Hard to describe rear derailleur problem that probably has a simple fix

[jrenaut]

I have a new chain, a freshly cleaned not too old cassette, and a pretty new derailleur cable. I thought I had everything adjusted properly – shifting is pretty crisp. But under heavy torque (restarted when you’re stuck in too high a gear, for example), it’s like something slips and there’s a horrible crunching sound. Often it does it multiple times until I’m at an appropriate speed for the gear and not mashing so hard.

Is the derailleur cable not quite adjusted properly, or is it something else?

[Steve O]

@jrenaut 195274 wrote:

I have a new chain, a freshly cleaned not too old cassette, and a pretty new derailleur cable. I thought I had everything adjusted properly – shifting is pretty crisp. But under heavy torque (restarted when you’re stuck in too high a gear, for example), it’s like something slips and there’s a horrible crunching sound. Often it does it multiple times until I’m at an appropriate speed for the gear and not mashing so hard.

Is the derailleur cable not quite adjusted properly, or is it something else?

Does it do this in only one gear or more than one? Generally your cassette will wear most on the ring you ride in most. If that’s the one this is doing it on, it might be that you need a new cassette even though it’s just the one ring (I hate that).
You can fiddle with the derailleur cable and see. Turn it 1/2 turn, go mash and see what happens. Then try again. Both ways. You’ll find out pretty quick if that’s the problem or not.

[jrenaut]

You know, now that i think about it, it may actually be 3,000 miles on this cassette. But I didn’t have any problems before I replaced the chain and cleaned the cassette, so I figured it was something I hadn’t gotten quite dialed in yet.

[trailrunner]

@jrenaut 195283 wrote:

You know, now that i think about it, it may actually be 3,000 miles on this cassette. But I didn’t have any problems before I replaced the chain and cleaned the cassette, so I figured it was something I hadn’t gotten quite dialed in yet.

I’ve had this happen before – new chain on older cassette causes skipping. The theory is that the chain and cassette wear together, and installing a new chain on a worn cassette will cause some skipping. Sometimes I can look closely at the cassette and see that teeth have worn.

On the other hand, when it skips, does it actually change a gear? If it does, that would indicate your cable adjustment isn’t quite right.

[Boomer Cycles]

How many times have you changed the chain on that cassette? If 2X or more, it is probably shot, too.
I don’t know how overdue you were on the old chain, but it will eat away at the teeth and create slop when you put on a new chain with tighter tolerances, and the new chain will not seat properly into the worn cog teeth, creating a mismatch. The crunching is the new chain trying to seat into the bottom valleys of the old cogs.

It is also possible that the problem is a worn chainring; same mismatch problem. Check to see if the teeth are pointed (“shark teeth”) on either the cassette or chainrings. As @steveo says, most of the wear is usually concentrated in the middle cogs on a rear cassette where you spend most of your time pedaling. Depending on your cassette, you may be able to switch out just the worn cog (if they are not riveted together). If you have a 2X or 3X chainring, you may only need to replace the one you spend most of your time on.

And also check your jockey wheels on the rear derailleur; the bottom one has the most tension, and also develops “shark teeth” from wear (and should be replaced along with the the rear cassette, when the time comes). The top ones ten to round off the teeth, which eventually creates chain slippage.

Sent from Boomer_Cycles via my iPhone using Tapatalk

[n18]

I was told by a bike mechanic that putting new chain on a worn cassette makes the skipping worse; which was what I experienced. 3000 Miles is the chain life I am getting, but if you delay changing the chain, you wear the gears too. You could get 10K+ out of a cassette if you replace the chain more often.

Also, have you trimmed the chain to the same length? Longer chains skip more. Here is a chain length calculator, but you need to multiply the number of links by 2 since each link is exactly 0.5″ long.

You can tell if gears are worn just by looking at them. Here are some pictures:

[ATTACH=CONFIG]20554[/ATTACH]
[ATTACH=CONFIG]20555[/ATTACH]

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