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I have often seen derailleurs on cheap MTBs to degrade in performance by showing hysteresis. By that I mean that to switch to a certain cog, one needs to shift one cog further and then immediately one back to get the chain to the desired cog.

What could be the cause for such degradation of shifting quality. Is this often a tuning problem, or wear, or the derailleur spring getting stuck with dirt and grime?

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3 Answers 3

up vote 4 down vote accepted

Here is the list of things I usually check for when troubleshooting jumping gears:

  1. Is the chain always goes up/down? In this case usually the adjustment is off. Try adjust the cable.
  2. Is the mech hanger bent? Sometimes during a crash you can bend a mech hanger. Most of the times you can bend it back, but there are cases where you need to replace the mech hanger for a new one. Usually they are cheap £5-15.
  3. Is the shifting very slow? Dirty outer cables and rusty inner can cause the problem with shifting. Replace inner cable and outer cable for new ones.
  4. Is the shifter clicking OK? I've seen cheap shifter wearing out and not "holding" clicks properly, so the chain would jump.
  5. Are the mech cogs (small chainrings) worn out? are they wobbly? Chain can only be placed properly if the cog is not moving sideways. If that is the case, replace the top cog. Also worn out cog can affect the shifting performance.
  6. Is the mech itself not bent or otherwise damaged? There are plenty of ways to destroy the mech.. usually that is beyond repair and you need to get a new one.
  7. If the mech is very-very cheap, it sometimes gets bent while in normal use (not MTB abuse). Only replacement will fix that.

Good luck! And I'm off to fix my wife's bike gears -)

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Keep in mind that this was the norm for derailers, pre-indexing. Indexing wasn't possible with the old style drivetrains because you needed to overshift slightly to force the chain to jump sprockets, especially when shifting to a larger sprocket.

What changed was mainly the addition of what I call "ramps" on the sides of the sprockets. These catch the pins of the chain and lift it up onto the higher sprocket. Without this feature indexed shifting would be impossible.

Thus, wear of either the chain pins or the "ramps" will cause poor shifting. Of course, there are also the other possible causes that Benedikt describes as well.

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In most cases it will be a combination of several of the factors you describe.

It can be a tuning problem. In this case normally the gear change will work in one direction but not in the other one. If it is only a tuning problem, it may be fixed quite easily by tuning your derailleur correctly. Also for this type of problem it is characteristic that it can occur in both shifting directions depending on the "direction" of detuning.

Unfortunately especially on cheaper derailleurs tuning may not be the only problem. The second one is friction either in the bearings* of the derailleur or of the derailleur cable in the cable hosing or both. In this case the problem is typically much worse in the direction where the the cable is released by the shifter and the spring does the work. Normally the derailleur should jump to its desired position with one sudden step, but if friction comes into play it may creep more or less slowly to its position which might give the chain not enough momentum to jump to the right cog. As cheaper derailleurs are built with simpler bearings they are more prone to such failure. A maybe slightly too weak spring may worsen the situation here.

Another point that you didn't mention is the wear of the derailleur bearings. The whole device consists of several linked parts that are aligned to each other to certain axes. If the bearings in the links are worn out in a way that there's some clearance, the derailleur may bend a bit, changing the alignment of the derailleur in respect of the frame and the cassette. In this case the chain may run from the derailleur to the cogs in an angle that hampers the shifting process.

*As pointed out in the comments there are no real bearings in the links between the parts. Instead the links consist basically of two surfaces that glide on each other. But there is a huge difference in the quality and durability of those surfaces and in how good they can withstand contamination that causes friction and wear.

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I've always found things to much much improved by a very good clean of the derailleur mech (probably strip down) and cable runs, followed by plenty of grease when dry. I would assume MTBs should be more dirt tolerant than my hybrid, but maybe not. Of course it could be cumulative bashing about with MTBs, either spoiling the adjustment or actually bending something. –  Chris H Jul 29 '13 at 14:27
    
What do you refer to as "bearings"? In my cycling career I've seen only experimental rear mechs with bearings. The rest of them were just connected together, so they can move. But, of course, there are bearings in cog wheels. –  trailmax Jul 29 '13 at 16:27
    
@trailmax Basically you're right, there are no real bearings such as ball bearings or the like, it's more or less two surfaces gliding on each other. But the performance depends a lot on the quality of the parts. On very cheap derailleurs the links may be made of plastic which wears out easily when contaminated with dust while higher quality ones are surely made of metal, maybe with some additional teflon barrel in between to reduce friction. –  Benedikt Bauer Jul 29 '13 at 16:35

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