The Chainlink is thrilled to introduce Scott Wilson as our new Technical Editor. Scott’s an MFA writing student at Columbia College as well as a seasoned professional bike mechanic. Scott’s “wrenching” experience includes bike shops, racing teams, and professional triathletes across the US. The aim of Scott’s technical articles is to explain in detail how bicycles and their individual components work...and in doing so, help you keep your own bikes running better and lasting longer. Scott’s premiere column will be a multi-part series shedding some light on one of the lesser-understood aspects of modern bikes; cables and cable housing.
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:
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:
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.
This is what shift housing looks like under the ferrule after a season of all-weather riding.
Here is the end of a section of brake housing. When it's freshly cut, the coil has a sharp end, so the mechanic who installed this piece ground it flat.
This shift housing was on the curved section leading to the rear derailleur. Notice how some of the wires stick out further than others? That's because there was a sharp bend in the route the housing had to take.
On the left is a used section of brake housing, peeled free from its outer casing. On the right is a new piece. Notice how the coil is spaced out on the older piece, especially where it bends.
Here is what an exploded piece of shift housing looks like.
Here, the cable has been sheathed in a nylon tube to prolong its life and keep debris out of the housing.