andrec

Today, I installed a new chain (SRAM PC-48). The old one was pretty stretched, but I don't think that the chainrings/cogs are badly worn. The shifting is decent (perhaps not as good as it was before, I'm not really sure; but then I did expect that since I'm matching a new chain to old gears). I did tune the derailleurs a little and lube the chain with White Lightning.

The problem is that when I'm on the smallest cog, and pedal with a medium amount of force (I'm not pushing as hard as I can, and I never stand to pedal), the chain skips... It happens:
- only on the smallest cog
- on both big and middle chainrings (didn't try the small chainring because I never use that combination)
- randomly, sometimes twice in a row, sometimes not for a few cranks of the pedal (all depends on pedalling force I think), so it's not a stiff link

I checked the small cog, and it looks surprising unworn, given that I only use the other cogs to accelerate from stop lights (and some hills), and spend most of my time on the small cog.

This has happened to me once before on my old chain around three months ago. It was before I learned to set the adjuster screws on the derailleur, so I just played with those because I knew that they affected the limits and thought that it was my problem. Well, I didn't know at the time which screw was high and which was low and which way to turn to move the limit in. The result was that I had the chain fall off several times before giving up and resetting the screws back to near where they were before. Got home that night and figure that I'd deal with it in the morning. I might have lubed the chain too, I can't remember. Anyway, the problem was gone the next day and never resurfaced until after I put on the new chain today...

Anyone have any insight into my current problem and how I might solve it (short of replacing cogs)?

And if I have to replace the cog, can I replace just the one? Is doing so bad for other parts of the drivetrain?

Thanks in advance!

I bought this bike used a year ago. In case any of this information is useful, it's a Rocky Mountain Fusion, with a date stamp on the frame from late 1992. Derailleurs are Deore LX, 7 speed G cassette on the rear. The original chain was presumably a Shimano something or other; it had "HG" stamped on the links. I suspect that all the parts on the bike were original.

andrec

A somewhat unrelated (I hope!) topic...

I was just cleaning my drivetrain (installed new chain not at home, so didn't have any cleaning supplies with me). It didn't help with the chain skip.

But I noticed that my middle chainring was not planar. The teeth themselves appear to be flat and round as far as I can tell, but parts of the ring were bent so that they leaned toward the small chainring closer to the inside edge of the ring. I'm guessing that this is to help with the shift, but have never noticed this before, and want to check to make sure that the ring is not deforming due to stress. Am I due for a cogset/chainset replacement?

Thanks again!

captsven

New chain on an old cassette slipping. Classic sign of a worn cassette. I bet that is the gear you use the most. Small cogs are more susceptable because they wear out faster.

Retro Grouch 

When you allow your chain to wear to the point that it has significant "stretch," it wears the other components to match. Unless you are very good at knowing what to look for, a visual examination of cassette cogs will not usually detect this wear.

When you replace a "stretched" chain with a new one, the new chain will no longer fit the worn cogs and tends to skip when you apply pressure to the pedals. This will be most pronounced on the cogs that you use the most because they will have received the most wear.

Considering the way that you say you ride, my prediction is that replacing just your smallest cassette cog will solve your problem. Your local bike shop will be able to get one for you.

HarryK

Agree with Retro Grouch. that cog wear is hard to detect. If you pull on the top of the chain, you may be able to see that only a couple of teeth on the cog are engaged due to the mismatch.

But, why ride on the small cog? It's prone to quick wear due to the number to teeth carrying the load. Also, the chain is under minimal wrap tension from the derailleur, so it tends to skip easier if the cog profile has any wear.

andrec

I was mostly basing my assessment that the cog isn't too badly worn because I can still see the corners of the teeth. I'll swing by a LBS and get someone knowledgable to take a look at it.

But why did I have this problem the one day with my old chain, and why did it go away? If that hadn't happened to me, I would have just assumed cog wear; but the one experience has instilled optimism that something is just badly adjusted.

I use the small cog because it provides a good cruising speed for me, and is minimal shifting for urban stop and go traffic. (And Vancouver is hilly, so there are many opportunities to ride with the large chainring/small cog combination.)

madpogue 

Just a stab, here. Is it constantly skipping, or only every few pedal revs? Since it's only on the smallest cog, and it's wrapping the chain in its smallest radius, it could be that where you joined the chain when you installed it, it's not pivoting freely enough. And as that spot passes over the cog, it doesn't wrap 'round the cog as expected and skips. Easy enough thing to diagnose visually.

andrec

The skip is constant if I apply enough pressure, and does not seem to be dependent on the chain. The chain looks fine, with no visible stiff links through the rear der.

Avalanche325

I would replace that one cog. It is hard to see the wear on the smaller ones. Don't look at the tips so much, but more the shape of the whole tooth. When worn they get shaped a little more like a shark fin without the point.

HarryK

I'd have the LBS check it out. If it comes and goes, could be the RD isn't providing enough tension, or the chain is a link too long.

If you find yourself always on the small cog/big chainring, you may find you're replacing the small cog on a regular basis. Perhaps you could get a different chainring for the bike so you have the gears you need...the chainrings are likely pretty worn also at this point.

bikex10

If it is not constant but seems to be timed it is probably a tight link.
You can locate it if indeed it is one by peddeling backwards and watch the derailer , it will jump when the stiff link passes. Put pressure side to side to lossen it.

damaged_goods

I have experienced the same problem. Simply replace the cog and chain at the same time should (due to wear) and all should be good.

Guest

I would contact Rev. Chuck. I'm going to give an alternate scenario, but since I'm new to this bike mechanics, I don't know a ton about this, but I know if Rev. Chuck reads this, he can elaborate.

First, there's some kind of tool you can get that kind of looks like 3 or 4 bike links on a steel stick. You can check for cog wear by placing it on the cog. If it doesn't lie totally flat, it is an indication that your cog is worn.

Second, it could be that your rear derailleur just needs to be adjusted. On the back of the derailleur, there is an H stop screw and an L stop screw. I can't remember which one, but one of them helps to adjust the derailleur so that it aligns correctly and the chain doesn't skip. You'll have to shift your gears to the smallest gear setting, adjust the screw, then SLOWLY click the rear gears as you SLOWLY hand pedal to ensure that the deraileur is correctly adjusted. I did this last week in my Park Tool class, and my bike runs very well now.

Sorry. I don't know a lot, but if someone can decipher this, they can elaborate and give a better explanation.

Koffee

DieselDan

A tight link will make itself known in the jockey pulleys. A worn out rear 11 or 12 tooth rear cog is not uncommon. A chain will grab each tooth more often on a smaller cog, casuing it to wear quicker.

Don Cook

It reminds me of a situation that happened to me a couple of years ago. I had just installed a new chain on a bike with a Shimano 9 speed setup. After 10-15 miles I noticed that on certain rear cogs I would get a chain "skip" every 2 or three crankset rpms. For the remainder of the ride I stayed away from those gear combinations causing the most aggrevation. Once I had it home and mounted for repair, I found that the chain pin for connecting and disconnecting the chain wasn't fully seated. As the rear cog would rotate, once in awhile the edge of the chain pin would catch an ajoining cog and lift the chain. This came to mind becuse you had just replaced your chain.

madpogue 

Yup, this is similar to the "stiff link" problem. Generally, if the skip is about once every wheel rev, it's a cog problem. If it takes several chainring revs, it's probably once per complete travel of the chain, thus a chain problem. Stiff links are also common on chain replacement; if one doesn't have a "master link" as you describe, the point where the chain was joined (with a chain tool) is probably the stiff link.

Don Cook

madpoque's post reminded me of the very slight length difference between a 8speed chain pin abd the 9 speed pins. When I purchased my first new road bike in many years (1999), I didn't know the difference. A LBS sold me a pack of Shimano chain pins for an 8 speed cassette. These pins will cause the same exact symptom as a 9 speed chain pin not quite seated all the way. In my case, once I removed the pins and compared their lengths (using a dial caliper) with the silver colored 9speed pins I realized what the problem was. A pnuematic die grinder applied lightly to the end of the pin took care of the problem.

A.troll

Well, Andre:

I'm not exactly fluent in Koffish, but I think I can interpret some of what she is trying to say.

The tool is a Rohloff HG/IG check. You can see it at www.rohloffusa.com Go to products > HG/IG check. You can buy one at www.biketoolsetc.com, and I'm sure other websites have them too.

The H and L limit screws on your rear derailleur determine how far it can travel in either direction. The H screw sets how far it can travel to the outside. If the limit screw was not adjusted well, it could allow the chain to shift off to the outside, or possibly, to chatter a little as it tried to shift off the smallest cog.

I think it's a lot more likely that your small cog is worn and needs replaced, boyfriend.

Hope this helps!

Trollie

RiPHRaPH

i'm going with: you've got whats called a crossover gear there. the largest chainring combined with the smallest rear cog. similarly, the smallest chainring combined with the largest rear cog can produce the same thing.

most will tell you not to crossover because it will cause chain rub/wear....

anyone else?

madpogue 

Um, those aren't crossovers. The largest chainring is furthest from the centerline of the bike, as is the smallest cog. The smallest chainring is closest to the c/l, as is the largest cog. In those combinations, the chain will be pretty close to parallel with the cog/chainring.

Crossover occurs when you shift to the smallest chainring and the smallest cog (creating a sharp "outward" chain angle) or the largest chainring and the largest cog (creating a sharp "inward" chain angle). In any event, the chain rub in those cases is usually on the derailleurs, not on the chainrings or cogs.

But yes, crossover is frowned upon (except, perhaps, by LBS mechanics trying to drum up business...).

Moisture

Reviving this thread from the dead . Got an identical issue to OP after trying out my 9 speed cassette.

The limit screw is set to stop at the second smallest cog (#8). It ONLY skips in this gear. Going back up to 7 or any other gear immediately stops the skipping.

I will try to play around with the limit screw a little and see if this helps. I am using a brand new 9 speed chain and a very lightly used cassette which has mostly seen use with new chain.

cxwrench

It can only really be the limit screw.

roadbikemaniac

If you’ve ruled out the chain link, chain wear, cassette wear, and derailleur adjustment, then take a look at the lock ring. Sometimes if the lock ring is the wrong one, it can be too wide in diameter, preventing the chain from getting a good wrap around the cog, causing it to skip.

ThermionicScott 

ZOMBIE THREAD ALERT! OP is long gone from these forums.