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I have a Merida Duke 650 hardtail mountain bike.

The issue with my bike is the chain continuously keeps coming out of gear while I am going down hill on bumpy surfaces.

Is this happening because my rear derailleur is too soft?

Is there a quick fix or trick to avoid this from happening?

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    Are you pedaling at the time or is the chain just bouncing out of gear on its own? – Glenn Stevens Oct 6 '15 at 2:47
  • While down hill ride; no pedaling. – 10K35H 5H4KY4 Oct 6 '15 at 2:48
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    The chain drops off where? On the front or rear? To the left or right? – John Zwinck Oct 6 '15 at 2:57
  • At Front and mostly yo left, but some thing right also – 10K35H 5H4KY4 Oct 6 '15 at 3:02
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    Have you noticed any chainslap ( the chain hitting the chain stay) before this happens? My first thought is maybe your chain is slightly too long and the rear de derailleur cannot keep it under enough tension from falling off. – Glenn Stevens Oct 6 '15 at 3:07
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You need to measure your chain for stretch, it may be too stretched and no longer meshing with the gears properly. There are tools for this or you can do it with a ruler--one foot of ruler should exactly match the distance between pins on the chain; if the pins are not lining up with the marks on the ruler it may be time to replace. If you do replace the chain, cut the new one to length according to instructions rather than according to the old chain length, in case it was too long.

Check your high and low limit screws on your front derailleur--perhaps they are set too far apart.

Also try tightening the "B" screw of your rear derailleur to put a bit more tension on the chain. The "B" screw is underneath the derailleur hanger and can be screwed in from the rear of the bike.

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    This is interesting... Will all chains line up with a ruler, or do you just know this based on the brand of bike the OP has? – JPhi1618 Oct 6 '15 at 13:47
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    All standard bicycle chains have 1/2" pitch (the distance between the centres of two adjacent rollers). So, 24 half-links should always measure exactly one foot, and all feasible amounts of chain wear will leave this off by less than a whole link. IIRC 1/16th of an inch over one foot is the threshold for replacing a chain (and 1/8th is the threshold for replacing the chain and sprockets). – Useless Oct 6 '15 at 14:36
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There is no easy and free solution for this, since your bike is not meant to be going downhill.

The best thing you can do is go downhill using the largest front chainring and a pretty large rear cog (but not the largest). This will ensure a pretty good rear derailleur tension which will most probably prevent the chain fall.

If you are willing to spend money on this you could convert to 1x10 or 1x11 (after making sure that the gearing will be good for your rides) and use a narrow wide chainring and a derailleur with a clutch mechanism.

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  • "your bike is not meant to be going downhill." is this specific to this model of bike, or hardtails in general? If hardtails in general, how did people manage going downhill prior to rear shocks? – Adam Davis Oct 6 '15 at 12:13
  • @AdamDavis my comment was specific to all XC hardtails like the one that the OP has It's a very nice bike but it's not meant to go downhill. One can of course do downhill on it, but it'll be problematic (dangerous, unsafe, not fun). Regarding the chain problem, it'll throw the chain any time, because it's not designed to protect for that. Have a look at the drivetrain of a real downhill bike that will throw the chain only when huge rocks smash something really hard underneath the bike (i.e, it'll never throw the chain): santacruzbicycles.com/files/frame-thumbs/v10cc_profile.jpg – cherouvim Oct 6 '15 at 12:19
  • @AdamDavis: There are downhill specific hardtails (for aggressive riding) that apart from the different geometry, sport a downhill specific drivetrain, with a bashring, chain device, cluch derailleur and narrow wide chainring. e.g: ep1.pinkbike.org/p4pb10007781/p4pb10007781.jpg – cherouvim Oct 6 '15 at 12:21
  • @cherouvim - so XC courses have no downhill sections, only uphill? – Rider_X Oct 6 '15 at 13:04
  • @Rider_X: They do. OP mentions "while I am going down hill on bumpy surfaces" and this is where his setup is problematic. Modern XC race bikes are on 1X11 as far as I know. – cherouvim Oct 6 '15 at 13:27

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