ScottCommutes

I am pretty good about working on bikes, but rear derailleurs are a bit of a mystery to me. I successfully changed out, sourced, and adjusted one once when its chain tension spring broke, but that is really the limit of my experience.

So I've been riding my wife's older 21 speed mountain bike every day to work while I tackle various problems with mine. It was gradually getting harder to shift into the biggest gear of seven on her rear freewheel until yesterday. All of the sudden, I was stuck in that big gear. The shifter moved freely through all the clicks, but the bike did not shift. I assumed the cable had broken, and rode it home to fix it - No biggie as I still had low/middle/high on the front shifter. Drivetrain is Shimano.

When I go to replace the broken cable, I discover that the cable is fully intact and the shifter is shifting again. However, now I can't get back onto the big ring in back unless the front shifter is on the smallest gear, even if I physically try to push the derailleur over.

Thoughts? Do you think I have a weak (stretched/failing) cable? Or do I have a weak derailleur spring or something? Or just more adjustment?

choddo

When were the cables last replaced?

FBinNY 

Let me confirm.

You couldn't get I to the big/big combination without crazy force. Then once you did, it was stuck there.

So, a question ----- did you replace the chain or freewheel lately? Maybe simply swapped the wheel from another bike?

I ask because the problem is most associated with a chain that's slightly too short. If it's not always been this way, then something changed.

ScottCommutes

The cables were last changed during the pandemic. My wife, however, hardly ever rode the bike - it stayed in the garage until I started riding it to work this past week.

ScottCommutes

Not exactly. I couldn't get onto the big ring on back from the small or middle ring on the front without excessive force. I did get stuck there.

Nothing whatsoever has been changed on the bike in years - it's my wife's and she barely touches it.

Andrew R Stewart 

By "biggest gear" do mean the tooth count or the pedaling effort? As they are opposites this clarity is very fundamental. I'll assume you mean physical diameters (tooth counts) of the rear cogs.

Cables can stretch (actually casing compression), rust, get gritty/grimy, develop kinks in either the cable or the casings, and slip at their attachment clamp. If the cables are routed outside of the frame it is fairly easy to follow the path and move the cable in each section of casing to test their conditions. Kinks act as a spring inline with the cable and make shifting into the lowest (as in effort, which is the bigger diameter rear cogs) gear less responsive. As choddo alluded to, replacing the cables and casings (and doing with the right parts and skills) can solve a lot of shifting issues without knowing more. I also wonder if the temps were freezing as water in the casing can freeze locking up cable movements yet freed up later when that water melted.

There are other reasons why shifting can be bad. Besides the usual stuff limit screws, chain/teeth conditions, hanger alignment. Most are either wear or incident caused. One often overlooked reason why shifting into the lowest rear gear (that big cog) is from guide pulley bushing slop becoming so great that the top of the pulley can rock away from the cog it's assumed to line up with when shifting. I still wonder about the BB play and shifting being of the same cause. Andy

Andrew R Stewart 

It seems that what has changed with the wife's bike is that it's being used and likely by a stronger and "higher milage" rider. Andy

ScottCommutes

You are correct to assume that I mean in the sense of physical diameter and tooth count.

I am about 99% sure that I can replace a cable competently - I have done that before several times on other bikes It seems like a no-brainer to start there, and I already have the parts.

FBinNY 

OK, so we move on.

It's possible that the RD rides too close to the freewheel causing interference between the jockey wheel (upper pulley) and largest sprocket.

There should be visible daylight and about 1/2" minimum free chain between the RD and sprocket. When there's less the RD is trying to jam the chain against the sprocket rather than leading it on.

So, shift to the 2nd biggest sprocket then eyeball the situation, checking that the pulley is forward and below the sprocket and free to move across without touching.

Andrew R Stewart 

Have you tried to shift the rear with only your hands pushing on the der, therefore removing the cable completely for the der's function? Do you have chain droop in the small/small cog/ring combo? Is the guide (the top one) pulley contact the underside of that low gear cog (that big one) and trapping the chain from lifting off the cog teeth (this would require the pulley to not be in contact)? Is there a lot of slop in the der parallelogram or the pivots? Andy

Iride01 

Some of the issues might be lack of use. And things just needed some movement to let them loosen up. However not being able to get back into the big cog on the rear, even when you push on it, might be that the low limit either was changed or never quite right in the first place and never presented itself till you used it.

Might be some other stuff going on too that is confusing the issue because there are really several issues. You might not need to change the cables yet. Maybe adjustment is all that is needed when you solve whatever is keeping you from being able to push the RD onto the big cog.

Why at first it wouldn't shift out of the big cog is curious. Because if the shifter was giving the cable slack to let it go to the smaller cogs, the spring tension should have moved it there. In fact, if the cable breaks, your RD will or should go all the way to the smallest rear cog.

With a 3x front, on the two smaller rings, the chain is at a better angle to get onto the big rear than when in the big front ring. So if the low limit is what kept the RD from going into the big rear cog when you were in the big front ring, it still might be that the angle of the chain on the smaller rings was letting it grab enough of the cog teeth to make the shift.

Mr. 66

I would flush all the pivots and pulleys on the rd with a light lube, and lube the chain.

Kontact

I'm sorry if I missed this, but is the chain long enough?

FBinNY 

Yes, that was confirmed earlier. The problem is happening on the middle ring.

Kontact

Do the cables go under the BB? Is the cable guide under the BB loose and swiveling?

MarcusT

I had a situation where the cable was getting caught up for no visible reason, until I removed the cable and looked at the housing. The person who assembled the bike cut the housing with a crude instrument and it left a spur . The cable was getting caught on this spur and eventually wear out the cable.

Andrew R Stewart 

Which is why I suggested tracking the cable path and testing each location the cable travels in any casing or around any guides with only one's hands moving the cable. Andy

Jicafold

I am with Andy. Cables and cable housing is cheap. I would replace both the cable and the housing with new quality items to include die drawn cables. When you cut the ends of the housing make sure that the opening is patent without any burrs.

choddo

Not heard “patent” in this context before. What does that mean?

KCT1986

Patent is sometimes used in the medical field regarding the clear passage through vessels.

FBinNY 

There's a big clue here that pretty much rules out anything cable related. Assuming the OP is telling us the whole story accurately (which rarely happens) the problem only occurs with shifting into the largest rear sprocket. That implies something within the RD or between the RD and sprocket. (see post 9 for my suggestion).

Of course, it could be a kink or fray in the cable binding qt a particular spot, but that kind of thing is rare.

To the OP

Try shifting by pulling the gear wire away from the frame along the down tube. If it shifts fine, then it's cable related, most likely in the shifter itself. If it's still difficult and binds, then it's the RD and/or freewheel clearance.

wheelreason

My HVAC mentor says " be a tech or a part changer, you can't be both". Detach the cable from the dlr and follow the multi meter readings...

ScottCommutes

For various reasons, I still haven't started messing with that rear derailleur yet. If you number the gears 11, 12, 13, 14, 15, 16, 17, 21, 22, 23, 34, 25, 26, 27, 31, 32, 33, 34, 35, 36, 37, I am only missing 11 and 21 (maybe 31 but who cares) - not a great loss in the grand scheme of things. Everything else works perfectly.

Anyhow, this weekend it stormed and I rode the bike through some flooded streets on the way to work, so now regreasing the bearings is a bigger priority than the derailleur......

Jicafold

The pandemic is not over yet. It may never be. So this is kind of vague. Regardless, as you say you haven't messed with the rear derailleur yet, consider removing it, giving it a good cleaning with a quality degreaser and toothbrush, lubricate, and reassessing it's limit adjustments.

MrGastrognome

This is a common issue I see during free bike repair clinics our local nonprofit holds. Especially OP's trigger shifter clicking and not pulling any cable to move the derailleur. Assuming the RD passes the range of motion test that Andy recommended, I would think it's old, gummed up lube in the shifter itself. Open up the shifter's cover (beware flying parts, losing tiny scres etc.) and flush the shifter's gears with PB blaster, solvent, boiling water or whatever you've got that will loosen up gunk while working the triggers up and down through the clicks repeatedly until the triggers engage pulley mechanism and it starts pulling cable again. Then relube and reattach the cover. Not foolproof but this technique has worked miracles on some very scabby old bikes.