Yet the United States National Highway Traffic Safety Administration recommends that “all cyclists wear helmets, no matter where they ride,” said Dr. Jeffrey Michael, an agency official.

Recent experience suggests that if a city wants bike-sharing to really take off, it may have to allow and accept helmet-free riding. A two-year-old bike-sharing program in Melbourne, Australia — where helmet use in mandatory — has only about 150 rides a day, despite the fact that Melbourne is flat, with broad roads and a temperate climate. On the other hand, helmet-lax Dublin — cold, cobbled and hilly — has more than 5,000 daily rides in its young bike-sharing scheme. Mexico City recently repealed a mandatory helmet law to get a bike-sharing scheme off the ground. But here in the United States, the politics are tricky.

SHAUN MURPHY, the bicycling coordinator of Minneapolis — which inaugurated the “Nice Ride” bike-sharing program in 2010 and expanded to St. Paul last year — has been pilloried for riding about without a helmet. “I just want it to be seen as something that a normal person can do,” Mr. Murphy explained to the local press this past summer. “You don’t need special gear. You just get on a bike and you just go.”

In New York, where there were 21 cyclist fatalities last year, the transportation commissioner, Janette Sadik-Khan, is always photographed on a bike and wearing a helmet. The administration of Mayor Michael R. Bloomberg has nonetheless rejected calls by Comptroller John C. Liu for a mandatory helmet law when New York’s 10,000-cycle bike-share program rolls out next year, for fear it would keep people from riding. Still, the mayor says helmets are a “good idea,” and the city promotes helmet use through education and with giveaway programs.

In the United States, cities are struggling to overcome the significant practical problems of melding helmet use with bike-sharing programs — such as providing sanitized helmet dispensers at bike docking stations, says Susan Shaheen, director of the Transportation Sustainability Research Center at the University of California, Berkeley.

But bicycling advocates say that the problem with pushing helmets isn’t practicality but that helmets make a basically safe activity seem really dangerous.

“The real benefits of bike-sharing in terms of health, transport and emissions derive from getting ordinary people to use it,” said Ceri Woolsgrove, safety officer at the European Cyclists’ Federation. “And if you say this is wonderful, but you have to wear armor, they won’t. These are normal human beings, not urban warriors.”