A bike-fixing conundrum (your help is needed!)
 

Folks, I am desperate to fix my sister's bike before I leave to go back to my studies this winter; I have only a few more days and then it will be history -- she might take it to a (*gasp*) bike shop to get it worked on, and then all will be ruined. The problem is this:

The rear deraileur is inclined only to stay in a select group of gears at a time. If it is in the smallest cog (highest gear), it can't go any lower than sixth gear. All attempts at keeping it in fifth gear are futile: it just want to drift back to six or seven. I reset the cable tension, and readjusted it numerous times (like 40), and it was to no avail. The limits are set fine and are not interferring.

Earlier, before I reset the cable tension, it was stuck in gears 3 and 4, because I had jacked the tension up with the deraileur barrel adjuster while out on a bike ride, away from a functioning 10mm wrench. It basically has a 'window,' if you will, of gears it's capable of working in. Shifting down doesn't work, because seemingly there isn't enough tension, which leads me to this conclusion:

I think the shifters are to blame. The bike hasn't been used in years, but it's a decent Yokota girls' mountain bike, with Shimano Alviro parts. The shifter in question is one of those 'rapid fire' style shifters with the lever that has two parts, one for your thumb and one for your index finger. It doesn't seem to be pulling enough cable when the time comes, I notice -- it just pulls a little, while your thumb is pushing on the lever, and the display reads what gear you should be shifting into, but then when you take off the pressure from the lever, it reverts back into the higher gear you were trying to get out of. Could this be the culprit? A janky shifter?

Do you guys read me at all? This is my first post here, and I haven't read any of the other ones so I don't know how they are supposed to go as far as tech talk goes. I hope someone here can give me some advice as to what the hell is going on with this thing, because it's driving me crazy. Thanks.

Are the cables/housing clean and lubed? Poor shifting in the rear is often cable related. Pay special attention to the loop that goes from cahin/seat stay to the derailleur.

Well I can pull it fine with my hand when grabbing the cable from the exposed portion on the bottom tube, but that sounds like somthing worth considering.

Also I forgot to mention that the barrel adjuster for the shifter is frozen stuck real good; my vice grips can't even get it to budge. (Maybe that's not so important, but it sure is annoying!)
Thanks for the tip.

Her Rapid-Fire shifters, which never were that great, are indeed shot. The internal ratchet is not holding and releasing properly, and most low-end Shimano units are not be designed to be repaired. Consider replacing them with friction-mode linear action thumb shifters or SRAM grip-shifts.

flairpin, unless you feel competent enough to strip it down it might be time to embrace the bike shop that you seemingly abhor so much. :)

The barrel adjuster must turn, for fine tuning of gear indexing is very difficult without it. You should consider removing the chain and cable and looking from the rear of the bike check exactly where the hi/lo screws are aligned. Those'll get you say, 90% of the way, the remaining 10% the barrel adjuster can sort out. If the barrel adjuster is seized, then maybe the shifter needs to be looked at as it's possible that there could be something wrong in the shifter that is causing the cable not to move correctly anyway?

Either way, from where I sit, I think you need to get it back to basics and take it from there. A new cable will do no harm. Don't neglect the b-screw that swings the rear mech closer to the cassette. As you can see, there are certain things that need to done or looked at.

Has the bike been bashed in one way or another? Maybe the rear der hanger is bent/buggered for example? I'm just trying to steer you in the direction of looking for serious mechanical problems.

Peter.

yeah, I checked for bends in the rear deraileur cage and hanger while doing this stuff, and it looked a tad bent (but what do I know), so I 'manually' readjusted it, if you catch my drift.

I haven't heard of physically removing all forms of tension on the r. deraileur in order to check for the adjustment of the limit screws. HOw does that work exactly? Because I already attempted to reset the cable tension (letting it go, and then starting all over) as best I could (keep in mind what kind of hell this can be when working without actual bike tools), and it didn't do anything but alter the 'zone' I was referring to of actual shifting range (from 3-4 to 6-7). From what I can tell without undoing the cable again, and removing the chain (which I can't do here w/o a chainbreaker), the limits are almost impeccable and aren't interferring with anything.

I will look into that one screw in the back you were referring to, but I don't think that's the issue. Changing the cable might help, but I have been out of town recently (hence the lapse in replying here), so I haven't been able to mess with it lately.

I appreciate you help, bokkie, but I am leaning towards the assumption that the real problem is in the shifters and or cable. And just so you know, I can work the tension pretty well with just the rear barrel adjuster, but the fact that the shifter-located one is shot is good enough reason to take it to a bike shop and have them mess with it. My sister is no bike rider, and I am weary about putting some friction shifters on the thing; Hopefully the bike shop will have the right grip shifters (man, those things are THE BEST). Okay, enough talk, time for the triflow to start flowing. Thanks for the help.

my friends bike was doing the same thing as was mine. I have no idea how we fixed it we just kept on adjusting the high and lowsets and it finally worked

That's right. The first aspect of setting the indexing correctly is to get those two puppies aligned as close as possible. For my SRAM XO rear mech it had to align directly inline with the biggest cog, and at the other extreme, it had to align to the outer face of the smaller cog. That gave me usable indexing and smooth changes almost immediately. I suppose there is the potential to pull the cable too tight when locking it down in the rear mech and by doing so you can bias the movement before you start! I had a little bit of bounce between say 4-5-6 but adjusting the barrel adjuster resolved that. I don't think I've suffered a missed index since I made those adjustments.

I must admit I am a big fan of grip shifters. The SRAM XOs are positive and actually having numbers for gear positions suits me, especially when I've been up-and-downing so often that I simply don't always know where I am.
:(

1.Flip the bike up-side down.
2.Disconnect the gear cable completely.
3.Pedal the bike and move the derailleur in and out with your hand.
4.Set the limit screws in this manner.

DERAILLEUR DIAGNOSIS:

Now ask yourself...is the derailleur still hanging up on the lower (bigger cog) gears and not dropping down? IF the deraileur is moving through its rang of movement with ease the problem is with either the cable or lever.

CABLE DIAGNOSIS:

Remove the cable. IF there is any sign of rust or extreme dryness anywhere on it...REPLACE IT. Its only a few bucks.

LEVER DIAGNOSIS:

This one is more difficult. It MUST be done with a NEW cable.
1.Install the cable, but only the part going from the lever to the frame.
2. Grab hold of the cable real tight and click the index finger part lever 7 times. This is to get hte lever into high gear.
3. Now, with lots of tension from you pulling on the cable, shift through the index positions one at a time.
4. There should be 6 difinitive clicks, one for each gear plus the gear you started in making 7.
5. Now shift back down, again there should be 6 clicks.

If there isnt 6 clicks up and down the lever is skipping over one of the ratchets because the ratchet mechanism is broken. off...

Sometimes its possible to shift to the lower gears with the broken mechanisn because theres still a little 'nub' for the ratchet to catch hold of.

If its ratcheting ok, blast the inside of it with some spray lube, lube the cable real good and you should all set...