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Chain skip....I'm baaaaaaaack ;)

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Chain skip....I'm baaaaaaaack ;)

Old 10-01-12, 04:08 PM
  #1  
stephr1
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Chain skip....I'm baaaaaaaack ;)

OK, after an absence of a month due to a recurring back/hip issue (twice) and waiting on parts (new freehub), here is where I am on this chain skip challenge (http://www.bikeforums.net/showthread...ga-continues):

I installed the new freehub. New freehub is tight and it solved the cassette wobble that developed so the cogs now rotate nice and straight and consistent (old freehub certainly had an issue). And rotation of the wheel while holding the cassette/freehub outer casing stationary sounds much more like I would expect: quiet and smooth with the usual tick-ticking (old freehub tick-ticking had become very soft, almost non-existent).

Also, using my high school geometry and understanding of topology, I realized that the truing of the hub would have nothing to do with the wobble I saw: ergo - rim is trued to the hub, not the other way around. The new freehub proved that

I went thru and did a thorough RD adjustment. Adjusted the Hi limit so the upper pulley is aligned with the top gear (7th). Adjusted the cable so that the upshift happens as expected (downshift is clean).

End result? Still getting the chain skip and occasional chunk-chunk (sort of) while riding on flat ground. Thought the freehub would be the problem source (wanted to offer kudos to the 2 people who identified it as such). No such luck!

Here's what I have now discovered: When in 7th (top) gear, I hear a kind of tick-tick-tick (with uneven timing and lower in frequency than the freehub...which may be the echo coming off the freehub) coming from the RD/cassette area. It seems as if the chain isn't settling solidly and immediately onto the cog (which would seem to be a possible source of the skipping). I've looked at the chain up close while hand-pedaling the crank (tho, actual riding with a load could make the problem much more severe) and it isn't obvious that the chain is doing anything other than laying directly onto the 7th cog as it comes off the upper pulley. Doesn't appear to be catching on 6th gear either. Nothing shows up as the chain missing any tooth landing, so to speak.

I checked the chain...no kinks or tight links. I took the lock ring from my old IG-60 cassette and compared it against the one on the bike. Pretty much same outside diameter. Checked the old 11T cog tooth penetration into the chain against the new 7th cog and it's pretty much the same (It would seem the new cog penetrates just a bit deeper). Thickness/width of the new cog is pretty much the same as the old one, maybe even a bit thinner.

I got onto SRAM's website and checked out the chain I bought (PC-870). Can't find any specs, but in a chart it shows this chain as listed for 8-spd cassette and another chain (PC70?) for 7-spd system. My understanding is that most 8-spd chains will work on 7-spd. cassettes, but not necessarily the other way around. Besides, my old chain was a PC-58 which the PC-870 supposedly replaces and I had that on an 11-28T cassette (new cassette is Shimano HG-30-7 B-Type...6th gear has built-in spacer, A-type does not). So is it possible the chain is not compatible with this cassette?

Anyone have any insight or experience into this?

The only(?) other possibility seems to be the RD. I've heard it 2 ways: parallelgram section could be a cause. I've checked mine and I guess I'm not sure how much play is too much. There's a little, but it doesn't seem(!) that would be the source. I spoke with a tech at the LBS (they do have a very lightly used RD left over from someone's upgrade). He said it would be a very rare thing to see an RD go bad and he's never seen one (tho, to be honest, he wasn't that old

Anyway, I'm thinking of springing for the new(er) RD, but I want to be sure that will solve the problem (and not waste money chasing this problem). As usual, I am always open to suggestions whether they lead me to the solution or not.

I have another, somewhat related question: In a Shimano tech doc for my FM-RH40 rear hub, the drawing shows some kind of spacer (can't be much more than 1 mm thick) between the wheel hub and freehub (looks like the freehub stationary part mounts against it). The few times I've taken off the freehub, I've never seen any kind of spacer come off (maybe it's forever pressed onto the wheel hub). Any comments about this? Thanks.

Thanks in advance...Steph

Last edited by stephr1; 10-01-12 at 04:13 PM. Reason: added info
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Old 10-01-12, 04:40 PM
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Here are the two most common causes of the problem you describe.

1- chain or sprocket wear, especially if the sprocket is more worn than the chain (search, new chain on old cassette). Hint -skip is random, but happens under load, even low load, and is usually worst on smaller sprockets.

2- stiff or twisted link, but you checked. If you can't pin this down, check again by backpedaling while holding the RD pulley forward so the lower loop is slack. Look for any hint of stiffness. Hint, stiff link issues repeat every 2-3 turns of the crank.

3- high upper pulley. If the jockey wheel is too close to the cassette, it can affect how the chain spools onto a sprocket. There should be daylight, or about 1" (on a tangent) between the jockey wheel and each sprocket. Hint, this problem usually occurs only on middle and larger sprockets.

One of the three above accounts for almost all cases of skipping, but there's always issues of RD alignment and trim, which can cause random shifting or other skip like issues. (But I put this in a different category).
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Old 10-01-12, 05:14 PM
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Chain skip....I'm baaaaaaaack ;)

FBinNY,

New counting system?

Thanks for the input. By this time, people have shared with me what you have, but I appreciate it anyway.

1. Brand new (~300 miles now) chain (x-changed it once) and cassette. So, I have to believe they're in good shape.

2. At this point, I'm willing to look again. I don't think this is an issue, but I can't remember looking closely for any protruding pins (in 2 chains, tho?). Pretty sure I had checked the chain with it loose, but I'm getting desperate (and frustrated at this point).

3. My understanding has been that the upper/guide pulley should be set so there is fairly minimal clearance (1/4" or so) between the pulley and 1st gear. I would guess that distance changes (increases) as I shift to 7th gear, tho the RD pivot springs might moderate that a bit. I know I don't have have 1" (or should that have been 0.1"?) between the upper pulley and 7th gear. How critical is this?

In previous threads I've mentioned this seems like one of those subtle, hard-to-identify problems (well, maybe more the solution. The problem is pretty obvious The subtlety is beginning to wear very thin......

Maybe I should just pull the trigger on a new RD. If nothing else, it would eliminate one more thing....tho wouldn't necessarily get me any closer to the solution

Again, appreciate the input.


Cheers....Steph

Originally Posted by FBinNY View Post
Here are the two most common causes of the problem you describe.

1- chain or sprocket wear, especially if the sprocket is more worn than the chain (search, new chain on old cassette). Hint -skip is random, but happens under load, even low load, and is usually worst on smaller sprockets.

2- stiff or twisted link, but you checked. If you can't pin this down, check again by backpedaling while holding the RD pulley forward so the lower loop is slack. Look for any hint of stiffness. Hint, stiff link issues repeat every 2-3 turns of the crank.

3- high upper pulley. If the jockey wheel is too close to the cassette, it can affect how the chain spools onto a sprocket. There should be daylight, or about 1" (on a tangent) between the jockey wheel and each sprocket. Hint, this problem usually occurs only on middle and larger sprockets.

One of the three above accounts for almost all cases of skipping, but there's always issues of RD alignment and trim, which can cause random shifting or other skip like issues. (But I put this in a different category).
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Old 10-01-12, 05:33 PM
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Originally Posted by stephr1 View Post
FBinNY,

New counting system?


3. My understanding has been that the upper/guide pulley should be set so there is fairly minimal clearance (1/4" or so) between the pulley and 1st gear. I would guess that distance changes (increases) as I shift to 7th gear, tho the RD pivot springs might moderate that a bit. I know I don't have have 1" (or should that have been 0.1"?) between the upper pulley and 7th gear. How critical is this?
Yes, it's a new counting system, consider #3 a bonus. Actually that's what happens when you write on the fly.

The pulley sprocket clearance is critical. Note that I'm not talking vertical distance, but tangential distance (along the chain) so the 1" isn't a mis-print. The B-screw setting has to be checked on all sprockets, though if you base it on the largest it usually comes out OK (check anyway).

But if you have a Shimano or Campy derailleur, where the cage pivots between the pulleys (Sram has the pulley at the pivot) the upper pulley changes height depending on the chainring, it's necessary to check the B setting with all chainrings. (the last step Shimano's instructions which most people never bother reading).

If all else is OK, and it skips on larger sprockets
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Old 10-01-12, 06:31 PM
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We may be talking about the same thing.

If I understand you correctly, it's taking the measurement from the rear side of the cog (where the chain starts to leave it) to the front-facing (chainring side) point on the upper pulley where the chain has become fully engaged. Based on your statement, that would essentially be the distance of two 1/2" links. Not having actually measured that (tho, I will now since it's' new information for me), I think the closest distance of ~1/4" between upper pulley and cog results essentially in the same setting. I read somewhere that some specs set the closest pulley-to-cog distance at ~6mm.

The RD is an older Shimano MC-18 and the upper pivot point is just below the upper pulley (between upper and lower). So, where best should I set the 1" distance: 1st gear, 4th gear (middle) or 7th gear?

In one of my previous threads, someone stated they didn't think the B-adjust would cause the skipping I'm seeing. Maybe they perceived my skipping problem as one between gears, not in one gear only.

BTW - I've never seen Shimano's instructions for setting up a derailleur. Something else new to check into

Again, thanks for the help.


Originally Posted by FBinNY View Post
Yes, it's a new counting system, consider #3 a bonus. Actually that's what happens when you write on the fly.

The pulley sprocket clearance is critical. Note that I'm not talking vertical distance, but tangential distance (along the chain) so the 1" isn't a mis-print. The B-screw setting has to be checked on all sprockets, though if you base it on the largest it usually comes out OK (check anyway).

But if you have a Shimano or Campy derailleur, where the cage pivots between the pulleys (Sram has the pulley at the pivot) the upper pulley changes height depending on the chainring, it's necessary to check the B setting with all chainrings. (the last step Shimano's instructions which most people never bother reading).

If all else is OK, and it skips on larger sprockets
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Old 10-01-12, 07:16 PM
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The relative radial distance, and tangential distance depends on whether the pulley is forward or under the sprocket. If you use my tnagential measurement (eyeball is good enough) you can do it with the large chainring, and largest sprocket you expect to use. But you need to reconfirm by shifting to the small chainring, which will rotate the pulley forward and up changing the distance.

the right setting is the one that gives you about 1" tangential clearance for the closest pair.
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Old 10-02-12, 10:04 AM
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I need to check your suggested measurement closer, but before I went on a bike ride this morning, I took a quick look and it appeared I had the two link (1") distance between chain transition points.

Speaking of riding, I went out on my morning ride and decided to try an experiment. From the start of this problem I assumed it was originating from the rear section of the drive train (RD/cassette/freehub). Others had suggested it could be a chain ring issue. My thought at that time was that my chainrings have been in worse shape without any problems (but, then, the chain/chainring had worn in (or is it out) together).

I pretty much always ride in the middle chainring and only use gears 3-7 for most of my rides. This morning I decided to prove or disprove my assumption and use the large chainring (42T or 44T) for the whole ride. This was prob'ly only the 3rd time the chain had been on the large chainring, so it's pretty much in brand new condition. What I experienced was that I had all the same symptoms: occasional chain skip (or a kind of "hesitation") on flat ground, and complete chain skip going over one of the bridges (yet, when I shifted down to 6th gear, I had no skipping at all...which is also consistent in my normal chainring-riding gear). What was different is that it seemed to occur just a bit less frequently...but that might be my wishful thinking. After thinking about it, that possibly made sense because the chain is more parallel to the cassette/cogs (less lateral stress on the chain/cog connection) and I was pedaling slower for similar speeds.

So, no closer to a solution (but certainly less hair on my head

Cheers...Steph

Originally Posted by FBinNY View Post
The relative radial distance, and tangential distance depends on whether the pulley is forward or under the sprocket. If you use my tnagential measurement (eyeball is good enough) you can do it with the large chainring, and largest sprocket you expect to use. But you need to reconfirm by shifting to the small chainring, which will rotate the pulley forward and up changing the distance.

the right setting is the one that gives you about 1" tangential clearance for the closest pair.

Last edited by stephr1; 10-02-12 at 10:13 AM. Reason: Delete inaccurate term
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Old 10-02-12, 10:15 AM
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What's the bike? Yesterday I learned the hard way that biopace and a loose BB is a great way to wind up with a nice line of holes in your calf.

How's your BB?

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Old 10-02-12, 10:34 AM
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1999 Specialized Hardrock. BB (along with everything else associated with the drive train) has been replaced in the recent past and was one of the first things I checked. It's solid (as in no detectable movement...other than rotation, of course

Originally Posted by IthaDan View Post
What's the bike? Yesterday I learned the hard way that biopace and a loose BB is a great way to wind up with a nice line of holes in your calf.

How's your BB?

E:
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Old 10-03-12, 06:49 PM
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OK, a quick update.......

I tried another experiment. I replaced the new 11T cog (on the HG-30) with the one from my old IG-50 cassette. I had compared the 2 and, while the old 11T showed some good wear and tear, it did not seem that much worse than the new one (which got me to thinking about something else). I didn't expect to solve the skipping problem with this because I suspected the old 11T would not play well with the new chain. Sure enough, I had pretty good skipping.

What I did notice, however, is that the skipping pattern on the old cog seemed awfully familiar to the skipping on the new cassette 11T (obviously worse).

This gave me the idea that maybe, just maybe, the cassette I bought, while referred to as Shimano, might not have been...even tho the lockring had Shimano on it, none of the cogs showed the Shimano name anywhere. I suppose it's possible an overseas mfg'er could have bought the lockrings and made a cheap knock off of the Shimano product...and that's never happened before (I may surely be grasping at straws here

Anyway, I pulled the trigger on purchasing a new Shimano HG-41 11-28T so I can determine if the problem is in the cassette. If it's not, the last thing is the derailleur....sigh......

UPDATED UPDATE: I just came back from putting the new 11T cog back on. Took another look at the lock rings and while 1 says Hyperglide and the other says Interglide(?), they both appear to have the same dimensions to support an 11T cog. What I did notice that I hadn't before is that the distance from the outside edge of the lock ring to the bottom (closest point to lock ring) of the "valley" of the 11T cog of the new cassette is pretty close to the same measurement distance (1/32" or less) for the old 11T cog. Keep in mind the old 11T cog has 2K+ miles on it. It sounds suspicious to me, but I don't really know what that distance should be. I'll have to see what this looks like on the new cassette when it arrives.

Anyone have any knowledge, insight or comments about this dimension? Thanks....

Cheers....Steph

Last edited by stephr1; 10-03-12 at 07:39 PM. Reason: New information
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