphia's parking crisis involves vehicles of the two-wheeled variety.

While the drumbeat during the Street administration was for more parking garages in Center City, the cry now is for more and better bike racks everywhere. That's progress.

Bike commuting is soaring, according to the Bicycle Coalition of Greater Philadelphia. I can attest to that. Some mornings at rush hour, the bike lane on 22d Street is jammed wheel-to-wheel, and the riders are stopped three abreast at red lights. It's starting to feel like the Tour de France out there.

There's no single reason why bicycle commuting has gone into high gear, though spiraling gas prices, concern about greenhouse-gas emissions, and an appreciation of messenger cool are surely contributing to the crowded bike lanes. Philadelphia's nearly flat central street grid is perfect terrain for bicycles. Besides, pedal power is often more reliable than taking SEPTA or driving. No wonder bicycles have become the transportation of choice for low-paid workers, like those who staff the city's restaurant kitchens.

The problem starts when the bikers stop. There just aren't enough bike racks on Philadelphia's heavily used, narrow sidewalks for everyone. Desperate bikers will lock to anything that won't move, like Rittenhouse Square's elegant wrought-iron fence or the railing around SEPTA's 16th Street concourse entrance. The tangle of metal is not pretty.

The Nutter administration hopes to improve the situation somewhat in the next few months. It just ordered 1,500 racks and expects to begin installation in November. The new upside-down "U" racks will bring the sidewalk total to 2,600, distributed through the entire city. It's a far cry from the 10,000 the Bicycle Coalition says are needed.

It's true that the inverted "U" rack favored by both the city and the Bicycle Coalition does its job with a minimum of fuss, and, at $300 a pop, is relatively cheap. But as a design, it lacks zest. Nor is it as efficient as it might be.

The time has come to build a better bike rack.

The main drawback is that the inverted "U," like the popular post-and-ring model, parks just two bikes. Given the competition for space on the sidewalks from street signs, parking meters, honor boxes, fire hydrants and trees, there's a real possibility that Philadelphia could run out of room on streets such as Chestnut and Walnut, where demand for bike parking is most intense.

A study that the Bicycle Coalition produced for the Nutter administration recommended creating clustered bike parking. Nothing radical, mind you, like the three-level bike garage next to Amsterdam's train station. Rather, the coalition wants the city to repurpose car spaces on select blocks for bikes.

There are two ways the cluster, or "bike corral," could work: in the street or on the sidewalk. Either way, you can fit four "U" racks and eight bikes in a standard car-parking space.

For the street version, the city would have to install some kind of protective bumper to shield bicyclists from motorists. To my mind, extending a section of sidewalk makes more sense because the raised curb offers natural protection. These sorts of corner extensions, called bump-outs, already exist on Chestnut Street, where they were created to make the crossings easier for pedestrians.

In a quest for the perfect bike rack, New York's Department of Transportation is sponsoring a design competition and has just named 10 finalists. None of the competitors found a way to park more than two bicycles, but the designs are a lot more fun to look at than Philadelphia's brownish, upside-down "U."

Clearly, it's a tough design problem. A successful rack must provide a parked bike with upright support, and it has to be muscular enough to resist metal-cutters. The rack should also be made so bicyclists can easily thread their U-shaped locks - there seems to be an alphabet theme here - through both the wheel and the frame, to prevent theft.

That tricky locking style is one reason bicyclists long ago rejected the ubiquitous "comb" racks. To lock securely, riders must drape the front wheel over the rack, but the maneuver damages gears and cables.

Perhaps one way to ease the parking crunch is to increase off-sidewalk parking by installing cluster racks inside garages and office buildings. Why is it that the Comcast Tower offers only a single rack on its sprawling plaza, given all its boasts of greenness, public access and transit connections?

Some public institutions provide no bike racks at all, according to the coalition's report. This ignominious bunch includes Pennsylvania Hospital, La Salle University, the museum's Perelman addition, the Mann Center, and the Academy of Natural Sciences.

The easier it is to bike and park in Philadelphia, the easier it will be to reduce car traffic and live up to Mayor Nutter's aspiration of making Philadelphia the greenest city in America.

The city Planning Commission has just started work on a pedestrian and bicycle plan, though it will likely take more than a year to complete. In the meantime, bicyclists may have to keep spinning their wheels in search of a place to lock up.