| Accessories | Bicycles | Parts | Specials | Tools |
|---|
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 the cheapest 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.
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. Lengths of housing are also commonly used when the direction of pull of the cable must be changed.
At first glance, many people assume that cable housing is made of plastic. Actually, it is steel, and the plastic is a covering to protect it from moisture, and to keep it from scratching the paint of the bicycle.Traditional cable housing is a tightly-wrapped spiral of steel wire, sort of like a small-diameter Slinky. It has no particlar 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.This 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 a bundle of wires running pretty much straight along parallel to the housing. They are held in place by the fact that they are 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.
A very common source of excessive cable friction or "sponginess" is improper cutting or failure to shape the ends of the cable properly.
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 immediatly after grinding. The heat from the grinding will partially melt the liner. By sticking the scriber in before it 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.
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 of 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, 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 seat stay, 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 down tube! 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 down tube, 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.
SRAM, the makers of Grip Shift, have a couple of special accessories for difficult rear shift cable installations.
- The "Bassworm" attaches just in front of the vulnerable rearmost loop of housing that runs to the rear derailer. This clever device serves two functions:
- It seals the front of the housing, making it impossible for grime and grit to enter.
- It also provides a fully adjustable "helper spring" to augment the return spring in the rear derailer for situations where you cannot reduce friction sufficiently to provide crisp upshifting.
- The "Nightcrawler" is a somewhat simpler variation on this, providing a bellows seal for this loop of housing, without the extra spring action.
In the old days, before the developement of plastic-lined housing, it was necessary to coat the inner cable with light grease or heavy oil.Modern plastic-lined cables have made the use of grease in-appropriate, 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 chain stay 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.
Much of the hardware associated with cables requires lubrication on assmbly.
- Adjusting barrels must have lubrication on their threads, if they are to remain usable.
- Anchor bolt threads should also be lubricated, lest the threads strip as you tighten them.
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.
At the risk of repeating myself, let me again urge you to pay careful attention to all aspects of cable routing. Care in cable installation is much more important than having the latest titanium doo-dads!
| Accessories | Bicycles | Parts | Specials | Tools |
|---|