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Hey guys I have a specialized mtb with the old oval chainrings and lately it's been locking up. When I start pedalling "hard" or faster the crank stops turning and then jumps forward a bit and generally is not very pleasant. Please note the wheels keep turning fine. Has anyone had this problem before? It's happened to me on another, thankfully cheaper, bike that was stolen a year before I built my fixie (which gives me no such problems) a couple years ago.

To recap, the pedals stop turning and then jump forward, stalling in a way, when I start to pedal harder, like up a hill or just on flat but faster. How do I fix this?

Thanks for any help! I really love this bike!

  • What gear combinations does this happen in? – BEVR1337 Nov 21 '15 at 0:20
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    Is there any slop in the cranks? When the "locking up" happens is the chain or chainring hitting the front derailleur? To escape the "locking up" do you have to pedal backwards, stop pedalling, or just push harder? – Móż Nov 21 '15 at 1:35
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    I would primarily suspect that the chain is getting jammed. This could be due to "cross chaining", improper adjustment, worn chain, worn cogs, too much chain slack, or something awry with the rear derailer. – Daniel R Hicks Nov 21 '15 at 8:40
  • Stolen bike? Fixie? What bike is having the problem and what is the gearing front and rear? – paparazzo Nov 24 '15 at 21:06
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This is most likely an issue with your bottom bracket.

To diagnose, remove your chain from the chainwheel so you can spin the cranks without the rest of the drivetrain being involved.

The easiest way to do this is to use your left hand to pull the rear derailleur's cage towards the front of the bike, creating slack in the chain. Then with your right hand pull the chain off away from the bike. You don't want to drop it in between the crank and frame as it's likely to get jammed.

If the cranks spin freely, with no "bad spots" and without creating a "grainy" sound, then your issue isn't with your bottom bracket and is likely a damaged/worn chain or rear derailleur cogs.

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If you're talking about an old specialized bike with biopace chainrings it may well be that the chain set is worn. Take a look at the chainrings and cassette and see how worn they are. Measure the chain and see if its stretched. I'd imagine you're going to find that the whole lot is just worn out and needs replacing. Whether you can find spares on ebay or a local store I can't say.

Generally, given that biopace chain sets stopped being made in 93 you (or you plus previous owners) have had over 20 years of riding out of it at least so it's pretty good value.

  • This is my guess too. And it could be jumping on cassette. Any worn component should be replaced. And if you replace anything then also replace the chain. With biopace you may need to replace them all to kind of keep things in synch. – paparazzo Nov 24 '15 at 21:11
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Not sure why the answer from @sully1900 was downvoted, it was a perfectly good answer on trying to diagnosing the problem.

You are running a fixie, so in addition to the other answers you were given, you may have issues in your rear hub. I had a similar issue to you in an older bike where my pedaling would inexplicably miss and shoot forward, however, I didn't have any "pause" as you describe. I solved this issue by replacing the springs and pawls inside the freehub. It is worth checking if you have the proper tools to take the freehub body apart.

See this article: http://dirtmountainbike.com/features/work-freehub-body.html#R91xq0wd1GMTHzRx.97

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it is necessary to to slow down the rate at which you pedal (I suppose we must create a differential, in terms of pedaling) in order to get the derailleur to shift. The differential must increase (in velocity--> accelerate) in order to downshift; it must slow-down to shift up.

if you've ever been cruising on a flat and up-shift; you may notice a delay between your shift and the ACTUAL shift...because you're pedaling, so what is the hold-up, right? most gears not shift gears until there is at least some change in velocity; nee the relative power we deliver to the pedals. same with hitting a steep driveway unexpectedly -- it's hard to downshift fast enough to deliver enough power (angular accel) to the pedals and thus downshift. usually I run out of velocity and have to jump off the bike if im blindsided by an unexpected steep-spot. sadly, I need to be going at least 1km/hr in order to maintain balance without a 3rd point of contact.

it may seem to you like it is stalling, when really the system is waiting on the differential described.

as reference - my system is an 18-speed - 2 up front, 6 in the back hybrid. the rear derailuer is brand new (and had to be custom made by shimano) -- I ate it hard and snapped the original.

  • You mean "the rider needs to ease off the chain pressure, else the tension of the chain overrides the deraillerur spring and stops the chain from moving up/down" If this was the problem, OP would have said something about problems changing gear. I doubt this is the root cause. – Criggie Nov 25 '15 at 2:37

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