Chain is skipping only on small cog. Is it possible to tell from these pics that it is worn? Bc to my amateur eye it looks ok.

Not really, but that rusty chain may have some stiff links which may cause problems on the smaller cogs since there's fewer teeth engaging.

You can see the increased pitch -- but enough to cause skipping? Just FYI, running chains dry usually causes excess wear.

No offense.... but that is what I was thinking. With all the rust, dirt, crud, and lack of lube.... I'd sort of expect it to skip or shift poorly. But not being properly seated in the drop-out could also be a cause.

There is this wonderful new invention for chains;
It's called "oil".

Sometimes the amount of collective chain/cog wear/miss meshing is smaller then you'll see. back in the early 1980s I worked at a shop that served many of the local firemen. There was one really big one who brought his bike in for service with a specific to a skipping problem. We did all the usual and installed a new chain and freewheel. We test rode it with good function. he picked it up and came back still with skipping. We went through at least 3 different freewheels and IIRC 4 chains, all new, before we came on the combo which he couldn't get to skip. Now to place a bit of perspective on this I'm only 66" tall and back then weighed maybe 125lbs. But my boss was the NYS road racing champ from a few years prior, stood 74+" tall and was still to be reckoned with in the local racing community. After the first come back he test rode every combo and each time we had to have the customer test ride it to see. After the second come back we did the rest of the combos in one day while the customer hug out, test rode and hung out while we changed something for the next test.

First make sure the other reasons for skipping are not present. Tight or twisted chain links. Broken or bent teeth. Proper high gear limit screw adjustment.

It is hard without a extremely worn cog to see wear enough to cause skipping. But the test to see is straight forward. Try a fresh chain. Then try a fresh freewheel. Then report back. Andy.

Doesn't look too worn, but sometimes cogs like that skip and others that look worse don't.

Thanks for the detailed response Andy, and everyone else for oil/clean the chain suggestions. I will try everything. Just wanted to make sure it wasn't obviously a worn cog.

Measuring the chain seems to say the chain isn't stretched. So if I clean and oil it up nicely with either chainL or wd40, and it still skips, would that be a pretty good indication it was the cog? Or at least that would be the next thing to replace before a new chain?

thanks again fellas

Dave, they are even in both dropouts but One of the dropout screws is stuck at that spot so the axle can't go all the way in. So I set the other even and that's the way it's been. Could that really cause chain skip?

Small cog looks worn to my eye. Pic 3 it looks hooked.

The smallest sprocket does show the wear pattern that causes skipping. The proof is the skipping itself.

The mechanism that causes skipping with new chain/old sprocket is tricky and subtle. It's not that the chain slips under load. It doesn't, which is why, this type of skipping is unheard of in single speed bikes. The problem is with all the available slack in the lower loop, it's easy for a chain to snag and ride up onto the teeth, then when that unengaged bit of chain comes around to the top you lip forward a bit.

As the sprocket and chain wear, the sprocket forms pockets because of the roller rolling up the back of the tooth ahead as it unfurls under load at 12:00. That's what causes the classic hook pattern you see near the bottom in the 3rd photo. The old chain doesn't skip because it's the one that caused the wear, so it's matched.

But when you put a new chain, and add tension the chain shifts forward on the sprocket, and it's shorter (unstretched) pitch doesn't allow the roller to swing in at 6:00 without bumping on tip of the tooth. If there were enough tension in the lower loop, or (like on a single speed) no slack to spare, echain would bump, and pop past and engage normally, though it might be a bit noisier. However without being forced in, the bump cause the chain to ride the tips and stay until it's phased to engage fresh 3-4 teeth back.

When skipping isn't that bad, you can ride with it and the chain will rapidly wear those corners down and ride OK, or you can use a Dremel to do the same, but it's a slow PIA process.

You can also predict skipping when installing a new chain, but need three hands, and a chain whip with new chain. Hold the wheel up and drape the chain of a chainwhip around the sprocket from about 2:00 around to where it drops vertically at 9:00. Lever the whip forward to load the chain and cassette, then swing the free end at 9:00 into and out of the sprocket. If it slips in and out freely you're OK. If you hear or feel some snagging as it swings past the tooth's back corner, expect skipping when you duplicate this condition when applying torque.

NB: its a rather old Sun Tour freewheel . they dont last forever .

Most people don't ride the small cog so much that it wears out first, so surprising that it's the one that skips.

Also, in some cases, people use the term "skipping" to describe a misadjustment of some kind.

+1 to skipping meaning different things, but the smallest sprocket is definitely worn.

As for how much people use it, IME from when I was in retail, it tended to be the first to go among non serious riders who had no idea what cadence was. Also consider that that is a 13t sprocket, so would be the 3rd one in on most modern cassettes. It was likely paired with a 52t chainring (Pre-Merckx), so the gearing was yet lower.

True, but if the rider used all cogs equally, the smallest cog would wear fastest due to the force being distributed across fewer teeth. In fact, the small cog wore out first on my Campy-compatible IRD cassette. That surprised the heck out of me. I replaced it with a Campy-compatible Miche cog, (since that was the only separate cog I could find).

I am a bit confused about this, I once looked at my cassette and thought the small cog was hooked and worn. Then I looked at this photo of a new cassette. The smallest cog looks more hooked than the OP images. Am I missing a subtle difference here. I would like to know as I try to prolong my drivetrain as long as possible.

https://media.performancebike.com/ima...14-NCL-TOP.jpg

It looks to me as part of the design - the larger cogs are less hooked. How can you tell? The "teeth" do seem a little thinner.

If you look at a sprocket -- any sprocket -- that only runs in one direction, as does a freewheel cassette, you come to realize that the chain only runs aginst the backs' of the teeth. That means you;re free to do whatever you want to the front sides, as long as the back cradles the roller so it won't climb up, and the next roller can swing into position without touching the next tooth.

Since the rollers are attached to each other, each travels in an arc with exactly 1/2" (0.500") radius and has to clear the tip as it swings in. The new sprocket meets this condition, even though the front of the teeth were sculpted away.

Now consider a worn sprocket. As the chain wears the pin wear allows the links to move apart (stretch) so the worn pitch may be 0.505". As this worn chain runs on the sprockets the arc that the rollers is now larger, so while it wears the backs of the teeth based on that pitch, it can sweep by a corner based on that 0.505" radius. All well and OK, until you install a 0.500" pitch chain with a smaller swing radius. It needs 0.005" remover from the unworn corner so it can swing into position as it engages.

This is hard to see with the naked eye, but the wear pattern is visible to anyone used to seeing sprockets, and can be confirmed as I describe using a loaded chain and testing whether the rollers touch the corners.

If you're having a problem visualizing what's happening, find a door that opens close to a piece of furniture. Move it so the door has the minimum toom to swing by the piece without touching. Now move the furniture over another fraction of an inch and try to open or close the door. If you moved it only a hair the door can swing by, but it touches the corner as it sweeps past. Any more and it won't swing past at all.

BTW- look ar the new sprocket (back of teeth only) and compare to the teeth visible at 6:00 in the OP's photos and you should be able to see the worn pockets on the teeth, and the beginnings of the offending hook. Hint, compare the worn teeth to those on the neighboring sprockets, and the difference becomes more obvious.

I see it differently. The teeth on modern cassettes barely come to the 1/2 way point of the roller to facilitate shifting whereas single speed cogs have longer teeth. Just a slight amount of wear at the top rear edge of the shorter teeth can cause a new chain to jump. This occurs with a new properly adjusted derailleur and with the big ring tensioning the lower run so there's virtually no chance of the chain not feeding onto the cog properly. You can observe it from behind on a trainer and the chain looks perfectly aligned with the bottom of the cog when it happens.

thanks for the info Francis, very educational as usual.

Interesting explanations.

In practical terms, replace the freewheel and chain. Replace them again before they get this bad.

Some people spend too much time using the smallest sprocket, which is bad for the knees and the drivetrain.

If it were my bike, I'd go ahead and order the new freewheel, but try grinding off the hooks on the sprocket teeth in the meantime, like FB alluded to. Here is an article that discusses it from the standpoint of cassettes, but has more discussion on the topic: More Miles from Your Bicycle's Cassette

The "hooking" action on Chris's smallest cog is pretty pronounced, so this has a good chance of eking out a few more miles.