drchinn

The bike in question is a 1992 7 speed Cannondale R600. In the last month I have crashed it and added a new chain. Two days after adding a new chain I noticed that the two smallest cogs are "skipping," either in the small or big chain ring. By skipping I mean that the chain normally wraps around the rear cog and then consistently every once in a while (depending on how fast I am pedaling) the chain bumps up and then goes down again. I took a video and posted it to youtube, hopefully it embeds properly. Any thoughts? I should also add that the I made my chain a half link shorter (or is that one link?) because my chain tool was being a jerk. One more think, it is most likely not a derailleur issue, I spent a good hour adjusting it and it didn't do anything to the problem. All the other gears are synced up just fine.

operator

Freewheel backwards and look for the derailleur jumping. Indicates a stiff link at that location.

drchinn

If that were the case, wouldn't it skip on all the gears? Also, the chain is new but the rear cog is original, perhaps its worn out?

UnsafeAlpine

It really does look like a mis-adjustment of the derailluer. Try tightening or loosening the cable adjuster a bit to set if there are any changes. Looking for a tight link is also a great idea.

DieselDan

You may have three problems at the same time:
stiff link
derailer maladjusted
worn cassette and chain

Your video shows a stiff link. Your thread description is of a worn cassette and chain, which usually shows up in the two smallest cogs first, as there are less teeth to spread the wear over.

drchinn

I already spent a ton of time adjusting it, then had two others try adjusting it, no one could fix it with adjustment. As for a stiff link, it's a new chain so shouldn't it not have that issue?

Here is another video if it helps.

UnsafeAlpine

Even with a new chain it may be a tight link. The tool you're using may or may not install the pin with the correct tension. Grab each link and move it back and forth. If one feels as though it won't move easily, that could be your culprit.

Honestly, it really looks like a misadjusted derailluer to me. You said you crashed and had to replace the chain. You way also have a tweaked derailluer.

BCRider

The two smallest rings are not very tolerant of even the slightest wear. If this is a very old cassette then it's likely that the teeth of the smallest cogs are worn or deformed enough to let the chain jump like that. It's a common failure mode.

They may have worked tolerably with the old chain because the wear of the old chain worked with the wear of the teeth. But now with the new chain it's less tolerant of the wear on the pressure face of the teeth and it wedges the chain up and off.

A new cassette will fix all this.

Squeazel

You need to get the right sized chain. See how the chain is having trouble releasing from the front of the cogs? It's sticking on the teeth, and I can't tell if the chain is too wide (sticking between the two low cogs) or too narrow (the cogs are wedging between the chain plates and sticking). If your bike is a 7-speed, get a 7-speed chain. Likewise 8 or 9 speed. There is no evidence that the cogs are worn out or that there's a tight link, looking at the video.

BCRider

OK so I looked at the video.

You're backpedalling it. You can't tell diddly from back pedalling. The chain lifting up can easily be due to the chainline depending on which ring it's on at the crankset. You need to set the derrailleur up with the chain running in the forward direction. If you set it by backpedalling then it WILL be out and you'll need to adjust it over again.

Does it climb off the teeth like this when you pedal forward? If so then you've got some serious issues.

drchinn

Only on the two smallest cogs. On both videos I had the chain rings lined up too.

drchinn

The chain I bought was a Shimano chain and it said on the box that it was for 6/7/8 speeds. On the front ring it says that it needs a narrow chain and I pointed that out to the shop mechanic. Perhaps he didn't realize something? I can always take the bike back to the shop, but I really want to figure it out for myself.

UnsafeAlpine

Why did you replace the chain after the crash?

ehilge

One thing that you might want to double check is the derailleur hanger if you crashed on the right side of the bike. The hanger is the piece that connects the derailleur to the frame. It could have bent in as a result of the crash. However, this does normally result in all of your gears having issues. It does seem more like a cassette issue to me. As mentioned before. Your old chain probably wore into the old cassette. Therefore the new chain does not mesh properly with the old cassette resulting in skipping normally under higher torque. Did you have issues with the shifting only after you replaced the chain or also immediately following the crash?

drchinn

No issues after the crash, just after replacing the chain. I'm leaning to the cassette being the culprit.

drchinn

Because it was exceptionally squeaky even after lubrication. I figured it was worth a shot.

BCRider

If it jumps around the same way even when you pedal forward (the climbing and skipping would be more along the bottom where it comes onto the teeth instead of the top) then you really should get the derrailleur hanger checked for alignment. Bends that you can't see with the eye will produce all sorts of odd shifting and chain flow issues.

As for the chain lifting up when you backpedal as you showed in the video if the freehub is running a bit sticky for whatever reason it will tend to make the chain do what you show. If you look at the teeth of the two smallest rings you'll see that the leading edge of the teeth at the top of the sprockets are angled back while the back edge where the chain pulls on when pedalling forward are more vertical. So if the freehub is a little sticky you'll find that these angles will lift the chain and you can get what you show in the video.

If you unmount the wheel and give the cassette a snappy spin backwards does it spin for a good 2 to 4 seconds or slow to a halt quickly?

DieselDan

Did you replace the cassette with the chain? A new chain on a worn cassette will skip. I can see a stiff link in your video, which was probably manifested itself when the chain was installed.

Squeazel

Yeah, maybe that's it- I never heard of an especially narrow 7-speed- maybe Cannondale did something different to keep a narrow hub spacing? If you have a pal with a 9-speed SRAM chain you could use (SRAM has quick-links, so they're easy to take off), it might be worth an experiment.

Mr. Underbridge

I vote hanger. I'd be more likely to believe it was a worn chain/cassette if it did it in the heavily-used middle gears, or if the chain was actually slipping under load. Here, we have a chain that's jumping a bit as it catches a tooth funny when pedaling under no load. Big difference.

The fact that it's doing it on the smallest cogs is telling too, because that's where the chain's most sensitive to alignment. My guess is that the chain isn't jumping because of a stuck link, because it's doing it way too often, much more than once per chain revolution. Rather, it's catching one of the profiled teeth on the 11 or 12t cog due to the hanger being misaligned. Add in the fact that there was a recent crash, and there's where my money is.

Note: I particularly recognize this behavior because my own bike used to exhibit the exact same thing, and it definitely has a bent hanger I haven't bothered to get fixed yet. I was able to tune most of it out, but every bent hanger displays different characteristics so no guarantee for you. You might be able to get rid of some of it by tweaking the b-screw to push the RD body away from the cassette, which will ease up the angle of the hanger misalignment.

cascade168

Yah. Geez, if you crashed, the first thing to check is the derailleur hanger. It takes all of 5 minutes to check and correct, if you have the ole DAG-1. I use this tool almost every day. Any shop can do this for you quickly and usually on the spot.

edit: Any time you have a situation where most of the cogs are shifting fine and a couple can't be tuned in, that almost always indicates a bent hanger.