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If I remove the (rattling)plastic disc -

  1. How often should I check the limit screws on rear derailleur?
  2. If the shifter cable tension is less than optimal, would this be a possible cause to drive the chain in between the spokes and the rear cog-set?
  3. Excluding the case of debris getting stuck in drive-train; are there any other adjustments besides above two and true-ness of rear wheel that may cause the problem of getting chain between cogs and spokes?

Derailleur in question is a Shimano Deore.

Update: Details- For mileage, I commute about 110 miles/week.

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With a good quality derailer, in good condition, you should never have a problem once the limit screws are set. But any time you're having shifting problems you should check it. And, of course, any time some force has been applied to the rear end (eg, bad fall) you should give it a cursory check. Or just blob some construction adhesive onto the shield so it doesn't rattle. –  Daniel R Hicks Aug 20 '13 at 22:16
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4 Answers

up vote 2 down vote accepted

It's really up to you how often to check your adjustments, but I'd say it's not an "every ride" kind of check, unless you're giving your drivetrain a workout! If there's any damage to the derailleur, hanger, or rear wheel, you may want to start there as well.

To specifically address your questions:

  1. You should check your limits any time you have a drivetrain issue. That could be poor shifting, dropped chain, or inability to shift.
  2. Shifter cable tension should only affect how easily/quickly the derailleur moves. However, there could be a case where too much tension could cause the derailleur to move through the last shift enough to stress past the limit screw, but I think this is unlikely.
  3. The rear wheel could be out of dish (centered from side-to-side) which might have moved the cassette further in. Readjusting the limit screws should take care of that, but you might still have shifting issues. (I had the same problem this spring)
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Aaron mentions the hanger, but I would add it as a bit that should be looked at anytime you have drive train issues. A bit hanger can through everything out of whack and you often don't realize you did something to bend it. Other than dirt, this is the first thing I suspect when there is RD issues. Downside, you need a special tool to really line it back up...for this, visit your LBS and they should be able to check for cheap. –  Ken Hiatt Aug 20 '13 at 21:59
    
If the cable outer is not properly seated in the cable guide it can significantly increase tension so worth checking, but I don't see this pulling the derailleur past the upper limit. –  DWGKNZ Aug 20 '13 at 22:08
    
Also just to add to Kens comment a hanger can be bent by something as innocuous as laying the bike down gently on the drivetrain side. It may not show any damage and be bent. –  DWGKNZ Aug 20 '13 at 22:29
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  1. I don't believe these would move without external force but worth checking at every service.

  2. No, the upper limit screw should prevent this from happening. Cable tension can be adjusted (most shifters and some RD have barrel adjusters) to optimize shifting between gears. Outer limits are fixed.

  3. 3 I can think of:

    • derailleur alignment, is the hanger bent
    • is the wheel in the dropout properly
    • is the derailleur cage bent
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I wouldn't bother checking until you see a problem, like difficulty shifting onto the biggest or smallest ring/sprocket, or the opposite: shifting off it. When this happens, it is not a "show stopper" problem. You can probably complete your ride, or even just pull over and make the adjustment right there.

There is usually a warning that the derailleur is going out of calibration. Before it becomes very difficult to shift to a gear, it first becomes slightly difficult and it progresses from there.

Regarding point 3, perhaps you're missing the plastic guard ring which goes between the casette/freehub and the spokes. Which is not to say that this is what prevents slippage, but it does prevent contact between chain and spoke.

By the way, one potential problem affecting the front derailleur, or at least some types, is that the entire assembly can rotate around the frame post to which it is clamped.

The front and rear derailleurs interact. Notice how it is easier to shift onto the leftmost sprocket when you're on the leftmost ring. So it cannot be ruled out that some rear difficulty is actually connected to a front problem.

One last piece of advice, and that is to install a rear derailleur protector, if you don't already have one. This part which installs on your wheel axle, held in place by the wheel lug nut. It extends a curved steel bar over the derailleur which guards it from being knocked around.

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I'd say every week twice. Monday and Thursday are good.

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Welcome to Bicycles.SE! We're looking for answers with more detail. Please give us some reasons and explanation, not just a one-line answer. A short answer like this with no explanation is likely to be deleted. –  freiheit Aug 22 '13 at 16:19
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Building on what @freiheit has said, you might like to have a look at our help page regarding how to write a good answer. bicycles.stackexchange.com/help/how-to-answer –  jimirings Aug 22 '13 at 21:14
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Trust me, I'm an bike engineer. My answer is perfect as it is. –  Steve Aug 23 '13 at 6:55
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@Steve, it would be helpful to understand the reason for your recommendation. –  Akshay Aug 23 '13 at 14:10
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