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Gear shift and brake cables often show the difference between a hastily assembled bike and one which has been assembled by a mechanic who cared what he or she was doing.
Especially now that new handlebar designs seem to come along every week, a good mechanic must understand the theory of routing cables. One can no longer rely on a couple of rote "rules of thumb" for routing cables correctly.
Although people pay a lot of attention to what kind of derailers and brakes are fitted to a particular bicycle, good cable installation practices are more important than most differences between different brake and shift systems. The most expensive brakes and derailers will work poorly if there is excessive friction or play in their control cables. Even cheap brakes and derailers can usually be made to perform satisfactorily if care is used in installing the cables.
The great majority of service problems with brakes and gears are the result of cable friction, not deficiencies in the levers, calipers or derailers.
Cables used on bicycles are in two parts. The inner wire is made of twisted strands of steel. The outer housing is also made of flexible steel, usually wound in a spiral. The inner wire runs down the middle of the housing. Both parts are equally important: neither can work without the other.
Isaac Newton said "For every action, there is an equal and opposite reaction." In the case of bicycle cables, this means that there cannot be a pull on an inner cable without an equal push on the housing. The housing gives the pull of the cable something to pull against.
That's how the cable can apply force to one arm of a sidepull brake, and the housing, to the other arm. Looking at the brake, you will see that the cable pulls, and the housing pushes -- equal and opposite.
Or, think of a tug-of-war: The two teams are pulling on opposite ends of the rope, like the brake lever and the brake. The ground underneath is placed in compression, like the cable housing.
To save weight, many bicycles substitute the bicycle frame for some sections of the housing. This is done by attaching "cable stops" to the frame or fork. A cable stop has a socket to receive an end of a cable housing, and a small hole or slot through which the inner cable can pass, but the housing can't. The "push" of the housing is transferred to the frame, so the inner wire can run bare until it gets to another cable stop facing the other way, where the "push" from the frame is transferred back to another length of housing.
This "bare cable" routing can be done anywhere that the cable runs in a straight line and doesn't have to bend. Housing must be used from the handlebars to the frame, to accommodate the turning of the handlebars as the bicycle is steered. Housing must also be used where a cable moves the two arms of a brake in opposite directions. Sometimes, the inner wire is stationary and only the housing moves -- see this example. Lengths of housing are also commonly used when the direction of pull of the cable must be changed.
Traditional cable housing is a tightly-wrapped spiral of steel wire, sort of like a small-diameter Slinky. It has no particular strength in tension (pulling) but it cannot be compressed because the coils of wire are tight against one another.
Through the 1970's, the inner wire ran right through the steel spiral housing, usually using grease for lubrication. Modern housing, however, has a plastic liner which surrounds the inner wire. This considerably reduces the friction. Some high-end cable systems, such as the Gore-Tex "Ride-On" cables, extend this liner even along the areas where there is no housing. These systems also have a special friction-reducing coating on the inner wires.
With the advent of indexed shifting combined with handlebar mounted shift levers, it developed that conventional housing was a source of imprecise shifting. This is because the effective length of the housing changes as it is bent. This is not a problem with brakes: Although sometimes it will be noted that rear brakes may drag slightly when the handlebars are turned all the way to one side, you can't turn the bars that far when the bike is actually in motion.
The small variation in housing length was too much for reliable indexed shifting, however, so Shimano introduced "S.I.S." housing, now widely copied by other manufacturers. This type of housing does not consist of a single spiral-wound wire, but instead, it has a bundle of wires running pretty much straight along parallel to the housing. They are held in place by being sandwiched between the plastic housing liner and the plastic outer covering.
"Compressionless" housing doesn't change length significantly as it is flexed, so the indexed shifter is able to communicate the correct setting to the derailer, even as the handlebars are turned, and the loops of cable housing bounce up and down due to bumps.
Warning: Since compressionless housing relies on plastic to hold it together, it is not as strong as conventional spiral housing, and should never be used for brakes! The loads applied to brake cables can easily cause compressionless housing to rupture and burst, causing a complete and sudden loss of brake function.
The German company Nokon manufactures cables with housings consisting of small, rigid, interlocking aluminum segments, with a low-friction plastic liner. When installed, the cable in its housing looks somewhat like a string of pearls. This system is index-compatible but allows much more flex than ordinary "compressionless" index-compatible housing, and also can be used for brakes. The interlocking ends of the segments keep flex below the limit where the inner wire would be damaged. Nokon cables are very expensive but worth the price in demanding applications, especially where the cable must be flexed sharply. One important application is where a cable must pass the hinge of a folding bicycle. There are other "string of pearls" cable housings, but as of this writing they do not work as well as Nokon.
Even when the housing is cut cleanly, the end is not square and perpendicular, due to the pitch of the spiral. Careful mechanics will grind or file the end of the housing so that it is flat and flush. The best tool for this is a grinding wheel, but it can be done with a file if you don't have access to a grinding wheel.
When you cut the housing, the end of the plastic liner also gets cut, and often gets squashed flat. You can use a scriber or a sharp awl to open it up and round it out. If you use a grinding wheel to dress the end of the housing, have your scriber right at hand so that you can open up the plastic liner immediately after grinding. The heat from the grinding will partially melt the liner. By sticking the scriber in before the liner cools off, you can not only round out the end, but the shape of the scriber will actually flare the end a bit for a smoother transition.
|Raw Cut End||Finished End|
Some people who don't have access to one of these cutters will use a hand-held grinder (such as a Dremel tool) with a thin abrasive "cut-off wheel."
It is not necessary to grind the ends of compressionless housing, because, if you cut it with an appropriate tool, it comes out flat. It is still usually necessary to open up the end of the plastic liner with a scriber or awl.
The final loop at the rear derailer is short and has a nearly 180 degree bend. "Compressionless" housing is normally used for this. I've taken to bending the piece of housing to the approximate shape it will be used in before cutting it.
If you cut the housing straight, all of the longitudinal wires come out the same length, so when you bend it, the end of the housing acquires a slanted face, since the wires on the inside of the bend have a longer way to go around the curve. It is my belief that cutting the housing while it is bent makes a smoother, more reliable connection at the end of the housing.
Ferrules are particularly important with compressionless housing. Even so, individual lengthwise strands can sometimes poke out of a ferrule or housing stop next to the inner wire. You may have to look closely to notice the rogue strands. They cause deteriorating, erratic shifting. Generally, pre-cut compressionless cable housing with tight-fitting ferrules is immune from this problem. Smaller-diameter (4 mm) compressionless cable housing will generally fit into housing stops without removing the ferrules.
The traditional treatment is to solder the strands together at the end of the cable. Ordinary rosin-core electrical solder does the trick. This treatment allows the cable to be pulled through the anchor bolt and housing to reinstall or relubricate it.
Solder adheres only to cables with tinned (galvanized) strands, not to stainless-steel cables -- which can usually be identified by their shinier appearance. Some stainless-steel cables are sold with the strands welded together at the end, but that only works if you don't trim the cable. The more common treatment is a small aluminum cap, which can be pressed onto the ends of a cable with pliers. The cap will prevent you from scratching yourself and will keep the cable from unraveling, but must be removed and replaced with a new one every time you disassemble and reassemble a cable..
It's good to leave the end of a cable long by a couple of inches so you can trim it back if it has started to unravel. Moderate unraveling can often be reversed by laying the strands back over one another, working from the middle toward the end of the cable.
To ensure firm contact of the housing against the stop inside the brake lever, the cables should be fully connected and put under tension before they are taped down. One good way to do this is to use a toe strap to hold the brake lever tightly applied while securing the section of housing that runs along the handlebar. It is good practice to use electrical tape or other adhesive tape to secure the cable housing against the handlebar. If you do so, it is easier to apply the normal handlebar tape afterwards, or to replace the handlebar tape at a later date.
The rear brake cable should go on the side of the stem opposite the front brake cable...this way you will not have to allow so much extra slack in the rear cable, since the handlebars can't turn as far in the direction that will tighten the rear cable.
Nobody knows exactly why this is. My theory is that it is based on the reasonable idea that you should be able to have your primary braking hand on the handlebars while making a turn signal with the appropriate hand -- coupled with the erroneous idea that the rear brake is the primary brake.
I prefer to set my own bicycles up with the front brake controlled by the right lever. This allows me to signal and stop at the same time, and also lets me use my stronger, more skillful hand for the more critical front brake. (I rarely use my rear brake.)
Since this is the opposite of the prevailing national standard in the USA, I would never set up a bicycle this way for a customer without a specific request to do so. I have an article on Braking and Turning which addresses these issues in more detail.
This worked quite well, until mountain bikes came on the scene and made granny gears a standard item. The problem was that the extended cage of a wide-range front derailer would interfere with the rear gear cable.
Unfortunately, this routing tends to degrade shifting somewhat. Locating the chainstay cable stop down below creates a sharper curve for the final loop of housing, and also exposes the entrance to that loop to crud splashed up by the front wheel. The bottom-bracket guide, whether over or under the bottom bracket, is also exposed to sprayed mud and crud from the front wheel...a particular problem for off-road cyclists.
The rear runs down along the seatstay, and the front runs down the back of the seat tube. When this style first arose, in the early '90's, the front derailer was a problem, since existing front derailers were intended to be operated by a cable pulled from below. Early top-routing schemes used brazed-on pulleys on the back of the seat tube, a rather mono-buttocked solution, in my opinion. This problem has been solved by the ready availability of "top-pull" front derailers.
Cable runs either over or under the bottom bracket can sometimes contribute to "autoshifting", spontaneous upshifts of the rear derailer under heavy load. This issue is addressed in a separate article.
Most bicycles with handlebar-mounted shifters run the rear cable on the right, the front on the left. This causes some awkwardness in routing the length of housing from the shift lever to the frame stops. Due to the need to allow these housings to be long enough to permit the bars to be turned all the way back and forth, the housings often wind up making a reverse bend--for instance, the rear will go from the shifter, which is on the right, swing forward and cross over past the centerline of the bicycle, then back over to the right side of the top tube, before heading down the down tube. These extra bends increase friction, and the fairly forcible contact between the housing and the side of the top tube can damage the finish.
A neat solution to this is to run the cables "criss-cross" style: The rear runs from the lever, (on the right) around the top tube, and to the cable stop on the left side of the downtube! The front cable crosses over similarly from the left side of the handlebar to the right side of the down tube.
The bare cables then cross one another under the middle of the downtube, making an "X". The cables may touch where they cross, but they will do so very lightly, since they are both straight...the tiny bit of friction at this crossing is more than offset by the reduction in friction in the smoother-flowing cable housings.
This technique does not work with over-the-bottom-bracket cable routing, but is doable with most newer bikes that have under-the-bottom-bracket cable routing and cable stops mounted toward the bottom side of the down tube.
This site also contains an extensive article on Derailer Adjustment.
New cable too short? This problem can occur when you can't get a long cable for a tandem or when you need to re-use a front cable at the rear There are commercial cable splitters, for bicycles with separable frames.Two cable anchor bolts in a strip of metal drilled at both ends also will do the trick. In an emergency, two lengths of cable can be spliced together with a square knot. Bend each piece into a tight "U" shape a couple of inches from the end, then loop them together to form the knot and pull hard on the cable with the lever to tighten it up. You may have to readjust it more than once. Highly flexible braided cable may need to be bent into a "Z" shape so it can be knotted once more in order to hold.
Where a cable end faces upwards so rainwater can run down inside the housing, the water can rust the cable, but worse, in cold weather, the water can freeze and disable a brake a few minutes after taking the bicycle outdoors. The boot used where the cable of a direct-pull brake crosses between the brake arms can be used here to keep water out. Oiling the cable also helps.
Modern plastic-lined cables have made the use of grease inappropriate, because the viscosity of the grease makes for sluggish cable movement. This is a more critical concern with modern brake and gear systems that use weaker return springs, and with indexed shifting in general.
Many manufacturers now recommend against using any lubrication on cables. It certainly should be avoided in the case of sealed systems such as Gore-Tex ®. Bicycles used in wet conditions, however, will often benefit by the application of a bit of oil, more as a rust-preventive than as a lubricant. The area of particular concern is the short loop of housing which carries the rear derailer cable around from the chainstay to the derailer.
Some bicycles provide awkward cable routing which forces housing to enter cable stops/adjusting barrels at a fairly sharp angle. This is particularly common on rear cantilever brakes. It often helps to put a bit of grease on the bit of cable that runs through such fittings.
Brake cable anchor bolts are the most important fasteners on a bicycle. They are small, and many of them have holes drilled through them, so it is easy to strip/break them, but...
If you don't get the anchor bolts tight enough, the brakes will appear to work properly in normal use. Then, someday a bus will cut you off, and you will squeeze the brakes extra hard to make a panic stop...just when you need the brake to work their best, the cables will slip and the brakes will fail completely, with no warning. How to test that cables are secure? Grab each brake lever in turn with both hands, and squeeze hard. Shift cables aren't subject to as much strain, but still, shift to the position with the cable tightest, and pull hard on the lever to check.
As just mentioned, cables can slip if not securely anchored. Housing can get bent; cables and housings can rust and seize up. These problems are obvious and call for replacement.
Cables generally don't wear, but they can fail due to fatigue when bent back and forth repeatedly, the same way it is posible to break a paperclip. Cables usually break at an end or where they pass over a pulley. Usually there is no symptom until the cable parts: prevention is by replacing cables on a schedule, or at least checking them frequently. Carry spare cables!
A pulley large enough to avoid fatiguing the cable would be at least a couple of inches across -- impractical for shift levers. A handlebar-end shifter, as shown in the photo below, may give a warning: frayed cable strands may prick the cyclist's fingers, or impair shifting as they push against the housing end stop.
A cable which has begun to fray due to repeated bending
Indicator chains of Sturmey-Archer internal-gear hubs have solved the fatigue problem for more than a century -- and so, instead, Sturmey-Archer cables usually fail where they pass over the pulley sector in the shifter.
A compact, lightweight chain isn't strong enough to resist the cable pull of a brake, and so a pivot at the cable end is needed instead. The cable-end socket inside a drop-bar brake lever is free to rotate, and so the cable can align istelf with the direction of its pull. Flat-bar levers generally rely on the cylindrical cable fitting to make this adjustment. Lubrication of the pivot is important.
Many brakes and derailers attach the cable with an anchor bolt that does not allow the cable to align itself. This problem is generally worst with front derailers. Bending also occurs where a transverse cable passes over the yoke of a cantilever or centerpull brake.
A shift-cable failure usually doesn't lead to a crash, but a brake-cable failure very easily can. This is one reason that every bicycle other than a track-racing bicycle needs two independent braking systems. (The drivetrain of a fixed-gear bicycle does count as one, if the cyclist is skillful in using it for braking.)
(David Gordon Wilson has written an article covering the issue of cable fatigue.)
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