Bicycle Mechanics - Rear derailleur cable tension - from too lax to too tight in a couple of shifts

Hello all. I'm hoping someone might be able to suggest where the problem with my rear shifting system lies. I've tinkered with the whole system for hours, followed setup advice in books (limit screws, indexing, B screw, etc.) and taken the bike to a mechanic, only to have the shifting fall apart a few blocks afterward.

I've replaced the following parts with brand new items:

1. Rear derailleur itself (Altus for Deore)
2. Derailleur hanger
3. Housing
4. Cable
5. Chain (length was determined using a few online calculators)

Whether it's the old Altus or the new Deore; the new or old chain, the problem is this: the cable has to be set so loose at the smallest rear cog that it almost strikes the chain in order for me to be able to shift upwards at all. By the time I'm close to the larger rear cogs it is almost impossible to push the shifting lever (and I wouldn't try). The cable by this time has become way too tight.

If I try to readjust and loosen the cable a tiny bit, then shifting around the smaller cogs is completely useless - the rear derailleur doesn't move at all because the cable is so loose.

Take it back to the mechanic. Or at least post a picture of the RD/cog area of the bike so we have some idea what you are dealing with.

At first hearing, this sounds like a compatiblity problem. Is the shifter also Shimano? Does the number of cogs (6/7/8) match the shifter?

Sounds like your low limit screw is too tight.

I'm thinking you did not upshift the shifter to the last gear before you anchored the cable to the derailleur. If you have any doubts, unscrew the anchor nut for the rear derailleur cable and press down the rear shifting lever repeatedly until you know it can upshift no further than re-attach the cable and see if it works better..

Thank you. The shifters match the 8 cogs in the back - both parts came with the original bike and they're Shimano too (the old cassette *was replaced at the lbs with the exact same one about five months ago due to wear). I'll try loosening my low limit screw now and see if that helps.

Onespeedbiker...sorry, I missed that reply. I'll try this as well.

Find the adjustment procedure on the Park Tool web site and follow it exactly. Can't fail.

In that procedure, you set the limit screws first, without using the shifter, independent of indexing.

Find the adjustment procedure on the Park Tool web site and follow it exactly. Can't fail.

In that procedure, you set the limit screws first, without using the shifter, independent of indexing.

Start at the beginning as if it is a new installation. DO NOT skip any steps. Adjust screws and barrel adjusters in 1/4 turn increments maximum, 1/8 turn preferred so you don't overshoot.

A couple of things you have said make me wonder about the chain length. I'm a proponent of the big/big, bypass the derailleur, figure the shortest place you could rejoin the chain, and add 1 inch of chain to that.

I've also seen some very strange things occur if the shifting cable isn't attached to the rear derailleur at the right place. They seem to be pretty finicky about exactly where the cable is attached.

The cable should not be loose with the chain on the smallest cog. There should be some preload on the cable.

Try this: shift the chain to the second-to-smallest cog. Stop the wheel, then click the shifter into high gear. The cable will go slack. Loosen the hold-down bolt, remove the cable slack (don't pull hard), and tighten the bolt. Now turn the cranks- the chain should shift into the smallest cog.

This will give you a baseline adjustment. Now fiddle with the barrel adjuster so the derailleur pulleys align with the cog. (It should be close.) Keep checking to see that one click of the shifter moves the chain from the smallest to the second-smallest cog and back again. Once that's done, check the rest of the cassette. It should engage/disengage all of the cogs equally.