Mysterious Chain Skipping

[Neil Smith]

Dear All,
put new chain on and got skipping so did the usual and replaced the rear sprokets. Still skipped. Various mechanics looked at it and checked:
a: drop out - it was true
b: indexing
c: b- adjustment screw
d: fiddled with the plastic adjuster on the housing leading the cable into the rear derailleur going.
e: even tried a new chain.
I should make clear that this skipping only occurs under pressure when I cycle up hill.
STUMPED

[AMuzi]

If the chain was severely worn, look at the chainring teeth.
New teeth will be symmetric, worn will show a decided 'wave'
shape.

--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971

[James]

If everything is new, did you check for a stiff link? You can usually
find them by turning the cranks backward (with the chain on the middle
of the cassette so the chain isn't at a huge angle WRT the chain rings
and cassette sproket). The stiff link will lump through the rear
derailleur. Best to use the middle or small chain ring so there is less
tension in the chain. If you find a stiff link, flex the chain sideways
at that point.

Do you use a removable "quick link"? Some are designed to be installed
one way. They have a concave and convex side. When installing the
quick link in the bottom run of the chain, make sure the concave side is up.

Does it only skip on one or two sprockets? One of the smaller ones?
Sometimes the spacer between sprockets is a largish diameter, and
interferes with the quick link.

I have had trouble mixing chains and cassettes from different
manufacturers. I couldn't get a BBB (I think) cassette to work with a
Campy chain. I believe it was a question of tolerances. The Campy
chains are typically slightly shorter when new, where as perhaps the
cassette was designed for a chain that was slightly longer. Check out;

http://www.cantitoeroad.com/docs/connex_by_wippermann/2010_chainwear_test_10speed.pdf

Note the difference in initial chain length of various brands.

--
JS

[Sir Ridesalot]

Was the old chain skipping? How's the tension on the derailleur cage fore and aft? Side to side tension okay on the derailleur? I was wondering if maybe your derailleur was worn too. Check the jockey and idler puuleys to be sure they are okay. Derallur is lubed okay and isn't sticking anywhere? Cable and housing are sound?

Good luck.

[Neil Smith]

The derailleur is the only variable that hasn't been changed. How do I check/adjust the side to side tension. How do you tell if the derailleur is worn?

[Sir Ridesalot]

If the chain is the right length and sags along the top run then the cage return spring is weak. If there is slop in side to side movement of the derailleur or if the derailleur is fairly easy to push sidways then the inside return spring is weak.

If the old chain wasn't skipping then it's likely not a derailleur issue. If the rear cluster was badly worn then, as A. Muzi said it might be the fron t chainrings that're the problem. I had that problem n two different bikes and changing the chainrings cured the skipping. On one bike the skipping happened with moderate load and on the other the skipping only happened under heavy load.

If you could post a closeup image of your front chainrings teeth the experts here could probably tell you if they are the likely problem. I use Flickr or Photobucket to host images and link to them here.

Cheers

[Duane]

Usually happens when you put a new chain on a worn cassette but you say
you've replaced the cassette. And you've tried a second chain. What do
the chain rings look like?

[Duane]

Oops, I see Andrew suggested the chain rings already.

[Dan O]

http://groups.google.com/group/rec.bicycles.tech/browse_thread/thread/f71c333c03a56faa/9e94c8c5b06a6f0b?hl=ene94c8c5b06a6f0b

(Unrelated side note that our absent professor took that opportunity
to suggest I wasn't riding properly, that "It's very common for new
cyclists... ", and "There are also lots of older cyclists who haven't
learned... ")

Anyway, back on-topic, as I recall the setup with new Nashbar 7-speed
freewheel and new SRAM 6/7/8-speed chains continued to give trouble -
seemed to be catching the sideplates on adjacent sprockets (much worse
under load) - until I soon replaced the old derailer with a brand new
long cage Deore XL. Things got even better still when I upgraded to a
(used, but not too worn) Dura Ace 7-speed freewheel.

[Neil Smith]

I finally tracked it down. It was the middle ring and the middle ring only that was worn on the front crank. Cycling up a gentle incline several times I was able to test all three rings the 2 outer were fine. This was a first for me.

[Sir Ridesalot]

Glad you figures it out. That must be yhe ring you use the most.

Cheers

[(PeteCresswell)]

Per Neil Smith:

>I should make clear that this skipping only occurs under pressure when I cycle up hill.

When this happened to me some weeks back, I thought my geared hub had
some sort of internal damage.

Then I realized it was the chain skipping.

Turned out that I had swapped chain tensioners and the new one was not
applying sufficient chain wrap. Swapped the old one back and the
problem went away.

IIRC on my der bikes, there was a chain wrap adjustment....

Also, I suspect that lack of lube can also push a marginal situation
over to skipping under load.
--
Pete Cresswell

[Duane]

Mr Muzi got it in one. Glad that it's fixed.

[Andrey Tarasevich]

On 6/14/2013 2:47 PM, Neil Smith wrote:
>
> I finally tracked it down. It was the middle ring and the middle ring only that was worn on the front crank. Cycling up a gentle incline several times I was able to test all three rings the 2 outer were fine. This was a first for me.
>

While I believe you when you say that the issue only occurs on the
middle ring, I don't see how your conclusion about the middle ring
itself being the problem makes any sense.

The chain angle, tension, friction and other parameters will be
different between the three chainrings, which is a perfectly good
explanation for why the problem might not reproduce on other rings. In
other words, the middle ring might just happen to create the most
favorable chain configuration for the problem to reproduce. The ring
itself might be perfectly fine, even if it looks more worn than the
others. Don't be surprised to discover that replacing your chainrings
does not solve anything.

Anyway, chain skips under large loads you say? What about the first and
the most obvious explanation that immediately comes to mind in such
cases: your frame is not sufficiently stiff. It deforms under loads,
which causes the chain to skip. Your older chain just happened to be
"matched" to your setup (accidentally or deliberately) in a sense that
it resisted the problem due to some other lucky combination of parameters.

I'm surprised to see that no one in this thread mentioned this most
basic, common and essential explanation so far.

[AMuzi]

And yet on modern equipment a worn out middle ring is not at
all unusual and the most likely case.

[James]

It is extremely unusual on my modern equipment. I don't even have a
middle ring to wear out!

I'll have to order a new 53t soon. The 39 is still fine.

--
JS

[Dan O]

I use my middle ring a little bit (for the steepest climbs, starting
off cold, and toodling on the sidewalk). Even of aluminum alloy, it
will probably outlive me. The small ring has never been used except a
couple of times to make sure it works. The big ring (not *that* big
at 48 teeth) is getting quite a profile.

[Frank Krygowski]

One friend came to me with the exact same problem. He'd paid an
amateur bike mechanic to put a used triple crank on his old bike,
since he now lives in a very hilly area.

Once I got to see the bike, I found the used crank had the most badly
worn middle chainring I'd ever seen, with teeth like vestigial shark
fins. I was amazed it worked with even light pedal force. But pushing
even a little hard caused the teeth to disengage from the chain, every
time. No frame flex or anything else required. In fact, while on
that chainring, there was no way to pedal hard enough to cause the
frame to noticeably flex!

- Frank Krygowski

[Andrey Tarasevich]

I notice that in your message you are talking about "chain skipping",
yet the repair measures you listed above strongly suggest that the issue
is actually "autoshifting".

So, could you, please, clarify this? What is actually happening? Does
you chain skip over the teeth of the same cog? Or does your chain skip
from one cog to another by itself?

[Andrey Tarasevich]

On 6/20/2013 8:26 AM, Frank Krygowski wrote:
>
> Once I got to see the bike, I found the used crank had the most badly
> worn middle chainring I'd ever seen, with teeth like vestigial shark
> fins. I was amazed it worked with even light pedal force. But pushing
> even a little hard caused the teeth to disengage from the chain, every
> time. No frame flex or anything else required. In fact, while on
> that chainring, there was no way to pedal hard enough to cause the
> frame to noticeably flex!
>

Apparently, you are talking about a completely different thing.

The information the original poster provided so far seems to indicate that:

1) The problem occurs in the rear, not in the front. It is the cogs that
skip, not the chainrings.

2) I'm not sure about this part, but it appeared to me that what the OP
was talking about was "autoshifting", not "chain skipping". The original
list of measures the mechanics took in their attempts to fix it does not
like there were attempting to rectify a genuine "chain skipping". I
suspect that the OP misused the term and referred to autoshifting as
"chain skipping". Again, I could be wrong, but only the OP can clarify that.

If the problem is indeed autoshifting, then frame flex is the first
culprit that comes to mind.

If the problem is actually true chain skipping, then I don't understand
why they were looking at his rear (RD indexing? RD limit screws???) if
slipping occurred in front.

[Sir Ridesalot]

The OP saidthat the CHAIN SKIPPING not auto shifting ONLY occurs whenthe chain is on the MIDDLE CHAINRING. Thus, the problem was not because of worn cogs, or frame flex.

Cheers

[Dan O]