A friend recently sent me the video embedded below in which a cyclist suddenly loses control and takes a hard fall. My friend had told me that he had figured out the reason for the loss of control, but it was not obvious. My friend asked me whether I could figure it out. Can you?

OK, now you have seen it. What caused the crash? There is a short strip of black tar on the road, but the cyclist is entirely past it before he loses control. He doesn't collide with any of his companions...

If you look carefully at the slow-motion segment of the video below, you will very likely see what caused the crash. Hint: no film was used at any step from the original shoot to the display on your screen. Every step was digital.

Did you see that jagged black line flailing around in the image? As I said, it is not dirt: no film was used in the making of this video.

It's the bicycle's chain, which broke, leaving the pedals free to spin. The cyclist was standing out of the saddle, and suddenly lost support from the pedal he was pushing down.

A chain's breaking is only one of several kinds of drivetrain failure which can lead to a crash. Lets look into them now.

How can drivetrain-failure crashes be avoided, or made less bad? Here are things you can do:

• Make sure that your drivetrain is in top condition. Check that the chain runs smoothly by turning the cranks backwards. Also turn the cranks forward with the rear wheel off the ground, and shift through the gears. Check for stiff chain links, bent sprocket teeth, bent chainwheels, loose crankbearings, misadjusted indexing. Keep track of wear. Replace the chain and sprockets before they threaten to make the chain skip.
• Standing while pedaling carries a risk. Sometimes, to acclerate or climb, you must accept the risk of a chain's coming off a sprocket, jamming or breaking -- but staying seated is safer, especially when riding at a high speed. Also see our page about standing while pedaling.
• Release force on the pedals when shifting. Shifting under power is possible with modern, ramped sprockets but still abuses a chain. Yes, I know that I'm a bit old-fashioned: I've been shifting derailers since before Hyperglide, and I'm tuned in to the feel of the drivetrain through my feet. I pedal lightly until the chain is running smoothly. You can do this too. Sure, if you're a racer, you may have to shift under power to keep up or stay ahead. It's your choice.
• Wear a helmet and cycling gloves. The cyclist in the video was wearing a helmet, but he must have skinned his left hand badly in this crash -- no gloves. Ouch!

And here are some considerations about equipment .

• Starting with 9-sprocket cassettes in the mid-1990s, and continuing with 10- 11- and 12-sprocket cassettes, the side plates of chains have become thinner, and chains have become increasingly prone to failure. This development is largely a triumph of marketing one-upmanship over rational design. There is much more to gain by using a wide-step double or triple crankset than by increasing the number of rear sprockets beyond 7 or 8. Oh, I know, shifting is easier without a front derailer, but awkward shifting is overcome with practice. Again, it's your choice. 7- and 8-speed cassettes and chains are still available. No available freewheels have more than 7 sprockets.
• As the side plates of chains get thinner, connecting them becomes trickier, and there is more potential for damage and a weak connection. Shimano chains, 9-speed and up, must be connected using a special link pin, different for each chain width, and a special tool. See the section on connecting chains in our main article on chains for more details.
• A freewheel or freehub can fail so it freewheels forward.
• The problem isn't limited to derailer-equipped bicycles. A non-derailer chain can come off the sprockets if it is not adjusted periodically to account for wear. A single-speed freewheel or internal-gear hub can freewheel forward. (See Jobst Brandt's comments). A threaded fixed-gear sprocket can unscrew. With a coaster brake or fixed gear, loss of the chain disables braking: so the bicycle should have a front brake as well, as described in our article on braking and turning.
• Belt drive, too, can slip, and a belt can break. A carbon-fiber belt must always be kept in line, never twisted, crumpled up or levered onto the side of a sprocket. This abuse will break fibers, leading to failure. To be reliable under high load, the belt must be tensioned higher than is usual, making it feel draggy when the crank is spun by hand. (The drag feels like a worse problem than it actually is in use, when the load on the belt is higher.) Belt-drive manufacturer Gates is offering a "snubber" device to keep the belt from lifting off the sprocket at the rear wheel.