Bicycle Mechanics - Rear derailleur/gear cable - tension coming from where?

Hi guys,

The problem I'm having is that the derailleur is not pulling the chain down onto the smallest cogs. When I change the shifter up to 8, the cable is untensioned on the top tube but the chain is stuck on the 3rd smallest cog. Occasionally a bump in the road will knock it down another cog, but generally it's only going 1-5 (on the shifter numbers).

It appears that there is tension on the cable somewhere between the top of the seat stay and the derailleur itself. Where to I start to debug this? Does this sound like a cable guide problem, or do I need to reset the limit screws (which appeared to be correct)?

TIA.
Al.


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More information...

I've recently replaced my rear gear cable with a new one. I originally did not dial it in correctly, so while the H & L limits seemed correct, it didn't seem to be indexed correctly - chain jumping back and forward between middle cogwheels. A guy at the LBS took a look at it and, in front of me, reconfigured it from scratch. He had it shifting perfectly through all gears. I took it home, worked great on the ride home.

Next day, after about 35-40 miles I had some trouble with the cable. The plastic sheath had popped out of the guide on the back end of the top tube (4300 runs the cables across the length of the top tube and down the seat stay). I released tension on the cable (into lowest gear) and put the cable back into place.


Revelant bike spec: Trek 4300, STI integrated brake-shifters, Deore front mech (2-3000 miles) , Alivio rear derailleur (3-4000 miles), new Shimano 8spd chain, new rear Shimano 8spd HG cassette, original front gear cable (3-4000 miles), new back gear cable (~80 miles).

If the cable is loose at the top tube then see if it is tight over the seat stay. If loose at the seat stay, then is it tight at the derailleur. Ths sticking should be in the last piece of housing the cable went through before it got tight. If the cable is loose all the way to the derailleur then the derailleur is suspect. My first guess would be the loop of housing running from the derailleur up to the lower end of the seat stay.

If the cable is loose at the top tube then see if it is tight over the seat stay. If loose at the seat stay, then is it tight at the derailleur. Ths sticking should be in the last piece of housing the cable went through before it got tight. If the cable is loose all the way to the derailleur then the derailleur is suspect. My first guess would be the loop of housing running from the derailleur up to the lower end of the seat stay.
Do you drink sports drinks? Do they slop out the top and down the downtube or seat tube? I tmay be that the stickiness has dried off on the bottom bracket cable guide causing the cable to stick. Changes to larger cogs are always fine; changes to smaller cogs iffy like yours. Clean out the guide and the cable, and lube with, say, a silicon lube.

You replaced the cable? How did you cut the excess off? Did you do this before threading the cable through? If so, it may be that an errant thread of cable has worked back into the loop of outer from the chainstay to the derailleur. This can cause similar symptoms to what you have.

There could be other problems associated with this.

No to sports drinks, I'm a plain h2o fan. I didn't cut the cable pre-install (sounds like a real bad idea)... but I forgot that I had to replace the final section of cable housing because I lost the previous one. It does sound like something to do with that section, I'll remove it and make sure the cable is lubricated there for sure. Lithium grease ok to use on that bit?