Bowden cable

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Bowden cables controlling an automobile throttle.

Invented by Frank Bowden, a bowden cable is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable (most commonly of steel or stainless steel) relative to a hollow outer cable housing. The housing is generally of composite construction, consisting of a helical steel wire, often coated with plastic, and with a plastic outer sheath.

The linear movement of the inner cable is generally used to transmit a pulling force, although for very light applications over shorter distances (such as the remote shutter release cables on mechanical film cameras) a push may also be used. Usually provision is made for adjusting the cable tension using an inline hollow screw (often called a "barrel adjuster"), which lengthens or shortens the cable housing relative to a fixed anchor point. Lengthening the housing (turning the barrel adjuster out) tightens the cable; shortening the housing (turning the barrel adjuster in) loosens the cable.

Variations

The original, standard Bowden cable housing consists of a close-wound helix of round or square steel wire. This makes a flexible housing but causes the length to change as the housing flexes. Because on the inside of the bend the turns of a close-wound helix can't get any closer together, the bending causes the turns to separate on the outside of the bend, and so at the centerline of the housing, there must also be an increase of length with increasing bend.

In order to support indexed shifting, Shimano developed a type of housing that does not change length as it is flexed. This housing has several wire strands running in a multiple helix, with a pitch short enough that bends in the cable are shared by all strands, but long enough that the housing's flexibility comes by bending the individual strands rather than twisting them. Another consequence of the long pitch of the helix is that the essentially parallel strands are only bound together by the plastic jacket, and so this type of housing cannot withstand high cable tension when bent, and should not be used for brake cables.^[1]

Some applications such as lawn mower throttles, automobile manual chokes, and some bicycle shifting systems require significant pushing ability and so use a cable with a solid inner wire. These cables are usually less flexible than ones with stranded inner wires.