A chain (514) is selectively engageable by a chain-side engagement element engageable with a sprocket-side engagement element of a respective one of the sprockets (512). A chain transferrer is provided for causing the chain to engage a selected one of the sprockets. Each of the sprockets has a respective sprocket middle plane perpendicular to the axis.The chain has a first enveloping side plane and a second enveloping side plane spaced from the first enveloping side plane along the axial direction. Both the enveloping side planes are substantially orthogonal to the axis. A virtual chain middle plane is substantially orthogonal to the axis.
