Ave

My first post on BikeForums.net!

Mechanical question: I’m using a 10 speed cassette (12-32T Shimano Deore XT) along with a triple crank in front (Shimano Ultegra 30/39/52T), KMC 10 speed chain, Shimano 105 10 speed rear derailleur. Shimano 10 speed thumb shifter on the handlebar.

I’ve used this setup for a while and it has worked well. In recent months, I’m finding that the chain does not drop cleanly into the smallest cog of the cassette.
It’s a nuisance. At home on a workstand, sometimes it drops fine, sometimes not.

When I bought the 105 rear derailleur, I was told that it was designed for a maximum rear cog of 30T. But I found that it worked fine with my 32T granny cog.

Prior to installing the 12-32T cassette in the last year, the largest cog I had used was a 30T - with no issues such as this.


I’m a DIY type of person and have done most of the work on the bike.

I checked the hi and low limits to be sure they were OK.
I replaced the rear cable housing (in the back) and shift cable. The front cable housing seemed fine, no resistance at all when moving the cable through it.
I’ve loosened the tension on the rear cable to the point where the chain will still move cleanly up to the second smallest cog. No further.
I’ve been using the chain for a while but it still has a lot of life left according to my measuring tool.
The cassette does not appear to be worn.
All other gears/cogs shift up and down cleanly. It’s only the smallest cog that I’m having trouble with.

Could the problem have anything to do with:
a. The spring in the rear derailleur stretching a little further than designed to reach the large 32T granny cog?
b. Wear on the chain or cassette?
c. Other?

Thanks, I appreciate your feedback!

fietsbob

When are you demanding the shift? Is the chain tension near zero?
getting momentum, spinning, going into the gear before you start up?

Aubergine 

woodcraft

The most common issues are friction in the rear cable loop- make sure that the length is right

to avoid any angle where it goes in to the RD, & hanger alignment- can be bent inward. The alignment tool is worth the investment if you do your own work.

A pro tip that I have seen is to add a washer under the RD mounting bolt to move it outboard slightly (reset the limit screws).

This gives a bit more spring tension at the outboard end.

chicagogal

Frayed cable? That is almost always the answer for me when shifting suddenly gets wonky.

Ave

Thanks all for the quick replies.

Fietsbob: When I demand the shift, it is sometimes under pressure and sometimes relaxed. It does not seem to make a difference.
It also doesn't change when using the middle chain ring or the large chain ring - happens with both. All other cogs shift fine - the small cog is
the issue.

Aubergine: I do have a rear hanger device to check and see if the derailleur is aligned properly. I'll check that.

woodcraft: Over the years, I've tried to cut new rear housing the same length as the original - maybe a slight variance over time? How do I make sure the length of the housing is right? It makes a nice loop and goes straight into the rear derailleur. Thanks for the tip to add a washer under the RD mounting bolt! I may try that.

Other suggestions are welcome!

base2 

Cable fray in the shifter.

fietsbob

waiting late , and it wont drop because you are on the hill, you can make a U turn go down, get it into the climbing gear

make another U turn and give the hill another go..






Front indexed .. ? flush old grease out and apply fresh in the shifter..




....

Maelochs

When he specified "smallest cog" I assumed he meant the 12T.

Ave

I installed brand new cable. Also, the old one was not frayed. Thank you.

Ave

Fietsbob: The shifter is 10 speed indexed. There is no grease in the shifter that I'm aware of?

Maelochs: Yes, the smallest cog, 12T, closest to the derailleur.

Maelochs

Not sure what to say here .... Most Shimano derailleurs can handle a couple teeth more than rated .... and this had been working for you. I would think that it wouldn't take much spring to bring the derailleur back down with no cable tension ... but i suppose it is possible the derailleur is just old. Does it swing far enough when you move it by hand?
@fietsbob was, I think, speaking of the STI shifters, for people with integrated shift/brakes, or the thumb-shifter body. Sometimes they gunk up inside but some WD-40 can blast out the old grime and crud and grease and rejuvenate them.

fietsbob

Awful lot going on in there to have dry metal to metal friction..

Ave

Maelochs: Interesting to learn that most shimano derailleurs can handle a couple of teeth more than rated. That's good. Derailleur is not that old, maybe a year or two.
When I release the cable tension and move the derailleur by hand, it does drop down into the small cog OK. But I think I'll also check that again.

I might also consider the suggestion to open up the shifter and spray with WD-40.

Thanks!

Iride01 

If you have inline adjusters for cable tension, you might be taking too much slack out of the cable. Or you might have pulled and clamped the cable too tight if you readjusted the old one or installed a new one. Or, you might just need to tweek the limit screw adjustment a tad.............

Maelochs

if you can move the derailleur enough by hand that it shifts but it won't on its own, I'd think the cable was a little too tight.

I always try to recall, "down the cassette, looser, up the cassette tighter," when I am trying to fiddle with my barrel adjusters to good effect. Then I forget which way loosens and tightens and have to start over.

Ave

Thanks for the additional comments and suggestions!

Update: Yesterday, I decided to install a new chain along with a new cassette on another rear wheel I have ready and waiting.
Rolled around the neighborhood and still discovered the same problem, not dropping consistently and cleanly into the smallest cog.
I worked with the barrel adjuster making it too loose and then tightened just to the point where the chain would lift nicely to the second cog.
But could not resolve the fundamental problem, which leads me to suspect the 105 rear derailleur's ability to move the chain down. I then removed
the derailleur to see if a small washer could fit around the bolt and noticed there was a tight clearance - I may take the derailleur to a hardware store and see if I can find the appropriate washer, and then after installing, I would reset the hi and lo limit screws.

Another idea: I have a new 105 rear derailleur waiting as a backup. Perhaps I should install that one and see if there's a difference?

I have not yet done anything with the thumb shifter which seems to work really well and has done so for thousands and thousands of miles.

Maelochs

Thanks for the update, sorry for the frustration.

having a big big cog shouldn't affect the ability of the derailleur to drop to the smallest cog. The only issue with too big a big cog is that the derailleur might not be able to stretch the chain far enough--it has to move in and down to get the chain around the big cog, and if the lower arm is too short it simply won't fit. Usually a little space can be gained by adjusting the B-screw. Still, the derailleur is returning to a "rest" position---untensioned---when dropping to the smallest gear. The size of the big cog shouldn't matter.

if it was me I'd look really closely at anything which might be impeding the derailleur's swing. I would probably blast WD-40 into all the derailleur joints and work them a lot in case some old grease gummed up. I would probably use some language best not used around children.

If the derailleur is thousands of miles old, its main spring might simply be worn out. i'd try the new one, and also consider the washer ... squeeze another few miles out of it maybe. I have never looked into rebuilding a derailleur, but maybe that would be a fun winter project ... for someone else, not me, but who knows?

As I type this, i have an unlocatable ticking noise in one of my bikes ... it is easier to commiserate with you than to spend another hour (probably the 20th or 30th) doing everything i can think of to find and fix that problem

Iride01 

When you had the cable loose and the chain still wasn't dropping down fully, did you adjust the limit screw for the high/top side on the derailleur?

CliffordK

I've had the issue. I always thought it was a poor design of the new indexed shifters, and the first click should pull twice as much cable. Friction shifters, one can just let the cable go slack, and has less problems.

I like the idea of adding the washer. I may try it.

Setting your B-Screw to get the upper jockey wheel as close to the sprockets without touching through all gears may help by minimizing chain flex.