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Sync chain
From WikiPedalia
The sync, or "timing" chain on a tandem connects one bottom bracket to another. There is a synch chain for each stoker, so, for instance, a triplet has two synch chains.
Sync chains generally connect equal-sized chainwheels, so the mechanical advantage is 1:1. This is true for all sizes of sync chainwheels, as long as they are both the same size. Smaller sync chainwheels, however, put a greater stress on the sync chain, the frame, and the bottom bracket bearings, so it is a mistake to make them too small in an attempt to save weight. Typical sizes are in the range of 38 teeth and up.
See also my article on Tandem Sync Chains.
