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Why is the chain connecting the front and rear cranks of a tandem called a timing chain. This brings to mind big-block V8s and leaky timing chain covers that plagued my younger years.

What's the history of this term?

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My take is that a timing chain on a motor keeps the values in synch with the crank. On a tandem the two riders need to be in synch. If you install the chain incorrectly the rides are not in synch (time). – Paparazzi Jun 3 '14 at 13:53
Another reason for having a timing chain is that getting power out of the stoker's crank is hard if there's only one long chain that barely touches the stoker's chainring. Two separate chains means much better engagement with teeth. Also, the Timing Chain doesn't have to flex sideways and can be a more-stout 1/8" width rather than a 3/32" width. – Criggie May 21 at 3:23
up vote 13 down vote accepted

The Timing chain helps keep the front and back riders pedals rotating at the same time. Depending on your preferences you can set them as "in phase" where both the front and back cranks are at the same point in the rotation, or as "out of phase" where the front and back cranks are offset by 90 degrees. This is advantageous as when one of the riders reaches the dead point (where the cranks are vertical), the other set of cranks is at horizontal allowing good power transfer. You can set up different levels of out of phase. You can set it up so that one rider is only a few teeth ahead of the other rider, so that both riders aren't in a dead zone, but the pedals are more or less aligned.

You can even set up both riders with different size chain rings, which would allow one rider to pedal at a faster cadence than the other rider. This presents problems when cornering, as it's hard to get the pedals into the proper position so that neither the front nor the back will hit the ground.

Sources and more info:
Pedal Setup
Poll: In Phase vs. Out Of Phase
Out Of Phase Cranks

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Out of phase also reduces stress on the drivetrain and frame. Tandem riders are more prone to snapping chains,for example (and does anyone run 11 speed chain/cassettes on a tandem?) – Móż Jun 4 '14 at 9:15

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