The U-shaped steel bike rack is surely the unsung workhorse of urban street furniture. U racks - more accurately, upside-down U's - are so plain, so recessive, you don't really see them until you need one. (And then, invariably, they're full.) U racks are cheap. Their streamlined form takes up very little space. Except in extreme circumstances, they're also indestructible. All that makes them a nearly perfect design.

But beautiful? Not really.

As urban bike culture grows, so does the impulse to reinvent the bike rack. Dozens of cities have held competitions in the hope of discovering a new design that performs like a U rack but looks great, too. Mostly the results have just been silly. We've seen racks shaped like dogs, guitars, eyeglasses, coffee pots, and kitchen whisks, many of them no more functional than those fiberglass cows and bears that began appearing a few years back on city sidewalks.

Still, the challenge of building a better bike rack is irresistible, and last fall the Bicycle Coalition of Greater Philadelphia succumbed. With support from the city's Office of Arts, Culture and the Creative Economy and the Knight Foundation, it put out a call to local artists and architects for more artful, eye-catching solutions. The nine winning designs were fabricated and are on display in City Hall's exhibition space in Room 116 until June 17.

The good news is none of the prototypes are indulgent art objects. The contest rules stipulated the racks had to be capable of efficiently locking two or four bikes, depending on size. To judge the results, I brought my own bike to put the racks through their paces. A hardware-heavy, '70s-era three-speed with fenders, kickstand, and side baskets, it's a challenge to park in the best of circumstances. If my bike could be secured easily, anyone's could.

The only winner that gave me trouble was the Pretzel, by Peter Everett Brown and Barbara Ann Sprague, architects at BrownSprague. Its P-shaped, Corten-steel loops are so thick that my standard-length bike lock (also a U) was barely long enough to fit through the wheel, frame, and bike rack. The loops also seemed uncomfortably high. Too bad, because their intertwined Ps could be an understated city icon.

As with many things in life, you can't go wrong being too thin.

For pure elegance, nothing came close to Koi, a slim, stainless steel panel designed by Juliet Whelan of Jibe Design Architecture. It's so different from any existing design you might not initially recognize its coolly modern form as a bike rack.

Whelan laser-cut a slice of silvery metal in a wave pattern, and the varied openings create a multitude of locking positions. She also embedded the panel interior with LEDs that shimmer across the surface when a bike is attached, a nice perk when you're locking at night. If the bike world needs a new standard, this may be it. Anything made of stainless steel, however, would be more expensive than the sturdy U rack, which costs the city a mere $150, installed.

To survive the city's tough sidewalks, of course, elegance has to be balanced by hardiness. Though the polar opposite of Whelan's approach, two winners walk that line pretty well by drawing from the city's industrial past: Bike Rack, the generically named design from Warren Holzman and Iron Studio, and Nathaniel Ross' Philadelphia Bike Terminal.

Holzman's design cleverly blends Philadelphia's gritty blacksmith tradition with the mathematical elegance of the famous Fibonacci spiral, yet still manages to come close to the U rack in functionality. The complex welding of Ross' Terminal recalls the lacy steel roof trusses of Frank Furness' lost Broad Street Station. What's nice about the design is that it offers so many points of contact for locking bikes.

Collin Robinson's bright yellow Spiral Bicycle Rack appears to do the same, though I couldn't be sure because it is housed in a vitrine for the City Hall exhibit. Inspired by an Alexander Calder jewelry design, it also achieves the simplicity and durability that all street furniture needs. The version here is glossed in yellow paint but could work as well in bare metal.

The rest fall into what might be called the "cute" category - Kathleen Fruge-Brown's blue Bird, Ralph Tullie's Bike Cloud, Carin Mincemoyer's Partly Sunny, and Joe Norman's Pedal Power. It's hard to imagine any of these yielding a new prototype, but a couple, like Norman's, might work well in a park setting. Pedal Power is also designed to be expandable, which means you can create a meandering line of green leaves.

The public will be able to test-drive the racks this summer when the winners move to a pop-up beer garden run by the Pennsylvania Horticulture Society, most likely in Penn Center Plaza, between 15th and 16th Streets. After that, they'll be installed in sympathetic locations around the city, including the Cira Centre, Boathouse Row, and Sister Cities Plaza.

But the real problem with bike racks in Philadelphia isn't design, it's supply. Although the Nutter administration has added nearly 1,500 racks in Center City and University City, and retrofitted some 1,600 meter stands, the parking crunch is often severe in places where millennials congregate - in Fishtown at the Frankford Hall-Johnny Brenda's nexus, around Rittenhouse Square, and along East Passyunk Avenue.

One wonders whether the city would have been better off spending the $100,000 in donated money for the competition on more U racks. Michael McGettigan, owner of Trophy Bikes, dismisses the whole idea of artist-designed racks. "I don't want my house plumbing to express an artistic urge. Same with bike racks," he argues.

It's good to see some institutions, like Drexel, Penn, and the Barnes Foundation, start to dot their campuses with racks. These tend to be a stainless-steel version of the city's U rack. Though they cost four times as much as regular steel, you might even call them beautiful.