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I have a mountain bike - a Stumpjumper hardtail frame with a Shimano drivetrain - where I'm getting a good amount of chainslap. Before I start fiddling with stuff, I'd like to understand the problem better.

This is a new-to-me bike that's only recently been built from a mix of old and new parts.

Drivetrain Wheel

(I've put up more pictures of the bike here, but these are the two that most clearly show the drivetrain.)

I believe I have two options here:

  • Increase the tension on the chain (somehow), or
  • Pull links out of the chain until all is well.

What are the advantages and disadvantages of both? Or are there other strategies I'm missing? Or is my time better served by simply wrapping the chainstay and living with the problem?

(While this isn't strictly relevant, I'd be remiss if I didn't point out that the front derailer also has issues - I can't get enough cable tension to get the chain to shift into the big ring up front.)

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Well first, obviously, make sure the chain's not too long. But I remember having a bad chain slap problem with my old Nishiki after the airline trashed the Suntour triple derailer and I was using a Shimano long frame. Eventually went to a Huret Duopar and the problem (and others) went away. But those options are no longer available. –  Daniel R Hicks Sep 19 '11 at 15:42
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4 Answers

up vote 1 down vote accepted

Chain slap is a result of slack in the relatively longer length of the chain required to accommodate the wide range of gears on the typical mountain bike cassette. You essentially have two options:

Treat the Symptom

Wrap the chainstay with an old inner tube or use a neoprene chainstay guard.

  • Pro This will prevent damage to the chainstay and will quiet the obnoxious noise.
  • Con The chain is still slapping around.

Buy and install a chain guide.

  • Pro These are supposed to help with both chain slap and dropped chains.

  • Con Expense; I've heard that there are frequently compatibility/installation issues. (I've never bothered to use one)

Treat the Problem

First, your suggestions of increasing the tension (by replacing/tightening the derailleur) or removing a link or two from the chain may work, but you need to be careful that you don't shorten the chain too much.

  • Pro Cheep. May improve shifting performance as well.
  • Con Easy to screw up and end up destroying the derailleur. May not help much.

Many mountain bikers don't need the full wide-range of the typical mountain cassette. If that is you, your best bet to reduce chain slap is to invest in a tighter range cassette and a short or medium cage derailleur. This will take much of the slack out of the system.

  • Pro Best bet for reducing the chain slap. May result in more efficient gearing and shifts.
  • Con Expensive if you don't have the parts laying around.

My General Recommendation

Wrap the chainstay first, check the length of the chain and derailleur tension second and then if you can go tight-range/short-cage.

My Specific Recommendation for your bike

You picture shows a long cage derailleur, and a 'mega-range' cassette. In your specific case you could start with swapping out that cassette with the huge jump to the granny gear with a more rationally stepped one and just shortening the chain

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Let me introduce you to the B-Screw...

Look at how that derailleur is hanging. The parallelogram is angled skywards rather than downwards. This is because the B-Screw at the back of the derailleur is not screwed in enough. This screw contacts the derailleur hanger's little 'spur' and you may want to twist the derailleur body clockwise to make it easier to do up the screw. Tighten it all the way, then go into the big cog and back it out so that the top pulley is close but not touching the sprocket. This will put the B-Screw in its correct position, putting a sensible amount of tension on the chain.

As for your front derailleur not making it onto the big cog, the problem may be with the bottom bracket. These come in different lengths and if the cranks or b/b have been repaired/upgraded over the last decade, due concern for the chainline may not have been given. Take out the one that is in there and put in a cartridge b/b of the same shell size with a shorter length. The next size down may be all you need but, without seeing the gap between the cranks and the chain stays I can only guess. Obviously you want the cranks to be an equal distance from the chain stays on each side with enough room in there for the triple chainset. the derailleur (front) will probably need adjusting by then, this has the outer plate parallel with the chainring and 1mm above the outer chainring teeth.

To make the bike look really good and eliminate chain slap problems in style, consider going on eBay to get a genuine Shimano 'Shark's Fin': enter image description here

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Will try the B-screw. The front derailer issue is almost certainly simply a matter of cable stretch, haven't gotten around to tightening the cable yet. (This front derailer makes it a true PITA to adjust, would have gotten a different one had I realized.) –  Neil Fein Sep 20 '11 at 3:41
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I've made some of these adjustments, but haven't had time to fine-tune and road test; will report back on how it works. –  Neil Fein Sep 25 '11 at 17:19
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With a racing bike I had many years ago I just increased the tension of the derailleur spring. This kept the chain length the same (so I didn't need to worry about any geometry changes making it harder to change gear) but increased the chain tension.

Not quite the same as your MTB, as I obviously wasn't jumping off things so chain slap was not anywhere near as much of a problem, but you could look at that as a solution.

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If you are feeling the chainslap only while careening down a steep bumpy downhill ( not doing alot of cranking) try using the middle or big chainring,this raises the derailleur and makes the chain less likely to slap as it is farther from the chainstay.Of course this is not a band-aid for driveline misadjustment.

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