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Cross Chain Issue with Chain Jamming in Tension Pulley Cage
#1
I am building up my bike that is quite a bit customized as far as components. Right now I am having issues where the chain is running off the rear derailleur tension pulley and jamming between the pulley wheel and cage when ever it is cross chained in the small cog/small chainring position. The cross chaining is definitely pulling the chain off the pulley wheel. It doesn't happen right away and takes a few turns of the crank before this occurs.

I don't plan to ever crosschain but was wondering if there is a resolution to this. The only reason I want to resolve this is if I accidently cross chain during a ride due to inattentiveness, I do not want my chain jamming and throwing me off the bike and damaging the derailleur.

My current set up is Shimano FC-5703 53/39/24 triple crankset, RD-M771 Deore XT rear derailleur, FD-6703 ultegra front derailleur, KMC X10.93 chain and a custom 13-32 10 speed cassette. I plan to make this my travel bike, hence the large range of gearing and mashup of components. Every other chainring combination runs fine except the small/small 24/13 combination. I know that the small 24 chainring is causing the chainline angle to be more extreme than a 30 chainring would.

Does anyone have recommendations on if there may be a fix? Please don't respond with "well don't cross chain." I don't intend to...just trying to eliminate any chance of a chain jam and accident if I somehow am not paying attention to cross chaining.
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#2
The problem may not be directly related to the cross chaining, but rather the amount of chain that the M771 SGS can take up. It is rated for 45T. (53+32)-(24+13)= 48T - 6¼% beyond its rating.

As soon as you go beyond specification limits, you will run into unexpected issues.....

I would swap the 53T for a 50T, and shorten the chain.

Why would a 24T granny cause a worse chainline than a 30T? The 24T is 6T worse for chain take up.
Nigel
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#3
Thanks for the reply.

For some reason I was thinking that the chainline with a 30T might be longer resulting in a less drastic chainline angle. But maybe the 6T takeup moves the pulley more forward and would be even more angle.

I know there are alot of compromises to make it work right by changing gearing. But trying to see if there are other options that will work before resorting to that? Are there other rear derailleurs that may work? Is this something indicative of a hanger out of alignment?

However, suppose I do go with a 50T. The only change in the small/small configuration would be a shorter chain resulting in the pulley cage moved forward. The chainline would be shorter with a steeper chainline angle that would still cause the chain to jump off. You stated it wasn't a chainline issue, but that is the only reason I can come up with why the chain tries to run off. What you prescribe may be within specifications but I am not envisioning how this would fix the issue. It seems that the specifications are only so that there is enough chain takeup for both large/large and small/small configurations which doesn't seem to be my problem. My derailleur cage is neither pulled completely straight in big/big nor does the derailleur fold up on itself in small/small. Maybe I am missing something. (edit: Does the pulley cage move more inward towards the frame if it is pulled further forward? I am not knowledgeable about how the derailleur parrallelogram changes. Moving the cage inwards would make my chainline straighter, thereby keeping the chain from trying to jump off. Is this what it would be doing?)
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#4
You might look at how the whole crankset lines up with the rear cogset. If the crank is sitting too far inboard, that would increase the chain angle on small-small. You could fix with a longer BB spindle.

Only other thought is if the pulley wheels are worn. Worn pulley teeth will allow a chain to slip off easier. You could try new pulleys.
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#5
Thanks..so maybe I may need a bigger spacer behind the crankset? How do I know what the correct crankset chain line should be?

Brand new components so worn items shouldn't be an issue.
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#6
In general, the center chainring of the crankset should be inline with the center of the rear cog. Different cranks have different offsets so it's possible that you ended up with one that is sitting farther inboard and that is increasing the angle on the chain.

If this was the case, you can adjust the chainline a tiny amount with spacers behind the cups of the bottom bracket. But if you have to move it more than a mm or so, you'd probably need to get a new BB with a longer spindle.

But again, this is all speculation, so YMMV
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#7
Upon further consideration - I am going with "don't cross chain" Smile

I generally set up my bikes so that the granny ring is only useable for lower (larger cogs) half of the cassette, and much beyond that, the chain is striking the middle cog. They are set up so that the big ring in the front works well all across the cassette, and the middle ring may or may not work well with the smallest cog at the back - possibly with the chain striking the big cog.

One reason for primarily using the biggest chain ring is that there is less tension on the chain for any given amount of torque at the wheel. Another is that the larger the number of teeth - especially at the back - the less force on any given tooth.

When down shifting; I am always going to bigger cogs on the cassette first, then the chain rings at the front.

Our T50 is equipped with 26-44-54 at the front and 11-34 9 speed at the back. The 44-54 jump is only slightly larger percentage than the spacing of the cassette - yes lots of duplicate gears. The 44-11 comb does not work well, and I avoid the 44-13 comb too.

For a single bike, for me - everyone is different - about 100 gear inches is a good top gear. For a tandem, it is quite a bit higher.
Nigel
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#8
I sure enjoy reading this post and all the possibility's of what may be the problem, But more my concern is this. when someone builds a bike one would go step 1,2,3,4, in order etc.etc. But when the OP asks a question such as and I quote:" Is this something indicative of a hanger out of alignment?" that is the first thing to do when prepping the frame to build. This would mean it was never done or one would have never asked in the first place and thus doing things in the order of or whatever 2,3,4,1,.
I highly suggest you not change a thing except dis connect your cables, align your hanger and not by eye but with a tool, reset your H L limit screws and go from there. but to answer the question, Yes it can be indicative of a mis aligned hanger
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#9
Thanks in hindsight, I should've checked everything first. This was a relatively new frame that had no issues before that was sent out for a S&S coupler retrofit. I didn't have a reason to think the hanger which was non-replaceable and seemed durable could be misaligned. But maybe it could have been through shipping. Plus I was anxious to get building and since I am not a bike shop, I did not have a hanger alignment tool just laying around. I think you can understand someone getting anxious to build. It only occurred to me after I experienced the issues.
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#10
I totally understand. It is the first thing I check when there is a drive train issue in the rear. I hope it helps your problem, but you have to rule out one thing at a time and its a good place to start. I chuckled when you said you did not want to hear"do not cross chain". but if everything else is fine and this is your only complaint, well Ok, I will not say it Smile
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#11
Hanger mis-aligned could cause this. But if it was that mis-aligned it should also make it pretty much impossible to get your rear shifting adjusted right. You made no mention of that. But I'm assuming you have indexed shifting and that it IS shifting fine.
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#12
(04-25-2014, 10:40 AM)painkiller Wrote:  I totally understand. It is the first thing I check when there is a drive train issue in the rear. I hope it helps your problem, but you have to rule out one thing at a time and its a good place to start. I chuckled when you said you did not want to hear"do not cross chain". but if everything else is fine and this is your only complaint, well Ok, I will not say it Smile

Haha. Yeah, I've read enough posts to realize that's a common reply. But yes, it's my only complaint. I'll definitely try to take it into a bike shop to check the hanger.
(04-25-2014, 04:20 PM)DaveM Wrote:  Hanger mis-aligned could cause this. But if it was that mis-aligned it should also make it pretty much impossible to get your rear shifting adjusted right. You made no mention of that. But I'm assuming you have indexed shifting and that it IS shifting fine.

I was wondering if a misaligned hanger would cause issues for all the gear combinations. For me, I only have issues in small small. The issue isn't automatic I have to spin the crank sometimes and wait for it to jump off. Should it be a problem for all the gears? If it is slight misalignment, does it only affect the extreme cases?
I also found out that my 10 speed shimano mtb cassette doesn't require the 1 mm spacer to fit 9/10 speed hubs. Only road cassettes do. I got excited and removed it thinking it would place my cassette inward helping the chain line angle. It helped reduce the occurrence of the jam but unfortunately not completely.
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#13
(04-25-2014, 05:53 PM)vkw Wrote:  I was wondering if a misaligned hanger would cause issues for all the gear combinations. For me, I only have issues in small small. The issue isn't automatic I have to spin the crank sometimes and wait for it to jump off. Should it be a problem for all the gears? If it is slight misalignment, does it only affect the extreme cases?

What I mean is that if your hanger is bent, normally you would have a lot of trouble getting your rear shifting to work well, regardless of the jumping off the pulley issue. If your rear shifting works well, that may mean the hanger is fine (but doesn't "prove" it)

I suspect the jumping off issue is a combination of chain line and weak spring tension from the derailleur cage. It sounds like moving the cassette inbound a small amount helped with supports my theory. But again, just a theory...
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#14
(04-25-2014, 06:11 PM)DaveM Wrote:  I suspect the jumping off issue is a combination of chain line and weak spring tension from the derailleur cage

Hmm, I know some derailleurs have two slots to place the spring post in to allow for different spring tensions. Maybe, I'll take a look tonight to see if there is an extra slot to increase tension.
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