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Cable and outer casing

It’s important to understand the differences between your gear and brake cables because they perform very different functions and require very different treatment. Here’s the lowdown on these vital components.

Cable and outer casing

  • Cable – The wire that connects a shifter or lever to the component – it’s usually silver.
  • Casing – The cover that supports and guides the cable.
  • Ferrule – A metal or plastic cap on the end of the casing that prevents the end of the casing from splaying out under pressure.
  • Cable stop – The part of your frame that the outer casing slots into – a slot in the cable stop allows you to slide the cable out of the cable stop for cleaning.
  • Cable end – A small metal cap that you squash over the end of the cable to prevent the cable fraying.
  • Nipple – The blob on the end of the cable that fits into the lever or shifter, stopping the cable from pulling through.

The outer casing used for gears is different to the casing used for brakes.

  • Brake casing is made from tight spirals of square section wire.
  • Gear casing is made from round section wire, formed into much longer spirals. It is covered in plastic (usually black), which protects the wire and keeps it in shape. It is lined with a Teflon tube (usually white) so that the inner cable moves smoothly inside.

The casings differ because they perform different functions. Brake casing must be very strong because it handles a lot of pressure. Gear casing must transmit a very accurate signal. When you shift from one gear to the next, you pull through mere millimetres of cable, so it is vital that the gear casing does not compress and influence the cable as you change gear or turn the handlebars. The long spirals prevent the gear casing from shortening under pressure, or as the sections of casing articulate with the bike. The advantage of the tight spiral wind in brake casing is that while it is strong, when it fails, it does so gently. Gear casing cannot take so much pressure, but that’s all right because gear levers are short, so you can’t exert much pressure on them anyway. When the casing does fail, it tends to be catastrophic – the casing splits open and you get no shifting at all. Were this brake casing, this kind of failure would be bad news because the cable is likely to break under a strong braking force – that is, when you need it most.

Getting the cable and casing right is a cheap task that greatly improves your bike. It’s worth changing the last section of outer casing (the piece that takes the cable into the rear derailleur) every time you change the inner cable, as it’s nearest the ground. More expensive derailleurs have a boot (rubber gaiter) protecting the end of the cable and so they stay cleaner, but this piece of cable costs so little in relation to the importance of smooth changing that changing it is worthwhile. The casing is really tough, so you need proper wirecutters to chop it to length – pliers won’t do. It is important to cut the ends neatly and squarely – if you cut raggedly or at an angle the casing deforms as you change gear, making your shifting sloppy. Once you’ve cut the casing, check inside to ensure that the lining hasn’t got squashed. If it has, open it out with the point of a sharp knife. Finish off the end of each length of casing with a ferrule. These protect the ends of the casing and stop them from splaying out.

Getting exactly the right length can be tricky. If the casing is too long, it adds friction to the cable, which again means sloppy shifting – the spring in the derailleur has to pull the slack that is created when you release cable at the shifter all the way back to the derailleur, so the less friction here the better. If the casing is too short, the inner cable will be constricted as it goes around tight bends. Care needs to be taken with sections of casing that join parts that move in tandem with each other; for example, the section of casing that joins the handlebars to the frame or that join the frame to the derailleur. Dual suspension bikes need extra care with the sections that connect the main frame to the rear end. Make sure you have enough casing to allow suspension to move without stretching the casing. Ghost shifting is often caused by casing that is restricted or stretched. Replace these sections of casing every time you replace the inner cable. When the casing is the right length, it approaches the cable stops parallel to the frame and looks elegant. Check casing regularly for splits, cracks and kinks. Damaged casing should be replaced straight away – it has a habit of being fine for a while, lulling you into a false sense of security, then it goes suddenly just when you’re farthest from home, leaving you to limp back in the smallest sprocket or chainring.

See also bike maintenance tips, tricks and techniques “Work the derailleur back and forth”