This is on a Raleigh m80 mtn bike. Truvativ blaze crank/chromo chainrings, deore front LX rear derailleurs.
When in the smallest rear cog and the middle chainring (mostly) the chain will act like the derailleur is out of adjustment and jump around. When put under load, the chain skips violently and sometimes starts to jump off the chainrings. Sometimes one link of the chain will get stuck on a shift ramp between the middle and largest chainrings. Also while under load there seems to be a noise like bad bearings coming from the drivetrain, and you can also feel vibrations under load that kind of feel like when a headset has been overtightened. The BB is smooth as butter without a chain. The chain does have a bit of sand and grime inbetween the rivets, but it should not be worn out.
The first time i noticed this i also noticed the BB was loose. I tightened it, and the problem was gone for half a ride. It started again, I checked the BB and it's still tight. Pedals are tight, crank arms are tight.
I did notice the middle chainring is slightly warped/bent. obviously the chainring needs to be replaced, but would this cause the problem i'm describing? Would this have worn out my chain as well? I have ridden a couple times like this but mostly stayed out of this gear that was causing problems.
I would replace that middle ring or remove it from the bike and try to straighten it. Removed, you can lay it on a piece of glass to check if you got it straight. Also check the other chainrings for burrs and file them off carefully.
Check your rear derailleur stops, particularly the one with the "H" next to it.
Check your chain for stretch. Just throw a ruler up against it and see how much longer it is from pin to pin at 12", or put it on the big ring and pull the chain away from the ring toward the front of the bike. If you can expose the end of one of the teeth, it's stretched.
Check the front derailleur cable tension. It sounds like it could be rubbing in the middle/small cog combo.
I forgot to add the most odd symptom. Occasionally I will hear this "ping" or ring sound. For the longest time I thought it was the ends of my shoe strings hitting a chainring until it happened while the bike was on the bike stand. I also just noticed a dull continuous resonating ringing sound while i'm pedaling. It kind of sounds like a quiet pitch fork. Only happened a couple of times now.
I replaced the chainring, and no change in symptoms. It only ever happens in the middle/small combo, every other gear combo rides and shifts perfectly. The limit screws are perfect as well. I am out of ideas here, other than replacing the chain, though I dont think a bad chain would cause a problem in only one gear combo.
The ringing sound, sort of like running your fingers around the rim of a wine glass, is most likely your disc brake rubbing.
As for the rest of your woes, look at the chainline when the bike is in the stand. The middle/small combo may be so angled that you are catching the edge of the second cog or the front chainring, or burrs on either.
If that's not it, my guess is your chain is stretched and/or your smallest cog is worn. Yes, a stretched chain will give you problems in just one gear combo, especially if it's the cog that has the least amount of teeth, therefore the first to wear, and if it's the middle ring which almost always wears out first due to every shift up or down going through it.
The way you talk about your chain makes me wonder... The info I pulled up that matches the specs you've given shows the M80 as a 2005 model. Have you replaced the chain since the bike was new?
yes this is an 05 model which I purchased just over a year ago on closeout. I have not replaced the chain, but I have less than a thousand miles on the bike. I keep the chain pretty clean as well, but from what i'm hearing it's the next logical thing to replace.
I dont think chainline angle is a problem as this problem is recent, and the chainline shouldn't have changed. Also when in the small chainring and small cog with an even steeper chainline angle (dont worry I know this is cross chaining and do not ride this type of gear, just as a reference on the stand) there is no problem.