Pssssssss! The hiss of a deflating bike tire often also deflates the heart of urban bike commuters. Although many people ride their bikes to work and school all over the world, few of them carry the pumps, patches, spare inner tubes, extra tires and tire levers needed to fix a simple flat tire.

Flat tires, especially on smooth urban pavements, are relatively rare and it's a pain in the neck to haul even a small bag of bike repair equipment around to the office or to classes all day, every day, just in case.

But woe to the unprepared commuting cyclist who finally ends up with a wobbly piece of rubber wrapped around a wheel. Carefully laid plans are thrown awry. Meetings are missed. Coworkers are left in the lurch. And the poor hapless cyclist is left with few options but to listlessly push the now useless bike - unless he or she wants to continuing riding and risk ruining the wheel rim - all the way to his or her destination.

Except this scenario never happens in Beijing, thanks to the city's ubiquitous corner bicycle repairmen and repairwomen.

They are the capital's unsung heroes, at least to commuting cyclists, and you can find one at almost every major intersection in town. At some intersections, you may even have a choice, because there might be two or more bike mechanics on different corners.

They all appear the same. A man or a woman with a cart that carries a large wooden box, which is usually flung open, revealing stacks of bicycle parts - extra chains, links, pedals and an assortment of odds and ends. Tubes and tires are stacked on top of the cart, a mini-tower of black rubber. And a few full-sized bike pumps lie on the sidewalk at the feet of the mechanic.

Although they often look the same, with similar clothing and similar cart setups, each is different. Most are friendly. A few are grumpy. Some refuse to take any payment if they help with only a small problem. Very occasionally one will try to rip you off and charge an outlandish fee for a basic, cheap bike part.

But the sheer number of mechanics is staggering. There must be, quite literally, hundreds of them across Beijing, possibly even thousands.

With such an overwhelming presence of mechanics throughout the city, you should never need to push your bike for more than a few hundred meters before you get a flat fixed, or a chain replaced, or even just get your bell to ring more clearly.

Of course there are limits to these curbside mechanics' abilities. If you have a complex problem with your bike, if your front suspension doesn't have enough give, say, or if you have a high-end ultra-light carbon racing bike you should probably take it to a bike shop.

But for an easy-to-solve, routine problem why not use your local sidewalk Mr or Mrs Fix-It? It's fast, reliable and cheap and it's kind of fun too. And there's something reassuring about these sidewalk mechanics' simple, no-nonsense approach to bicycle maintenance.

It's a phenomenon that a no-carbon-emissions commuter transport network is kept running by an informal network of equally humble, yet important curbside repairmen and repairwomen.

It's time Beijing gave these people their due - after all they grease the wheels of a good portion of the city's daily commute.

(China Daily 09/30/2010)