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Old 11-01-09, 03:02 PM   #1
AL NZ
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jumping chain on my boy's BMX - Help!

Hi all,

any advice would be appreciated
I am fairly able when it comes to bike maintenance, but struggling with my boy's bike

It is a 20" mini-mountain bile with 7 speed rear cluster (S-ram) and single 36T chain ring. It has only done about 200km, and now the chain jumps off all the time in the highest 2 gears (ie: 2 smallest rear sprockets). And I mean, All The Time. eg: 5 metres down the road

the bike has done a lot of off-road miles, so I have cleaned the chain, but it still does it. I took a link out as well, but that aint helping (on reflection, the chain jumps off even under tension when pedalling, so no point in removing links)

It has a cheap plastic outer ring, not really a proper bump ring, but this doesn't serve to guide the chain, as the gap is wide enough for the chain to fall between chainring and the plastic outer ring.

The chainline on a 20" is probably too angled in smallest sprocket, as with the smaller wheel, the crank and axle are much closer to each other, magnifying the chain angle.

But I am puzzled as to why this has started - it wasn't a problem for the first 2 months of ownership.
I've measured the chain - it isn't worn/"stretched".

I hoped cleaning it would allow it to mesh more cleanly with the chainring, but hasn't helped so far

My options seem to be

1. take off plastic guide and crudely sand it back to make it fit more closely (not my idea of a good solution)

2. buy a chain guide, but these cost! The most basic one I can see on the net which should work is
DMR Chain Cage Chain Guide, at 24 UK pounds, still quite an expensive fix

3. Have it out with the chain store (no pun intended) that I bought it from . However this seems likely to be an uphill battle. (from Halfords, UK)

Any bright ideas?

thanks...
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Old 11-01-09, 03:05 PM   #2
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It's a BMX bike? suck up the loss, try and tighten the wheel back as far as possible and if it is clearly a chain issue goto any walmart, or bike shot and buy a chain locally. LBS will install it for free usually if you buy it there.
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Old 11-01-09, 03:06 PM   #3
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Also, Check to make sure rear sprocket on tight and hub to wheel is tight. They could be shifting just slightly enough.
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Old 11-01-09, 03:26 PM   #4
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Good catch on the multi-speed. I didn't even notice that. Most likely just derailleur adjust, tension adjust as OSB says.
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Old 11-01-09, 03:27 PM   #5
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thanks, guys

the derailleur is not bent. It is not a problem with tension, as it is coming off the top, while pedalling (ie: the chain jumps off as it engages the front chainwheel, even when pedalling hard)

Do others think a new chain might fix it?

It was a semi-budget bike, 140 pounds UK sterling, but nicely spec'ed for a kids' bike. It may therefore have a cheap chain.

the rear wheel cannot be adjusted backwards - it slots into non-sliding lugs, and that fixes it's position
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Old 11-01-09, 03:29 PM   #6
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Check that the chais not too long as well - that will decrease overall tension in the smaller rear cog combos.
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Old 11-01-09, 08:02 PM   #7
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Originally Posted by OSB View Post
Its jumping off the lowest gear in the rear? Or in the front? If its the rear then the limit screw isn't set right.
Yes, I'm not clear where the problem is either.
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Old 11-02-09, 12:53 AM   #8
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the chain comes off at the front, at the top of the chainwheel, where it engages the chainwheel at at angle (when in highest gear, ie: smallest rear sprocket)

I have tried multiple settings of the rear derailleur set screw, from slack to full adjustment, with no difference.
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Old 11-07-09, 07:40 AM   #9
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after a thorough clean of the chain it was still jumping off at the top as it meshes with the top of the front chainwheel.
I have now discovered one of the chainlink pins was protruding on the outside of a link, so not connecting the link on the inner side, and allowing that link to bend laterally so it can easily not engage the tooth of the chainwheel. It is probably the one where I removed a link, but I put that back to the proper depth, so it must have worked loose. Also, it doesn't explain why it first was jumping off prior to me removing the link. It may have been like this all the time, and not actually be the link I removed.

So now the chain is not jumping when I test it by pedalling the bike off the ground.
The real Test will be when my son rides it.
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Old 11-09-09, 01:14 PM   #10
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Replace the chain. Protruding pin is not a good sign.
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Old 11-10-09, 02:01 PM   #11
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now fine, but will replace it soon
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