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Chain Jam

Old 05-15-20, 01:29 PM
  #1  
Pratt
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Chain Jam

No, not a new spread for toast. Shifting onto the biggest cog, while on the small chain ring caused a jam, the other day. Since I was going up hill at a really low rate of speed, The bike immediately stopped and I fell over. The second time, I was ready and did not fall over. The chain looked like it was riding up on the upper jockey wheel on the derailleur.
Chain rings are 46-36-24, rear cog is 30. Suntour cogs and derailleurs on a Bruce Gordon.
Any ideas?
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Old 05-15-20, 02:12 PM
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Could the derailleur or hanger be bent? Does alignment look good?
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Old 05-15-20, 02:26 PM
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I had that happen to me on my MTB after swapping out the BB for a sealed unit and having to modify (shim) the front DR to reach the outer ring. It came loose just enough to move (pivot) and throw off the alignment with the chain. After having the chain jam in the rear, I looked it over and sure enough the FDR was guiding the chain at an off angle to the rear cassette and binding the chain back there. I realigned the FDR and all is good with the world again. If I hadn't recently shimmed out the FDR for the reach, I never would have suspected it. Just another thought. Good luck,
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Old 05-15-20, 03:53 PM
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Suspect the hanger is OK, no other shifts feel bad. Maybe the FDR has an issue, I'll look.
Thanks, to you both
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Old 05-15-20, 08:47 PM
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I doubt that posting this, that the bottom of this problem will be solved. As a bicycle store owner for over 12 years and a mechanic for many more, I believe that if you are not able to figure out why the chain is jamming I can not imagine how someone on this forum will know (especially without even seeing it). I would though, recommend that you take it to your LBS and give them some hands on with the bike. (something that will never happen on this forum). hope you figure it out. Good Luck.
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Old 05-15-20, 11:09 PM
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how old be thy chain?

check that the links are all rotating smoothly. rust inside the bushings might be causing one link to bind.
won't notice on the big chain ring, but going around the pulleys, one link will stick and not rotate freely.

https://tse2-mm.cn.bing.net/th/id/OI...4?pid=Api&rs=1

Last edited by saddlesores; 05-15-20 at 11:13 PM.
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Old 05-16-20, 01:43 PM
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Thanks again. LBS is about 75 miles. Chain has <50 miles on it.
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Old 05-16-20, 02:20 PM
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Alignment issue (bent RD or otherwise)
Idler pulley/cog issue - bearings (if it has them, roller, ceramic?) goofed up? Bushing issue (if bushing instead of bearings)?
Debris stuck in the RD causing an issue with the above 2?

If you are having trouble seeing the chain alignment shifting put the bike on a stand and hand crank it slowly and watch the motion of the RD and chain alignment. Check the RD spring tension also. Too lose and the chain could be allowed to bounce too much.

Check the link count on the chain and see if it is identical to what you were using before. Chain length will affect the RD as it is also taking up the slack of the chain. Different length chains will give you different angles to the pulleys/cogs in the RD and could affect the shifting somehow.

Good luck.
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Old 05-16-20, 05:05 PM
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Originally Posted by KC8QVO View Post
Alignment issue (bent RD or otherwise)
...
If you are having trouble seeing the chain alignment shifting put the bike on a stand and hand crank it slowly and watch the motion of the RD and chain alignment. Check the RD spring tension also. Too lose and the chain could be allowed to bounce too much.

Check the link count on the chain and see if it is identical to what you were using before. Chain length will affect the RD as it is also taking up the slack of the chain. Different length chains will give you different angles to the pulleys/cogs in the RD and could affect the shifting somehow.
....
Yeah, do this. Assuming you do not have a bike stand, you can put the bike upside down on the ground to do that.

Or, hang it from a rafter in a garage from a rope.

If the chain is that new, maybe it is a couple links too short and you only found out about that now.
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Old 05-16-20, 07:49 PM
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Where did the chain jam?

If it was between the large sprocket and the frame it could be the rear derailer limiter screw needs to be adjusted inward.
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Old 05-16-20, 08:22 PM
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"Chain Jam" -- isn't that the name of some rock group?
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Old 05-17-20, 06:31 PM
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No, the rock group was what I landed on, on the dirt road.
Chain length looks ok, not stretched out or all doubled up on either the small ring/big cog, or middle ring/ big cog.
Jam was between RD and largest cog on the cluster.
I put the bike on the rack in the garage and tried various shifting combinations and they all seemed to work fine
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Old 05-17-20, 09:12 PM
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Originally Posted by Pratt View Post
Jam was between RD and largest cog on the cluster.
LOL. Cluster. What a cluster fluck.

Its a "cassette".

In any event, good test on the cranking. When you say it jammed between the RD and the largest cog on the cassette - you mean on the wheel side of the cog? Or the next smallest cog on the cassette? And when you un-jammed it - was the chain on the idler pulley/cog nearest the cassette? Or was it off of that idler as part of the jam?

Someone already pointed out the adjustable stop-screws on the RD mechanism. If the RD throws the chain too far "in" (towards the wheel/tire) it is possible the chain could ride up and over the teeth. That doesn't sound right to me, though, if everything was adjusted before and the only difference was the new chain.

I suspect there is something that is bent, or otherwise misaligned. I had that happen to me with my folding bike. I swapped the tires F/R earlier this season to balance the thread wear (will need to do that again soon - the small tires wear faster) and I managed to put some odd pressure on the RD maneuvering the wheel on and off. I had to reset the RD adjustment etc to get it dialed back in. Not much of a problem, but it was avoidable work if I was more careful with the taking out/installing of the wheel...
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Old 05-18-20, 05:23 AM
  #14  
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Originally Posted by KC8QVO View Post
LOL. Cluster. What a cluster fluck.

Its a "cassette".
......
There is some historical precedence for the word cluster. Decades ago in the era of freewheels, the word cluster was commonly used interchangeably with the word freewheel.
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Old 05-18-20, 05:36 AM
  #15  
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Originally Posted by Tourist in MSN View Post
There is some historical precedence for the word cluster. Decades ago in the era of freewheels, the word cluster was commonly used interchangeably with the word freewheel.
Yep, back in the day that's what we called them. And the chamois in your shorts was made from a deer...
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Old 05-18-20, 09:31 AM
  #16  
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Originally Posted by Pratt View Post
No, not a new spread for toast. Shifting onto the biggest cog, while on the small chain ring caused a jam, the other day. Since I was going up hill at a really low rate of speed, The bike immediately stopped and I fell over. The second time, I was ready and did not fall over. The chain looked like it was riding up on the upper jockey wheel on the derailleur.
Chain rings are 46-36-24, rear cog is 30. Suntour cogs and derailleurs on a Bruce Gordon.
Any ideas?
Where did the chain jam? Did it jam at the chainstay or elsewhere? Id suspect classic chain suck caused by a burr on a chainring.
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Old 05-18-20, 06:18 PM
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Static testing did not reveal any jamming, but the weather for tomorrow looks to be nice so I'll try to recreate the problem; on a nice level parking lot.
Wish me luck
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