I am running a 1995 seven-speed Alivio crankset and a 1995 Alivio 7-speed rear derailleur with a SRAM 9-speed cassette on a new, 9-speed hub. The shifters are simple friction. The rear derailleur has an adjustment barrel and the shifter does not.

The problem is that near the middle of the cassette, the chain skips back and forth between two cogs. It does this when the bike is on a stand and I'm pedaling by hand so I'm pretty sure it's not frame flex. I'm pretty sure it's the same two cogs every time and it's literally a back-and-forth, rather than just jumping to one and staying there.

Any ideas as to the cause? Is it a components mis-match (9-speed vs 7-speed) or simply an adjustment issue? Cassette and chain are brand new with probably a hundred or so miles on them.

Some possible causes are- The der guide pulley (upper) has too much rock/slop to maintain the chain's run onto the cog. This pulley is meant to have a small end play with out any significant rocking. There might be too much cable friction to allow the der to track where the lever wants it to be. the tighter spacing of 9 speed systems makes this der positioning more sensitive. The cog or chain might have a bent/twisted segment. The chain might have a pin sticking out too far or a side plate pulling off a pin, or if used a connectink link might be too wide. Some of the shifting quickness is determined by the cog to cog size difference. This is why, sometimes, the shifting is better on one portion of the cassette then the other portion. The der might not be hanging parallel to the cogs' plane. If the frame hanger or der cage are bent they can disturb the chain's engaging onto the cogs.

I assume that the chain and cassette are both 9 speed sized. The 7 speed der shouldn't be much of an issue IF you are using friction levers and also if the total range it moves through is sufficient for the 9 speed cassette. The wider cage might place the inner plate of the cage very close to the spokes when if the largest cog, but this is another issue/concenquence.

Are you able to hold the der (by hand either directly or with a little bit of cable pull) in a position that stops the chain from hunting between the two cogs? Can you spot a certain portion of the chain or cogs' circumfrences that the hunting happens at, consistantly? How about when back pedalling? have you tried a diferent cable/casing or rear der yet?

For a experienced wrench this shouldn't be too big a problen to figure out with careful examination. What that means as far as the cost to correct is yet to be determined.

Thanks for the suggestions. When I get home tonight I will examine the parts you mentioned and try some of the experiments to see if it narrows the problem down to something more specific.

The behavior you describe does sound like it is caused by a changing alignment of the drivetrain. I would only add, to the helpful suggestions you have already received, that it is possible that your chainring is bent or loose. (In addition, chainrings are frequently replaced with the cogs and chain as they wear together. Is it possible that your chainrings are worn?)

To test my theory, you might try wiggling the chainring, inspecting its "trueness" when rotated, removing the rings and inspecting them on a very flat surface, and seeing if the skipping occurs with every rotation of the crank or roughly thereabouts.

Good luck.

Andrew was pretty damn comprehensive, but incorrect IMO that you want lateral float on the upper pulley, since you're friction shifting (I don't even like it on indexed). Switch the Centeron-G pulley to the bottom, I reckon.

Also, I think cale is barking up the wrong tree; I have a hard time seeing bent or worn chainrings as the cause of your issue.

Here's what I suspect may be the cause, since pretty much the same thing happened to me when I switched to 9spd: excessive runout in your freehub. If you hold the cogs and spin the wheel, any oscillation should be barely noticeable, if at all. Mine was as bad as a crappy freewheel hub, so depending on where in its rotation the freehub engaged, the cogs would be anywhere from zero to a couple of degrees out, resulting in intermittent autoshifting.

Drove me nuts until I found the cause; since you're running friction shifting though, that rules out a whole class of issues I wasted a week or two trying to sort.

If it's a Shimano hub, you can swap out the freehub body pretty easily; pull the axle and use a 10mm allen key to get it off. There are two kinds of spline attachment it could be, and make sure to get an 8/9/10spd HG one.

Cassette runout would be more pronounced on the largest cogs, as the angle is constant but the radius is larger, causing more deflection.

Also re: freehub body runout- This is exactly why Shimano includes some upper pulley end play. To absorb/compensate for the various out of tolerances. Andy.

Whenever I encounter a rear shifting issue that doesn't respond to normal tuneing, the first thing that I do is to check the derailleur hanger alignment. That's what I would do with your bike too. Even is that isn't the problem you will at have eliminated it as a possible cause.

To do a quick and dirty at-home check, shift into a gear combination that makes your derailleur arm point straight down. Now prop the bike up against something and examine the derailleur from the back. If the derailleur arm seems to point toward the rear tire - that's it. You need to align the derailleur hanger. I can get a 7-speed to index reliably just aligning the hanger by eye but I need a gauge to align a 9-speed.

I didn't get a chance to ride last night but I put the bike on the trainer and it didn't consistently skip. I'll take it for a ride tomorrow and spend more time in that part of the cassette and see if it's still doing it.