Tuesday, March 30, 2010

Bicycle cable basics

Bicycle cables transfer motion from your brake levers and shifters to the brake and shifting hardware with very little friction loss. Unlike rods, cables also allow forces to be “bent” around corners, so a bicycle can have all of the braking and shifting hardware attached to the handlebars, which are constantly moving.

Without the cable’s ability to transfer motion around corners, all of the shifting and braking hardware would have to be installed on the frame, making it difficult or dangerous to operate. Cables are easy to install, adjust and maintain, and you will certainly be working with them when you create a new cycle, as it will normally require at least four cables for dual wheel braking and complete shifting.

A few unviversal bicycle cables
Figure 1 - A few universal bicycle cables

A bicycle cable consists of two main parts: an inner cable that consists of several twisted strands of flexible hardened steel and an outer sheath that is made up of a hardened steel coil wrapped in protective rubber. Often, there will also be a lead ball formed at one or both ends of the cable that allows connection to a lever or shifter. Bicycle cable is very flexible to a point, and then the sheath will be bent or friction will impair the motion of the inner cable, so there are limits to how tight of a corner a cable can be bent around.

On mass manufactured cycles, the sheath is often omitted as much as possible, with the bare inner cable running along the underside of the frame tubing. The longer the sheath, the more losses that will occur due to sheath compression so the running of “open” inner cable is actually a good design. Figure 1 shows a typical hardware store “universal” bicycle cable that includes a formed head at each end. This is the type of bicycle cable you will most often use in home built creations, and it will need to be cut to a custom length.

Cable ends often have a ferrule
Figure 2 - Cable ends often have a ferrule

Several bicycle cable sheaths are shown in Figure 2. The top two cables have an end cap (ferrule) that protects the end from damage and helps it seat into the hardware it mates with. When cutting your own cables, you will probably not have and end cap, but this is okay as long as you have trimmed the end properly using techniques that will be discussed later. The cable at the bottom of Figure 2 has been custom cut, so it has no end cap.

Brake cable is larger than shifter cable
Figure 3 - Brake cable is larger than shifter cable

Braking cables have a larger diameter inner cable and head than shifter cables since they have to endure a much greater amount of tensile (pulling) force. The two center cables shown in Figure 3 are obviously larger than the outer two cables; they are brake cables. Also, notice how much larger the brake cable head is compared to the shifter cable head.

At the top of Figure 3 is shown a badly frayed cable, which is a serious stabbing hazard and will need to be cut in order to use the cable. The hard steel strands are like needles, and will easily punch through a light glove and into your skin, so be careful with frayed cable ends or newly cut cables. Sometimes you can simply twist the strands back together if they have unraveled, but once bent, you will have to cut the damaged part off.

Cable is cut using large wire cutters
Figure 4 - Cable is cut using large wire cutters

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