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Making a Bike Shift Perfectly

Teaching someone the steps to adjust a derailleur is relatively easy. Once the limit screws and cable tension are adjusted everything should be perfect. But experienced mechanics will often shy away when someone asks to be taught how to adjust a derailleur. They know that it’s just not that simple. There are so many things that have to come together perfectly for a bike to shift right. If just one thing is wrong then adjusting the derailleur can be a horribly frustrating endeavor. This explains why graduates of bike mechanic schools aren’t necessarily qualified to be head shop mechanics the day after graduation. The teachers at those schools will be the first to tell you that instruction gives you the fundamentals, practicing them a few thousand times is what makes you good.

Over time, pro mechanics begin to develop an intuition about what might actually be causing a bike to shift imperfectly. There is this mysterious black magic that good mechanics seem to be able to perform on bikes. A lot of times they’ll diagnose something wrong with a bike without even really knowing how they knew what the problem was. They just feel where the problem is coming from.

That intuition is what makes a mechanic so valuable. There is no way to teach it and it can’t be learned quickly. The only way to get it is to learn the fundamentals and then practice for a few years. Eventually, you just find yourself adjusting bikes by feel instead of following any sort of step by step process. It’s also one of the hardest things to understand for those who haven’t gone through the transformation yet. Many shop owners and managers are frustrated by how hard it is to find a good mechanic and why they think they deserve more money than the sales staff to do the easiest job in the shop. Many others have come to accept that mechanics are simply a special kind of magician that can make bikes work beautifully but can’t explain why, and you have to have them around if you want to run a bike shop.

Whether they know it or not, a mechanic quickly goes through a mental checklist of hundreds of things when diagnosing imperfect shifting. When you take a step back and think about all the things a mechanic is considering during their diagnosis it is absolutely mind boggling. It has to start with faith in the components. A good mechanic doesn’t accept that the shifting isn’t perfect simply because the derailleur sucks. That’s not to say that there isn’t a performance difference between high end and low end equipment, or that another company’s product will always be perfectly compatible with everything else out there. However, if you are starting with a complete group from one manufacturer it’s pretty safe to say that everything will work. For the most part, mixing in components from outside companies will work without a problem as well. The bike industry is pretty lucky in that products that make it to market are generally well tested and reliable by the time they get there. Knowing the components are good is what tells the mechanic that the problem has to be somewhere else. The following is all about that something else.

Explaining the massive list of things that must be considered to diagnose imperfect shifting is no easy task. To begin, consider these six basic causes of imperfect shifting: A flaw in the transmission of the action at the shifter into action at the derailleur; Friction preventing the derailleur spring from effectively moving the derailleur to the next gear; The gear is a moving or misplaced target; Improper mounting of the derailleur; A frame issue preventing the derailleur from doing its job; A flaw, failure or incompatibility in the components themselves.

Transmitting the action at the shifter to the derailleur:

How do you get the precise movements of the shifter to make the derailleur move the same precise amount? For quite some time now the answer has been to have the shifter pull on a steel cable. In theory, as long as the cable doesn’t stretch, the shifter movement will be perfectly transmitted to the derailleur. That would be the only part of the equation if the shift cable could be in a perfectly straight line or run around nothing but pulleys, but it’s not, and it can’t.

The idea of running the steel cable through a flexible housing to allow the cable to go around corners and move with the turning handlebars has been around for quite some time as well. It works because when the shifter pulls on the cable, the anchor point at the derailleur has to get closer to the shifter. So either the derailleur can move, or something between the anchor point and the shifter has to compress. The housing resists compression better than the spring in the derailleur resists moving so the derailleur moves instead of the housing compressing. It is also possible to interrupt the housing and run bare cable in straight lines or through pulleys and guides. In these areas of bare cable there is no housing to compress, just cable stops that have to resist being pulled toward each other, a relatively easy job since they are attached to the frame.

Early versions of cable housing were simply a long flat piece of steel wound like a spring to create a tube and then wrapped in a plastic sheath. Later came the inner liner to reduce friction. Picture a modern brake cable. The wound housing did a good job of making the cable go around corners, even an almost 180 degree turn at the rear derailleur, but it had a flaw. The housing would compress slightly under the force applied by the shifter. A certain amount of the movement of the shifter was not being transmitted to the derailleur. Housing was kept as short as possible. Shifters were mounted to the down tube, cables ran over a pulley to get past the bottom bracket, only a rear derailleur piece of housing was used. Indexed shifting was also proving to be close to impossible because the compression of the housing robbed too much of the cable movement.

In 1984, Shimano came out with SIS housing and the first really effective indexed shifting system. The coaxially wound wires that replaced the wound flat steel allowed the housing to better resist compression forces, transmitting the movement of the shifter all the way to the derailleur very effectively. This compressionless housing led to an increase in freedom of design for frame and component engineers and is still the predominant means for translating shifter action into derailleur action.

There is still a mountain of things that can cause action at the shifter to be lost before it reaches the derailleur though. Failures of the cable and housing system are common causes of problems but anything that flexes between the shifter and the derailleur can be an issue. In a perfect system 100% of the action of the shifter is transmitted through the cable and housing into the derailleur. The following things prevent some portion of that shifter action from making it to the derailleur.

Failures of the cable and housing:

1. A crack or split in the cable housing outer sheath, allowing a section of the housing to compress. This is often under the ferrule where it can’t be seen.
2. A cracked ferrule allowing the end of the housing to move a bit instead of pulling cable.
3. A crack in a cable stop in the frame can allow the end of a piece of housing to move instead of pulling cable.
4. A kink in the cable causes extra friction but the kink will also have to be straightened by cable tension before the derailleur will move.
5. If a ferrule with an extended tongue at the end is used in a place where it doesn’t fit, like in the shifter or derailleur it may prevent the ferrule from settling up against the cable stop. The ferrule will have to be pulled into the stop before cable can be pulled.
6. The wrong size cable head may not sit in its cradle properly, effectively changing the size of the spool that the cable is wrapped around.
7. The wrong diameter cable effectively changes the size of the spool the cable is wrapped around since the center of the cable sits closer to the pulley axis if the cable is thinner.
8. If something gets lodged under the cable head inside the shifter then the cable may not sit in its cradle properly, effectively changing the size of the spool that the cable is wrapped around.
9. Brake housing or non SIS shift housing will obviously compress and keep all cable pull from being transmitted to the derailleur.

Things that flex and prevent all of the shifter action from getting to the derailleur:

10. A crack in the shifter housing near where the cable housing attaches can allow the housing to get pulled into the shifter instead of pulling the cable.
11. If the barrel adjuster is turned out all the way and the threads are no longer engaged it can allow the barrel adjuster to flop over instead of pulling cable.
12. If the cable is run through an under bottom bracket cable guide and is not seated in the groove properly then the cable guide may simply move or compress instead of pulling cable.
13. A thick chain stay protector that runs through the cable and frame and does not allow the cable to run in a straight line can get pushed out of the way and compressed instead of pulling cable.
14. Accessories mounted to the frame like water bottle cages and chain watchers that touch bare cables can flex out of the way or make the cable wander as is put under tension.
15. Some frames require special ferrules. If the wrong one is used the ferrule may flop around in the cable stop instead of being supported by it.
16. Using a 5mm housing ferrule on 4mm housing, or simply a ferrule from a different company that is supposed to be the same size but is actually a little bigger can allow the housing to move instead of pulling cable.
17. Not using a ferrule where one is require will allow the housing to flop around instead of pulling cable.
18. If the housing is pulled tight as the suspension compresses it will start to pull cable on its own.
19. If the housing gets pulled tight as the bars are turned it will start to pull cable on its own.
20. If the front brake pulls the shift housing out of the cable stop as the bars are turned it will start to pull cable on its own.
21. If a rubber boot that fits over the end of a tongued ferrule is used at the derailleur, the boot may get pinched in the derailleur mechanism and it will have to be crushed before the derailleur can move.
22. If the cable is not routed perfectly through the multi-pull pulley on the front derailleur the pulley size can be effectively changed.
23. A pulley used to change the cable direction (like one used to make a bottom pull derailleur work on a top pull frame) that has play in it will flex before pulling cable.
24. A crack in the frame can allow two cable stops to get closer together before cable is pulled.
25. If there is a long section of cable with no housing then normal frame flex can pull the cable.
26. A cracked cable stop on the frame can allow the end of the housing to move before cable is pulled.
27. Cables wrapped around each other inside the down tube of an internally routed bike will affect each other during every shift of the front or rear derailleur

Allowing the derailleur to spring back to the next stop in the shifter:

As if all those problems transmitting shifter action to the derailleur weren’t enough to deal with, realize this: that only applies to half the shifts, the ones where the shifter is pulling cable. For the derailleur to move the other direction the spring must be able to pull the cable back through the housing until it is stopped by the next step in the shifter. All of the previously mentioned issues still apply to the derailleur pulling cable through the housing but now friction becomes a giant issue as well. When pulling cable at the shifter you have the strength of your hand on your side. If it’s hard to pull the cable, just pulling harder can make the shift happen. When the derailleur pulls the cable back it only has the strength of the spring.

Friction in the cable and housing system is essentially how cable and housing wear out over time. Dirt gets inside the housing, the cables start to rust, the plastic sheath starts to get rougher, or the grease or oil between the cable and housing wears out or gets contaminated. The effect is slow, so often times the rider doesn’t notice that it’s happening. Often times at the race trailer a rider will ask to have a minor shifting issue looked at and we nearly break our thumb trying to shift gears. It isn’t until the rider is shown how easy it is to shift gears on another bike that they realize how much friction had crept up on them. Cable and housing should be replaced together as regular service about once a year for active riders.

For a mechanic, friction can sometimes be a tricky thing to diagnose. It is easy to expect that the derailleur moves the right amount when the cable is pulled and releases the right amount when the shifter releases cable. If you don’t feel the extra friction at the shifter it can just feel like the cable has too much tension on it. In the stand the bike will seem to shift to a larger gear fine but hesitate when coming back down to a smaller one. Removing tension from the cable seems to work because you are able to over-shift a bit to get the chain into the next bigger gear and it drops down to a smaller gear nice and quickly. The problem is that there is more to the equation when the bike is out in the real world. Eventually, the derailleur will be able to pull through that friction and then the derailleur will be adjusted to far towards the smaller gear. Also, while over-shifting may seem to work ok in the repair stand, it is not ideal for actual riding conditions. A simple test to see if cable and housing friction is affecting the shifting to a smaller gear is to shift to the smaller gear, wait for the derailleur to settle into position, then pull or push on the derailleur to tug on the cable a little more. If the position of the derailleur changed then it was likely because the derailleur was not strong enough to pull the cable through all that friction. A word of caution though, the test would be exactly the same for friction in the derailleur itself. So don’t spend time and money replacing cable and housing until friction in the derailleur is ruled out. Simply unhooking the cable and pushing the derailleur across the cassette and seeing how fast it returns gives you a pretty good idea.

There is a lot of technology out there these days that can help reduce cable and housing friction. It can make shifting better than it ever has been before and help old cables last a little longer. PTFE coated cables are the simplest addition to a standard kit and go a long way to reduce friction. They used to be famous for the coating coming off inside the housing and making things worse but that’s been pretty much figured out by now. GORE uses a different kind of coating on their cable to reduce friction. For the new 9000 Dura-Ace group, Shimano is introducing a cable wrapped in super slippery polymer strands. The cables are slipperier than PTFE coated ones and since the strands are independent of each other, if the strands start to come off in one area the integrity of the rest of the cable is not affected. Shimano has also made a specific cable grease for some time called “SP41” or “Special Grease.” SP41 housing comes with the grease already injected inside the length of it. The grease can also be purchased in a small tub. In this case the grease would be applied to an exposed piece of cable by putting a little grease between your thumb and forefinger and running it down the cable. This grease can be applied to a new system that doesn’t use SP41 housing or an old system where the friction has gotten out of hand.
Not all friction is caused by degraded, old cables and housing. There are some bikes that suffer from significant friction right out of the box. Sometimes bikes that have just had their cable and housing replaced suffer from excessive friction. Let’s look at several things that can keep the derailleur from pulling cable back through the housing.

28. Dirt inside the housing
29. Rusted cables
30. Contaminated grease or oil inside the housing
31. A worn out, rough liner inside the housing
32. Using a low end cable that has not been smoothed by drawing it into a round shape
33. Using a brake or non-SIS housing ferrule can allow the strands of wire supporting the housing to push through the hole in the ferrule making the opening for the cable smaller.
34. Pinching the housing when cutting it to length and not opening the end back up
35. A kink in the cable
36. A kink in the housing
37. A breaking or fraying cable
38. Seals in the ferrules are generally a good thing but do contribute to the total friction in the system. They can be eliminated in many places, especially on a road bike.
39. Tight bends in the housing
40. Housing sections that are too long, requiring the housing to bend in both directions and make an S
41. Sticky drink spilled on the cable
42. Sticky drink spilled on the derailleur itself (more common on the front derailleur)
43. Full length housing used – the more housing there is, the more friction there will be
44. Not all under bottom bracket cable guides are created equal. Independent testing shows that Shimano cable guides are the slickest and most durable out there.

Component engineers are certainly aware of all the issues between the shifter and the derailleur. In fact, there has been an incredible amount of advancement in the design of shifting systems over the past few decades to minimize the effects of cable travel loss and friction. Take for example the evolution of cable pull ratios.

This is an extremely complicated subject and it’s not practical to go into too much detail here. The things to realize though is that there are a lot of things that affect how easy it is to pull cable at the shifter and how easy it is for the derailleur to pull cable back. Basically, the shifter translates movement of your hand into movement of the cable. It might take 20 mm of shift lever movement to result in 3 mm of cable movement. At the rear derailleur, cable movement is translated into movement of the derailleur. That 3 mm of cable movement results in the upper pulley moving along a slant that follows the cassette, about 5 mm. The horizontal movement of the pulley has to equal the cog spacing (3.95 mm for 10 speeds). So the shifter gives your hand a mechanical advantage over the cable allowing you to overcome the springs in the shifter and derailleur and friction in the cable and housing. At the rear derailleur some of that mechanical advantage is taken away again and translated into increased movement of the derailleur. This means that the derailleur also has a mechanical advantage over the cable to help pull the cable back through the housing.

The principle at work here is that both the shifter and the derailleur have a mechanical advantage that helps them pull the cable. There are two ways to increase this mechanical advantage. Either lengthen the lever arms at the shifter and derailleur, or reduce the amount of cable pulled. Less cable pull means more compact, lighter components and less throw of the shift lever. More cable pull means a wider range of adjustment which reduces the effect of cable travel loss and friction. Furthermore, more leverage at the shifter or derailleur helps pull the cable through friction, less leverage results in less compressive force on the housing. It’s a delicate balancing act to say the least.
It’s the springs in the derailleur that actually pull the cable back through the housing when shifting to a smaller gear. The springs have more than enough strength to simply return the derailleur. If the spring is much stronger than it needs to be, then it will be able to pull the cable through more friction. However, the resistance from the spring will also need to be overcome when pulling cable from the shifter. It’s another delicate balancing act.

Over the years both mountain and road bikes have changed dramatically. When indexed shifting first came out, both road and mountain bikes only used a small piece of housing to connect the shifter to the top of the down tube and another to connect the rear derailleur to the chain stay. The friction in a new cable and housing system was minimal because of the short pieces of housing with gentle bends. Nowadays, though, mountain bikes use full-length housing that has to bend around suspension pivots and sometimes go in and out of the frame. Road bikes place the housing under the bar tape and then snake the bare cable through complicated internal routing schemes. The result has been a dramatic increase in drag between the cable and housing making the shifter harder to operate and shifts to a smaller gear slower and less consistent – even on a brand new bike.

The challenge for a component manufacturer is that in order to address these changes in bike design, they would have to change the cable pull ratios at the shifter and derailleur, which means changing compatibility. Imagine the nightmare if one day Shimano came out with a new 9 speed XTR rear derailleur that wasn’t compatible with any previous 9 speed group. Fortunately, other companies coming out with 10 speed mountain and 11 speed road drivetrains first allowed Shimano to make these changes with a minimal impact on compatibility. It was necessary to cut ties and compatibility with the past in order to evolve to meet the needs of modern bikes.

The gears are a moving or misplaced target:

The derailleurs still work exactly the same if there is no chain, crank, or rear wheel on the bike. All they do is move how they are supposed to move. Obviously, if the gears are not where they are supposed to be the derailleur won’t be moving the chain to the right spot. Here are some examples of things that can lead to the gears moving around or ending up in the wrong spot:

45. The wheel is not properly settled into the drop outs. This results in the cogs being in a different spot than they were when the derailleur was initially adjusted. It can also result in the cassette sitting at an angle which will make perfect adjustment impossible.
46. The wheel is not mounted straight in semi-horizontal or horizontal drop outs.
47. The quick release skewer is loose allowing play.
48. The adjustment of the bearings in the rear hub is too loose allowing play.
49. The bearings in the freehub body are worn allowing it to wobble on the hub.
50. The rear axle is broken.
51. The rear axle is bent.
52. The freehub body fixing bolt is loose.
53. The cassette lock ring is loose.
54. A crack in the cassette carrier.
55. A cracked cog allowing the others to move around.
56. A cracked cassette carrier.
57. A bent cog.
58. The incorrect spacer is used between cogs.
59. A cassette spacer is missing allowing two cogs to touch.
60. The number of gears in the cassette doesn’t match the number of gears in the shifter.
61. Worn bottom bracket bearings allowing the chain rings to wobble.
62. The bottom bracket cup is loose in the frame.
63. The crank arm fixing bolt is loose allowing the arm to wobble on the BB spindle
64. The external bearing preload is adjusted too loose allowing the entire crank arm to shift from side to side.
65. Damaged square taper or splined bottom bracket spindle that allow the crank to wobble on the end of the spindle.
66. A bent chain ring that wobbles as it rotates.
67. Loose chain ring bolts.
68. Chain ring bolts that are too long and do not properly secure the chain rings.
69. A chain ring is installed backwards affecting the spacing.
70. A crankset is used that that has the wrong chain ring spacing. Like using a 9 speed crank on a 10 speed bike.
71. A crack in a bottom bracket cup allowing play.
72. A crack in a crank arm allowing flex in the chain rings.
73. A bent crank arm spider.

Mounting components to the frame

If you hook a derailleur up to a shifter when neither is attached to a bike and you start shifting gears you will see the derailleur flounder around on the table like a fish trying to find its way back to water. There are actions in the derailleur that require equal and opposite reactions and without the frame holding onto the derailleur it can move any way it pleases. Most mounting issues are with the front derailleur because there are so many more variables there are a few things that can interfere with the operation in the rear as well.

74. The rear derailleur mounting bolt is just plain loose, allowing the derailleur to move around.
75. The rear derailleur’s B plate is not rotated all the way up against the B angle stopper tab on the derailleur hanger.
76. The derailleur hanger mounting bolt is loose where it attaches to the rest of the frame.
77. An E-type front derailleur mounting bolt is loose in the frame.
78. The front derailleur is mounted too high.
79. The front derailleur is mounted too low.
80. The front derailleur is not mounted parallel with the chain rings. If the derailleur needs to be mounted crooked to make shifting acceptable there is likely a frame issue or an incompatible component in the mix.
81. The front derailleur clamp band fixing bolt is loose allowing the whole thing to rotate.
82. The front derailleur clamp is a loose fit on the frame and allows the derailleur to rotate.
83. An E-type front derailleur is mounted at the wrong angle.

Frame Issues:

There is a large book full of specifications for a frame manufacturer to follow in order for components to work correctly. It contains all sorts of information like how far a road brake pivot bolt should be from the brake track, what the dimensions of the bottom bracket shell need to be, and what areas need to be kept clear in order for crank arms, gears and brake rotors to spin around. It also contains very specific dimensions on where the derailleurs need to sit in relation to the gears. Frames that don’t meet all of these specifications are very common. Often times it is because of some conscious decision on the part of the frame designer to give up some shifting performance in order to gain some other benefit. The most common example is failing to meet the minimum chain stay length. If the chain stays are too short it may be hard to use all of the cross over gears without the chain rubbing. If the chain stays are too long for a small size bike it may not handle well though.

Issues related to frame spec:

84. If the rear derailleur hanger’s B plate stopper tab is in the wrong position the derailleur will be rotated too far forward or back. This affects the distance the derailleur sits from the cogs and may make it impossible to set the B angle correctly.
85. If the rear derailleur mounting position is too low, then the derailleur will sit too far from the cogs making shifting slower.
86. If the rear derailleur mounting position is too far forward or back the chain will meet the cassette at a different point, making shifting slower.
87. If the rear derailleur hanger is too thick it may not be possible for the derailleur to shift to the largest cogs.
88. If the angle between the chain stay and seat tube is too small, then shifting will be slow and the chain may rub on the front derailleur.
89. If the angle between the chain stay and the seat tube is too large, then the shifting will be slow and the chain may run on the front derailleur. Note: it is relatively common for a full suspension mountain bike to have a chain stay angle that is too large. In this case the bike is designed so that the chain stay angle will be correct when the suspension is sagged. This may mean that when the rear suspension is locked out that the chain will rub on the front derailleur in certain gears, particularly in the middle chain ring and large cog.
90. If the chain stays are too short the angle of cross of the chain in cross over gears will be too great and the chain may rub on the front derailleur or the other chain rings. The minimum chain stay length is 405 mm for road bikes and 420 mm for mountain bikes.
91. If the chain line is incorrect and the middle of the chain rings does not line up with the middle of the cassette, then the chain angle in cross over gears may become too great on one side.
92. If a bike’s front derailleur mount (road braze-on, MTB direct mount etc.) is at the wrong angle the arc of the derailleur cages will not match the arc in the chain ring diameters. This will slow shifting on its own and also force the derailleur to be mounted too high.
93. If the spacing of the frame does not match the over lock nut dimension of the wheel, then the wheel may move in the drop out, the derailleur hanger may sit at an angle, and the chain line will likely be incorrect.
94. A suspension pivot can interfere with the operation of a derailleur. This is commonly seen on a bike with a pivot just in front of the drop out on the chain stay below the level of the axle. This suspension pivot would interfere with a properly adjusted derailleur so the derailleur is either moved back by the frame manufacturer or needs to be rotated back with the B angle adjustment. Either compromise will move the guide pulley too far from the cogs and slow shifting.
95. Some thru axles that thread directly into a nut captured by the frame require so much real estate that the derailleur needs to be moved down to make room for it. This moves the guide pulley too far away from the cogs and slows shifting.
96. Extra stout chain stays can be thick enough to interfere with the derailleur, forcing it to be moved back or adjusted back with the B angle adjustment.
97. If the cable stops are too close to the shifters or rear derailleurs the housing may not have a sufficiently gently arc to it causing excess friction.
Other issues related to the frame:
98. Flex in the frame can allow the rear cogs and front chain rings to move out of plane with each other causing the chain to rub. It is unlikely that shifting will occur under the loads that will flex a frame this much but it would definitely be slowed as well.
99. A crack in the frame can allow any of the drive train or shifting components to move and affect their performance.
100. If the derailleur hanger is bent, then the derailleur will sit at an angle and the derailleur will not move the correct horizontal amount with each shift.
101. Play in suspension pivots can allow parts to move around in relation to each other as well.
102. If the frame is not straight, then the chain rings and cogs will be out of plane and the chain can rub on either derailleur.

Component Issues:

A good mechanic never blames his tools. The same should generally be said about blaming the components when shifting is imperfect. However, there are some things to look for to indicate that a component is worn out, being interfered with, flawed in some way, assembled incorrectly, or just plain incompatible with the other pieces.

103. Play in the rear derailleur’s pivots. There is actually a relatively large tolerance for play in the pivots of a rear derailleur but eventually it will become so sloppy that shifting performance may suffer.
104. A bent B axle. If the B axle of a rear derailleur is bent, then the derailleur may sit on the frame at an angle just as if the derailleur hanger were bent.
105. A bent P axle. If the P axle of a rear derailleur is bent, then the pulley cage may move in and out as it rotates. This can cause the chain to rub on the cage or the pivot to bind and not spring back.
106. If the parallelogram of the rear derailleur is bent, the cable pull ratio of the derailleur may be changed and it will not move the correct amount with each shift.
107. If the pulley cage is bent, it may catch the spokes in the larger gears. The chain may also rub on the cage.
108. Excessive dirt in and on the derailleurs may prevent the pieces from rotating correctly or springing back to a smaller gear.
109. The front derailleur cage is bent.
110. Dirt inside the shifter can prevent the pawls from moving and prevent cable from being pulled or released.
111. Something underneath the cable head, like dirt that falls inside the shifter when changing cables, can change the cable pull ratio of the shifter.
112. Old pieces of a frayed or broken cable inside the shifter may interfere with the shifting mechanism.
113. Dirt and mud on the surface of the chain and gears may prevent the chain from settling all the way onto the gears causing the chain to skip or drop.
114. A worn chain may lose its ability to hold onto a gear and increased lateral flexibility will slow shifting.
115. A new chain on worn gear will not settle onto the teeth properly and may skip or drop.
116. A worn chain on new gear will not settle onto the teeth properly and may skip or drop.
117. A quick link in a chain has more lateral flexibility than other links of the chain. If this link happens to be the one to line up with a shift ramp or be the first to contact a tooth on the next size gear, the shift may be missed.
118. A stiff link in a chain may cause the chain to drop, skip or bind the rear derailleur.
119. If the derailleur is set up off by one gear, so that the second shift in the shifter moves the derailleur for the first time, the cable pull ratio in the shifter and derailleur will not match and the derailleur will shift as if the hanger were bent.
120. Some freehub bodies don’t use a wide tab to make sure the cassette cogs can only be installed in one position. If the gears are not lined up properly, the shifting ramps will not work.
121. Mixing cogs from different cassettes may result in shifting ramps not lining up right. It may be surprising how exact the specifications need to be. Not all cogs of the same size have the same shift ramps. They may differ depending on the gears they are designed to be next to. Furthermore, it may not be possible to chain the gear sizes in a cassette by simply swapping cogs from another ratio.
122. Installing a chain ring in the wrong orientation will cause the shift ramps to be in the wrong place.
123. Using chain rings that weren’t meant to go together can cause the shift ramps to be in the wrong position. For example, many road double cranks have A and B type large chain rings to work with different size small rings.
124. There are multiple lock ring sizes for cassettes. Using a 12 tooth cog lock ring on an 11 tooth cog will prevent the chain from settling onto the gear properly.
125. If an asymmetrical chain is installed backwards shifting will be much slower since the rear shifting features of the chain will be used during front shifting and vice versa.
126. If an incompatible chain is used shifting may suffer. The affect is often more noticeable with front shifting since the outside width of the chain is much more important there.
127. Using chain ring sizes outside of the specifications of the front derailleur can slow shifting since the wrong part of the derailleur will be lined up with the chain when it tries to shift. The tightest tolerances for chain ring sizes are on E-type derailleurs since they have limited range to be raised and lowered.
128. If a non-floating pulley is used in the guide pulley position the derailleur will have a hard time moving the chain from cog to cog.
129. If the cable is wrapped the wrong way around the cable fixing bolt at the derailleur the cable pull ratio will be affected.
130. If the pressure plate under the cable fixing bolt is oriented incorrectly the cable pull ratio can be affected.
131. The grease inside the shifter will eventually begin to thicken, especially if it is not used often. This can prevent the pawls from moving, preventing cable from being pulled or released. This can often easily be fixed by removing the old grease. However, since many shift mechanisms are very hard to access it may not be possible to simply spray degreaser on the affected pieces. In this case the shifter will need to be submerged in a gentle degreaser. In either case, the shifter still needs to be lubricated but it is not possible to re-lubricate it with grease. A liquid lubricant must be used instead. Shimano recommends a Molybdenum Disulfide lubricant. This lubricant will need to be periodically reapplied since it will not last as long as the original grease.

A final thing to keep in mind is the level to which something may be called “compatible.” Shimano often publishes an A and B level of compatibility for products where A level means that performance will be the same and B means that performance will be good but the appearance or quality of the product may not match what you are replacing. Often times riders will try something listed as incompatible and then tell anyone who will listen that even though Shimano says it won’t work it totally works fine. Those riders have different criteria for performance than Shimano engineers do though. They are also using a test sample size of one bike, so their results are generally not very scientific. An example is using a road rear derailleur on a 34 tooth mountain bike cassette. Some people may find that the derailleur works fine with that cassette. They may not be noticing that the upper pulley hits the bottom of the cogs in the larger gears though. They may also have a bike with a derailleur hanger that is horribly out of spec and happens to be wrong in just the right way to make this combination work. That does not mean everyone will be satisfied with the performance or that it will work with every bike in the world.

Similarly, other component manufacturers that list a product as compatible with another company’s shifting system or drive train may have different standards of tolerance. Just like different levels of components within a company’s line, aftermarket products have different performance levels. It is not necessarily safe to assume that just because another product is at the same price point and says it is compatible that it will work as good as the original product. A perfect example is with road crank sets. It may be called compatible because the chain rings are the right distance apart for a 10 speed chain. However, the current generation road 10 speed components from Shimano use wider front chain rings to make the chain move over further as they climb up the ring, decreasing the slope of the hill they have to climb. If another crank set uses a big chain ring that is not wider it will still work but the limit screws will have to be adjusted further in on the front derailleur which is designed to move the chain over further. This can cause the derailleur to hit the high limit screw before the shifter clicks into position. That means that the last bit of shifter throw is only stretching the cable instead of moving the shifter. This can make shifting back down to the small ring extremely difficult because the shifter is being operated under tension. Furthermore, the wide large chain rings on Shimano crank sets provide a performance benefit and make shifting easier. If an aftermarket chain ring doesn’t use the new wide shape it can never really be considered compatible.

In the end, this has been about the quest for perfect shifting, and that isn’t necessarily at the top of everyone’s list of priorities. If using a 34 tooth cassette on a road bike is more important than shift quality then go for it. If you use a different crank set because it’s lighter then you are making the decision that weight is more important than shift quality and that is a valid and respectable decision. One of the great things about working on and riding bikes is all the options that are out there. Tinkering with different combinations and finding acceptable ways to make them work is fun and makes riding more fun. Sometimes it even leads to changes in the direction of the industry so eventually you can indeed have your cake and eat it too.
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