Rear derailleur cable problem, or...?
 

My rear derailleur has started skipping gears and balking when shifting; since I bought the bike four years ago, I assume it's a stretched shifter cable. Trouble is, since the bike was new it's always dropped the second cog when shifting up – that is, when passing from third gear to second, it's always hesitated on second for a few cranks, then spontaneously fallen down to the highest gear. When I shifted back to second it would calm down and stay put. This was the only shifting problem I had until now.

Because I seldom use those gears and prefer riding to tweaking, I just lived with it. But now I find myself wondering if the assembler cut the cable housing to the right length, since I'd have to remedy that if I expect a new cable to solve the whole problem. The loop at the derailleur has always seemed flat; most bikes have a nice graceful curve there. (See photos, taken in highest and lowest gear.)

I'd appreciate any input on this issue. Could a too-short housing have caused the original spontaneous shift from second cog to first? For that matter, do you agree that the problems I'm having now are probably down to an old, stretched cable?


https://cimg3.ibsrv.net/gimg/bikefor...1943320ec9.jpg
https://cimg5.ibsrv.net/gimg/bikefor...18bc7fb5eb.jpg
Thanks.

Cables don't stretch. Undo the cable and pull it out of the housing, take a look at the housing ends. That's what happens. Check your hanger alignment. Housing length looks fine to me.

If it's only happening on one gear and shifting well in the others, the hanger is misaligned.

This is not correct. 99 times out of 100 the hanger gets bent in or twisted. This will have very little affect on the smaller cogs and will get progressively worse as you shift to larger cogs. It will NOT create a shifting problem in only one gear.

That doesn't agree with my experience. My old bike SRAM X9 rubbed on the 3 biggest cogs but shifted well on the others until I aligned the hanger. My current bike 12sp SLX rubbed on the side of the 2nd biggest cog and suffered from lazy shifting between 2nd and 3rd cog only until I aligned it.

Play around with your barrel adjustors and you should be able to get the shifting smooth again.

If the rear derailleur hesitates shifting up, you need to add some slack. Turn the barrel clockwise 1/4 at a time and try again.

If the derailleur hesitating when shifting down, you need to increase line tension. Turn the barrell counter clock wise 1/4 at a time, then try again.

You should find the barrel adjustor for your front derailleur on your shifter. You literally don't even need to get off your bike to make a successful adjustment.

if tinkering with these settings doesn't fix your shifting problem, maybe you bent something in the rear.

I'm sorry but that's the way it is. If the hanger is bent inwards the chain will get progressively closer to the next larger cog each shift. It will start to rub, then jump to the next cog. It's the only thing that can happen...it turns the situation into a geometry problem.
If everything is aligned properly the pulley cage is parallel w/ the cogs. If the hanger is bent inwards the line it creates is at an angle to the cogs and at some point will intersect the line created by the cogs. In the upper diagram the hanger is obviously straight, the chain will never rub or jump to the next cog. In the lower one it will. Not hard to grasp.

https://i.imgur.com/QM1NaoQl.jpg

Check the cable. Shift to the big rear cog, chain on that cog. Without turning the crank, shift to the small, now there's slack. Pull the housings free from the frame cable stops. Lots of slack. Move the pieces of housing and inspect all along the length of the cable. Push the head of the cable out of the shifter and inspect all around the swaged stop for any sign of a broken strand. Anything funky with the cable replace it. I would add oil and blow out the shifter a couple times as well. Check for any foreign junk in the shifter.

Check the hanger. I turn the bike upside down and look at the derailer cage in relation to the plane of a cog, bigger cogs with no chain are easier to see. You can pedal and shift while watching. Cage and cog should be parallel.

You may just be misadjusted as to the cable length by 1/2 turn or so on the barrel adjusters. Housings need to be fully seated in the ferrules, ferrules in the cable stops. Check the bottom bracket guide as well.

Lube the.cable as a first step. I use a light spray of wd40 at the cable ends. Tiny amount and watch it wick up the cable.
If the cable tension was good up to now how did it change?

4 years? Recable. Follow the instruction in one of any number of YouTube videos on how to adjust indexing.

But check your hanger alignment, too.

It is always tough to sort out when everything shifts fine except a single cog, especially since it has been shifting poorly in that 2nd position cog for 4 years.

If it were the first position, I’d be inclined to think you need to lube the cable because gummed cables don’t un-gum going to the 1st position cog as the rear derailleur return spring has less tension. It doesn’t hurt to pull the cable, clean the housing and install a new cable.

I’ve had a bad or poorly spaced cog in the past and for me it was more a matter of trying to figure out if that one cog spacing was off. If you have automotive feeler gauges you can check the cog spacing. Maybe re-visit the high limit screw.

It is frustrating and sometimes it is just going through each component to determine the culprit. If there was nothing glaring, and the hanger is fine, I’d start with checking chain stretch and replace it, if it didn’t fix it maybe even a new cassette.

John

Thanks for all the responses!
 

Sorry I couldn't get back to the thread until now...


In the meantime I did a complete rear derailleur re-adjust - deliberately messed it up, re-adjusted systematically. I'm still not delighted with the results - noises I don't care for, hesitations, last-minute refusals - but it's ridable.


In my experience, cables stretch as they get old, eventually becoming difficult to control. Normally I just replace them when troubles start, and I do plan to do that, but given that weird spontaneous shift I've had since Day 1, I wanted some advice on that first.


The chain is in great shape, stretch-wise - like new, actually. No doubt down to my weird obsession with chain maintenance, deep-cleaning it several times a season and compulsively wiping and re-lubing on the road. I would have replaced it anyway just to eliminate that possibility, if the original problem hadn't been there when it was brand-new.


I've been getting conflicting advice on lubricating cable housings. I've done it religiously since the 80s, each time I take the bike out, but now I see where some experts are saying you shouldn't (at least not until the cable is pretty old) because it creates gritty gum in the housing and messes up the action. Any road, my cable exits have been lubricated to within an inch of their lives up till now. I'll try the tricks suggested here to ream out that (apparently not too-shallow; thanks) hub bend.


The hanger checked out when I inspected it, but I'll look again.


As for those more recent shifting problems, that _is_ almost certainly down to age and nothing I haven't dealt with before, though this is my first indexed bike and it's fiddly. (For the young: the old friction shifters simply got mushier as the cable wore, requiring progressively deeper pull and tune, until at last they refused to stay where you shifted them. You replaced the worn bits - cable, housings, cogs, chain, jockey wheels - set the high and low preventers, and off you went.)


Anyway, thanks for the great advice; I've saved everything to a text doc and will run down the list.

With regard to your specific question about the length of the housing, the reason it doesn't have the big curve that you're used to seeing is that this somewhat new design (meaning within the last 15 years) orients the cable end roughly downward and rearward at the point of attachment to the derailleur. Most other derailleur designs have the cable end pointing forward and slightly down at the point of attachment, so the cable has to bend quite a bit farther around from the starting position of pointing straight rearward on the chainstay. To make that additional bend without introducing a tight radius kink, the path of the cable had to include a large curve. Some earlier designs addressed this same problem by using a pulley wheel to achieve a tight-radius bend rather than reorienting the cable attachment and lever arm.

I had the exact problem you described, except between 3rd and 4th gears. When I inquired about it on this site, I was told the cable inside shifter was likely frayed. I started to pull the cable, but noticed it wasn't frayed, and has only got about a thousand miles on it. What I did notice is the original Tiagra cable looked like it was greased, all except the last inch and a half, the part inside the shifter. As a learning experience, I tried rubbing a little grease onto the ungreased part, and reinstalled it. For the first time in a year it's not dropping shifts. I don't know why, and await the onslaught of posts on why you shouldn't do what I did, but for now I'm happy.

Thanks for explaining that! It's been bothering me. (Pavlovian response to all the bikes I saw prior to recently.)

Excellent info. Sounds like that might be my problem, too. I'll look for it when I change out my cable.