I recently bought a new Mountain Bike which works pretty well, aside from the fact that I have a problem with it which creates a pretty damn large safety hazard, not to mention creates a massive headache for me.

I'll be in the 7th gear, and when I start to stand up/put a lot of pressure on the pedals - I'm not really sure how to explain this - it feels like my chain slacks for a moment, and my pedals kind of jerk and then everything goes back to normal for a moment, before going back to slacking again.

This ONLY happens in the 7th gear. It used to happen sometimes in the 6th gear, but I messed around with my H/L screws and barrel adjuster and now it only happens in the 7th gear. I'm not sure what is causing the problem.

Does anyone have some advice for me? I've tried adjusting my barrel adjuster over and over again and I can't seem to narrow it down to being that.

Could it be a problem with the H/L screws?

I REALLY need to figure it out, because right now I pretty much can't use my 7th gear because I'm afraid one of these bad "jerks" will cause me to crash, as I've come close to doing it before.

  • 3
    Take it to back to the shop. Should like the free-hub / freewheel is slipping. – paparazzo Feb 4 '16 at 19:52
  • Messing with your H/L screws will never fix any shifting problem that isn't with your first or last gear. Those screws are basically stops that keep the derailleur from throwing the chain higher or lower than your first or last gear. They in no way affect the indexing. Generally, they should be set by whomever builds the bike and never touched again. – Deleted User Feb 5 '16 at 7:37
  • Unless you change the freewheel/cassette to have fewer cogs then you would need to reset the H/L to match it or if you want to lock out some of your smaller cogs maybe to simulate a 3x5 for some reason. – David Feb 6 '16 at 23:11

Does the bike have multiple chainrings? Make sure you aren't cross-shifting- that is, shifting into a gear that is too high/low for the chainring that the chain is sitting on. This can cause a problem like you've described- chain rubbing on front derailleur and constantly attempting to shift to put the chain into a more linear position.

  • This would be better as a comment, since you're really asking for clarification, but you don';t have enough rep for that. – Móż Feb 5 '16 at 0:37
  • That's what I figured, but thought it may be useful information anyway. Please remove if inappropriate. – RobinHorner Feb 5 '16 at 0:40

It sounds like what is suggested by Robin -- when the chain is on the extreme cog on the rear it "kisses" the adjacent chainring on the front and begins to shift rings. But there is not enough chain pressure to make the shift complete so the chain slips back to the ring you were on. Sometimes this is unavoidable for certain front/rear combos, but sometimes tweaking the derailer adjustments can fix it (though you might introduce some derailer noise for the errant combo).

You can also have symptoms similar to this if either the chainring you're using or the rear cog you're using is worn and "hooked". The chain will briefly snag and then pop loose. But a new bike would not have worn rings/cogs.

And as Frisbee suggests, it's vaguely possible that the freehub is slipping internally, though one would not expect that to be limited to a single cog.


If you have multiple chainrings, try it in the largest chainring, smallest cog combination. I am assuming the 7th gear is your smallest cog since you didn't mention otherwise. In the tallest (highest) gear, the rear wheel torque is minimal but possibly of the longest duration and the load is highest. One thing you could test to see if it is the freewheel/freehub slipping is use a lower geared chainring/cog combination so that the chain is straight (such as middle chainring in a triple and 4th cog in a set of 7), stand on the pedals and use as much force as you would in 7th gear but use the brakes to maintain a load so you don't build much roadspeed. If it still slips then it is likely not a crosschaining issue but rather slippage. I dont really see how the freewheel/freehub can slip unless it is defective. I am not a bike mechanic.

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