Shifter cables often go unnoticed during a ride. All we know is that when we hit the shifter, the derailleur moves the chain onto the next cog.
Not so fast.
Shifter cables are simple and sensitive components that actuate the shifting mechanism.
Bowden cables have been around for more than 100 years. There's a nylon, rubber, or plastic coating around a protective, flexible steel housing. Within the steel housing is an inner steel wire lined with nylon fabric for lubrication. At the end of the cable are metal stops by which it is moved by the shifter. The cable runs through hollow bolts attached to the derailleur and shifter (called barrel adjusters). The tension of the cable can be loosened or tightened by moving the bolt. Actuating the shifter pulls or releases the cable tighter through the housing, simultaneously moving the derailleur in tow.
Different than brake cables, the housing layer surrounding the inner wire in shifter cables has very minimal elastic properties. Instead, the housing consists of a cluster of linear strands. The structure is rigid and fortified. Shifting is about precision, and unlike braking (where pressure increasing builds up on the cable as more braking is needed), the strain on these wires is consistent for every shift.
On the low end, shifter cables can cost next to nothing. However, if you're serious about getting the best protective or lightweight housing, you will certainly notice a difference. Shifts will be crisp and exact, keeping your thoughts focused on the road and not the rear derailleur. The most expensive cables can reach over $200 on Amazon.com, but that's extreme. Extremely worth it.
Routing shifter cables around the frame of the bike is very important. Cables fitted to the proper length mean that the rear derailleur has the range of motion it needs. A cable too long will let the derailleur wander, sometimes leading to misalignment and ghost shifting when inconvenient. Having too short of a cable will limit the derailleur and it won't be able to move properly to shift through the gears. Many bike frames will have cable guides used to anchor the cable housing, helping the inner wire do its job.
Handlebar configuration influences cable routes throughout the bike. In general, the cables should be long enough to make wide, sweeping turns from the shifter en route to the derailleur. Any tight angles or extra slack will cause unwanted bending which puts stress on the cable or improper shifting.
The inner wire of shifter cables will be a diameter of 1.1mm. Outside diameter will either be 4mm or 5mm depending on materials used.
Length of the wire is cut as needed, which obviously varies on size of the frame and the path which it is routed.