Cables and Cable Housing Part I: Theory

When keen cyclists hear product names like “Dura-Ace” and “Super Record,” the first thing that comes to mind is images of precise, errorless shifting; a clean ‘click’ noise; and the feeling of smooth, efficient energy transfer. But what do you think when you hear names like “SP-41” and “CGX-SL”?

Shift and brake housing usually doesn’t get much attention, even though its job is vital to the operation of the components it’s connected to. You’ll never see an ad for the hot new racing machine that says “Now with SP-41 pre-lubed housing and PTFE coated cables!” But, when your shifter makes that click noise and the mechanism inside pulls or releases a length of cable, attempting to transmit an up or down shift to your derailleur, that communication must go through a stiff, low-friction, appropriately sized length of housing. Housing is a dynamic component and requires careful installation and regular attention to perform at its best. This article is a short explanation on what housing is, how it works, and how it should be treated.

What is housing? What does it do?

The job of housing is to hold the cable in place and give it something to push against. Think of it like this: you want to drag a rock to where you’re standing with a rope, but directly in between you two is a hole. You don’t want the rock to fall into the hole, so you thread the rope around the trunk of an adjacent tree, thereby altering the path of the rock but not changing where it ends up. Make sense? Housing allows the cable to move in a non-linear path between components.

There are two basic types of cable housing: shift and brake. You can tell them apart by their relative thickness to each other (brake is a little thicker), their relative flexibility (shift is a little less flexible), and by looking at their cut ends (brake looks like there’s a metal coil inside, shift looks like a bunch of wires surrounding the center). Why, you may be asking, are there different types of housing for braking and shifting? Well, there wasn’t always a difference. Only since the advent of precise, handlebar-mounted, indexed shifting (i.e. clicky shifts, as opposed to click-less friction shifts) has the type of plastic tubing that holds your cables in place mattered.

Brake housing (also called Bowden cabling) is designed to be strong; it’s constructed out of a helical steel wire sandwiched by outer and inner plastic or nylon or Kevlar. This way, when you grab a handful of brake lever with all your strength, the cable won’t bust out the side. The problem is that when you bend brake housing, that coiled wire effectively lengthens; if it’s connected to your derailleur the bounces and bends that happen during hard riding cause mis-shifts. You can test this by moving your handlebars all the way left and right and watching the rear brake tighten just a little. That little bit of movement is enough to confuse a derailleur. Shift housing, in comparison, gets its stiffness and strength from many steel wires arranged parallel to one another, but with a slight spiral, and held in place by a plastic inner and outer wall.  Due to this design, the effective length of shift housing (also called compressionless housing) doesn’t change nearly as much as brake housing when bent. Think of it like bending a Slinky versus bending a garden hose: the Slinky bulges out at the apex and the garden hose doesn’t. The downside is that shift housing isn’t very strong or flexible. If you somehow felt inclined to test this, you’d grab the shift cable that’s running down the length of the bike’s down tube and pull it as hard as you possibly can with both hands. The force of your pull would cause it to explode, hernia-like, out of the housing. Yikes! There are special types of compressionless housing made for brakes, but that’s beyond the scope of this article.

Does housing go bad? Why?

Housing wears out in several ways:

1. All the bending and movement the housing experiences during riding combined with the internal forces of the cable trying to burst free cause a lot of issues over time. In the case of brake housing, the coil unwinds and pushes beyond its plastic liner. The housing becomes less stiff when this happens and not only is braking quality affected, but also the unsheathed coil can rub on the cable, causing friction and a grinding feel at the lever. With shift housing, the little parallel structural wires migrate inside the plastic liners and try to escape out one of the ends. Complicating things is that the wires in shift housing follow a mild spiral that unwinds over time, but sometimes the wires take a bias towards one end of the housing and can even wiggle away from the other end, leaving it unsupported.  When you get a basic tune-up, the mechanic is mostly correcting for the effective shortening of the cable when the housing comes apart from the inside. There are effective lengthening forces at work too, but we’ll save that for another article. A little housing degradation is expected and built into the design of the ferrules that hold the housing at its ends, as well as the malleable inner and outer plastic walls, but everything has limits.

2. The environment is rough on precious little bike parts. All too easily water travels down the cable and into the housing where it sits, immune to evaporation, forever. Cables rust, inner coils rust, everything gets ruined. When a cable oxidizes it not only effectively shortens as the individual wires that compose it space out, but it causes a lot of friction: rust is not smooth.

3. The plastic outer covering degrades, especially when it’s exposed to UV light for a long time. When you store your bike, keep it away from windows.

There are a lot of other ways housing goes bad, including misuse and damage. I’ll let your imagination fill in the rest of the list. The short of it is that housing doesn’t last very long, maybe a season, maybe more, maybe less. Professional racers have their cables and housing replaced after every race, which is probably just a little wasteful. But there are ways to get a more mileage.

How do I prolong the life of my cables and housing?

Trying to make your housing last longer is kind of like trying to make your tires last longer; it’s a task the gods would give to Sisyphus. But, here’s a quick list of tips:

1. Use pre-lubed, quality housing composed of multiple layers of polymers, like Shimano SP-41, Jagwire LEX-SL, Campagnolo Ultra-low friction, etc. Note: housing and cables are brand-specific. Shimano/SRAM use a different standard inner width than Campagnolo. Also, Jagwire cables made for Shimano/SRAM are not the exact same diameter as their Shimano/SRAM brand-made equivalent, and even within the companies “slick” or “coated” cables are just a tiny bit thicker than their untreated brothers and should be paired with the appropriate housing.

2. A lot of mechanics will tell you to drop chain oil into your housing after riding through the rain. This isn’t always a good idea. Lubrication is a very exact science and certain lubes do not react well with the other lubes that might already be residing within the housing. I’d say you should consult a technical representative for whatever brand of housing you’re using, but those guys don’t always know either. Luckily, most companies sell their own cable and housing lube, so use that if you can get it. If you know your housing and cables did not come lubed, I suggest routinely dropping in a paraffin wax-based liquid like Boeshield T9, but that’s just an opinion, there might be better solutions that I haven’t tried yet.  

3. You can sheath your cables and housing in nylon tubes, basically sealing them from the outside world. The Gore Company used to make a “Ride-on” cable and housing system that did this, but it fell out of fashion. However, sometime in the future I’ll do a post on how to seal your cables by yourself.

4. If you notice a cable fraying, replace it. When cables come apart they chew up the inside of the housing.

5. Lastly, KEEP YOUR BIKE CLEAN PLEASE. Most of the repairs I see on a regular basis could have been avoided if the rider had wiped the bike down with a rag after every couple rides. Dirt, salt, water, and road debris are to bikes as flesh-eating fungal spores are to man.

These tips are all, to some degree, band-aid solutions. The best time to prepare your cables and housing for a long and productive life is when they’re being installed. For our next technical article The Chainlink will investigate the best practices for installing cables and housing, but also show some tips to increase performance. In the mean time, here are some photos to help illustrate the differences between brake and shift cables, as well as the effects of Chicago riding on them.

​Here, the cable has been sheathed in a nylon tube to prolong its life and keep debris out of the housing.