Riders frequently complain to me that they "push the shift lever and nothing happens." While there are multiple potential causes for this symptom, the inherent design of index-only shifters leads less experienced riders to suspect a malfunction.
Index shifters were designed by people who know very well how derailleurs work, to make shifting easy for people who don't know how derailleurs work. The plan went wrong because a rider benefits a great deal from knowing what the shifter is supposed to accomplish, and now whole generations of riders have never experienced friction shifting or lever-type shifting of any kind. They just learn how to push things to make the bike pedal easier or harder. It's magic. Don't ask. When it doesn't work, they ask a friend, or look at a YouTube video, or, as a total last resort, go to an actual bike shop to speak to someone they hope will know. Some of us do. Most of the time, these efforts lead eventually to a working bike, but probably not to an understanding of what it was trying to do in the first place.
Once cable-operated derailleurs were common, and the general configuration was basically standardized, there followed a couple of decades in which the derailleur-gear rider would haul back or push forward on a lever held in place by a set screw, to make the derailleur -- front or rear -- move laterally to place the chain in line with the desired gear combination. Even the most casual rider would figure out eventually that the lever pulled the cable and the cable pulled the doohickey that made the chain move. They might ride with the chain rubbing on the front derailleur cage until it wore through, but they could at least shift well enough to get around. If it didn't shift, pull farther and harder. There was an obvious correlation between lever movement and derailleur movement.
Given that not everyone has a good innate sense of spatial relations, companies did try to simplify the biomechanics. An example would be Suntour's notorious backwards-acting front derailleurs of the 1970s. Presaging Shimano's horrible Rapid Rise rear derailleurs, Suntour applied the principle to front derailleurs, so that the rider would push or pull the shift levers in the same direction, rather than opposite directions. The resulting devices not only shifted like crap, they engendered even more confusion in a world where the majority of derailleurs did not operate that way. Suntour also had a set of downtube shifters that bolted to a special mount. It slid up or down as the rear shifter was moved, to automatically trim the front derailleur. I missed Suntour when they went under in the 1990s, but remembering stuff like this makes me wonder why I did. I focused on the nice things they made. The Japanese component companies were always masters of the mixed blessing.
With any return-to-center shifting system, whether it's under-bar or brifter or barcon, the shifting movement has to include the degree of overshift any derailleur system needs in order to move the chain to the next cog or chainring. The derailleur has to travel a little "too far" and then settle back, to displace the chain from the gear it's on and climb or drop to the new selection. This is particularly true when moving the chain from a smaller diameter cog or ring to a larger diameter cog or ring. You have to push the chain up and make sure it gets a grip. Dropping the chain to a smaller diameter takes little or no overshift, because gravity is helping you. If you're unfortunate enough to have Shimano's Rapid Rise, then very little helps you, either way. Get rid of it as soon as you can, and avoid it like norovirus in the future.
When everything in a drive train is new, the system usually functions unobtrusively if it was properly set up and adjusted. But the more cogs you cram into the cassette, the more sensitive the shifting systems become. It all depends on the balance between cable tension and the power of the derailleur return springs. When things get dirty and worn, that balance gets harder to maintain. Buy a new bike! (A public service announcement from The Bike Industry). This is when you get the low-flow toilet syndrome.
You've probably become familiar with low flow toilets, given their proliferation. In the olden days, when toilets sent enough water down to flush a dead guinea pig with one pull, you could push the lever and walk away, confident in most cases that no leering zombie of a turd would be lying in wait when you opened the lid again. And no bloated, floating carcass of a guinea pig, for that matter. With a low flow toilet, you have to push and hold the lever until all the flushing noises have completed their cycle if you want to be somewhat sure that everything has left the bowl. And forget entirely about guinea pigs, or even smaller rodents.
With index-only systems, riders accustomed to other push-button devices in their lives just push to the click and expect the device to do the rest. This isn’t even entirely true of electronic shifting systems, let alone cable-operated systems.
For a cable system to operate with click-and-forget precision on the trip from smaller diameter to larger diameter, cable tension has to be high. If it’s too high, the derailleurs won’t want to move far enough the other way. Some systems currently in use already require ridiculously high tension. At their showroom best they might shift with something close to click-and-forget precision. Gradually, that will erode. A rider usually evolves with it, learning to push the lever fully past the click, and stay on it a little longer and a little longer. Eventually, the lag becomes too obvious to ignore.
Actual malfunctions will also show up as slow shifting in their early stages. These malfunctions include cables fraying inside the brifter and fraying inside the frame, if you have internal cable routing. Good luck finding a high end bike these days that doesn’t have internal cable routing. Of course you want it! Housing failure will also cause sluggish shifts. If the linear wires are failing at the end that plugs into the brifter, they can also burrow into the brains of the shift mechanism and cause damage.
Fully enclosed cables, with a continuous piece of housing from the shifter to the derailleur, will require a whole new length of housing just to cure a bit of fraying at the end, unless the housing was cut a bit long to begin with. Using a junction ferrule, you can sometimes add in a section, but every joint is a potential source of slop that can cause inaccurate shifting. You have to have room to make the connection and still get a smooth line for the cable to flow more or less unimpeded. Of course the currently fashionable and largely mandatory 4mm housing is a built-in impediment, but that’s just more value added by the bike industry.
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