# Chain drops to highest gear (small cog) when down-shifting



## ki5ka (Dec 17, 2006)

I posted this last year in drive train and have had no responses so I'll try it here. I think it might be a "tandem specific" problem related to frame flexing, but I'm not sure.

Problem: Chain suddenly drops all the way across the cassette to smallest cog when downshifting. Seems to happen exclusively when transitioning from a short decent to a climb.

The specifics from today, which are typical: 
Steep short decent followed by a short steep climb
I shift to small ring and middle of the cog (5th) in preparation
Complete decent and multi-shift for climb, backing off pressure for the shift and completing at least one full rotation
Apply pressure to climb and this ugly noise thing happens, occasionally accompanied by a broken chain (like the last time)
Derailleur is in 5th gear, chain is in 9th, as shown in pic below








We have been living with bad shifting for a long long time. Especially in middle gears, we will have unasked for shifts, half-shifts, or no shift when shifting. No end of "tuning" seems to help. One shift, there will be too much tension, the next, not enough. Can't figure this out.

-SRAM X0 shifter 
-SRAM X0 rear derailleur
-SRAM PC 991 Chain
-2 new cables
-3 different wheels
-Now with Chris King hubs and FunBolts for rock solid solidness ;p
-4 different cassettes
-Derailleur adjusted and checked for damage
-Same Captain 

Any ideas?


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## wschruba (Apr 13, 2012)

You don't happen to have a midship tensioner on there, do you? Is be willing to bet it's a chain tension issue that the rear derailleur just can't deal with.

I would also check the chainline to the rear hub, as well as the alignment of the jockey wheels with a machinists rule.


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## Okayfine (Sep 7, 2010)

How are you determining chain length? Rear derailleur tension spring stiff enough? RD set up correctly with respect to the b screw? Any chain tight links due to past miss shifting?

Your OP says you shift in preparation for the climb, but then that you also complete the descent and then multi-shift for the climb. Where is the shifting taking place?

Shifting the RD more than one cog at a time has been a recipe for broken chains for us. But since I've been doing the service on our ECdM, I haven't had problems with shifting. And I do a few dumb shifts per ride, no doubt.


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## ki5ka (Dec 17, 2006)

wschruba said:


> You don't happen to have a midship tensioner on there, do you? Is be willing to bet it's a chain tension issue that the rear derailleur just can't deal with.
> 
> I would also check the chainline to the rear hub, as well as the alignment of the jockey wheels with a machinists rule.


Nope, no tensioner, and I'm thinking chain tension as well as alignment is at play.


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## ki5ka (Dec 17, 2006)

Okayfine said:


> How are you determining chain length? Rear derailleur tension spring stiff enough? RD set up correctly with respect to the b screw? Any chain tight links due to past miss shifting?
> 
> Your OP says you shift in preparation for the climb, but then that you also complete the descent and then multi-shift for the climb. Where is the shifting taking place?
> 
> Shifting the RD more than one cog at a time has been a recipe for broken chains for us. But since I've been doing the service on our ECdM, I haven't had problems with shifting. And I do a few dumb shifts per ride, no doubt.


I think it is probably Sheldon Brown's advice for chain length that I follow. Straight on the largest combination plus 2 links? I always look it up when I do it, so I'm not sure that's right. [1 link, just checked. Maybe I should double check this on the bike]

Inadequate tension on the derailleur seems to be a likely cause. Is there a way to increase it?

I prep for the climb by shifting to the small chainring ahead of time, paired with the appropriate upshift in the rear, careful to keep in the middle gears, then shift only the rear as I hit the bottom. I try to make sure I get at least a full revolution in before there is tension, but it doesn't always work out that way. The pic you see from today happened when I was being particularly careful and it seemed like it should have been a good clean shift, but it still happened.


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## Okayfine (Sep 7, 2010)

Chain length test you mentioned is the standard.

No way I know to increase the RD tension. The new RDs seem to be following Shimano's Shadow Plus logic.

I know that with my chain length and RD, if I'm in granny and in the smaller gears of the cassette, there will be a LOT of slop in the chain and the RD can't take it all up. I'm using a mid-length RD cage, so you might check to see what your RD is. Perhaps moving up to the long cage will help especially if you have a short cage.

That said, you may just be in the wrong ring up front. Instead of granny up front and middle in the cassette (5 of 9), perhaps the middle ring and towards the big end of the cassette (2 of 9) would eliminate the slop in the chain and give you the same basic gearing.


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## wschruba (Apr 13, 2012)

If you disassemble the main pivot on the rear derailleur, there is a second, higher tension slot to put the spring in. I believe XO is held together with a machine screw stop? If you think the derailleur can take it, you can even remove the stop and try winding the whole thing forward another revolution. It is all a moot point, though, if there's chain droop in the smallest sprocket combinations. If you didn't want to change the largest chainring to enable a shorter chain, you would need a tensioner.

It's difficult to see from the fish-eye distortion, but it also looks like the derailleur hanger is twisted, which would also cause what you're describing.


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## ki5ka (Dec 17, 2006)

Okayfine said:


> Perhaps moving up to the long cage will help especially if you have a short cage.


Have long cage.



Okayfine said:


> That said, you may just be in the wrong ring up front.


It's a very steep climb followed by a switchback. Can't make it in middle ring, but good thought.


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## PMK (Oct 12, 2004)

I would ensure the cable runs are proper and of good length to allow no ghost shifts. In particular focus on the linkage area for the rear suspension.

Aside from that, I would suggest tossing out the much respected book and shorten the chain.

Considering it is a suspension machine, set chain length to allow use of all cassette choices in the middle, about the five smallest when in the big ring and about the five largest when in the granny.

Normally, if the chain is optimum, remove a set of links and reinstall the master if you run SRAM chains.

Honestly though, double check the cable length in the linkage area first, then mod the chain.

All the best with it.

PK


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## PMK (Oct 12, 2004)

wschruba said:


> If you disassemble the main pivot on the rear derailleur, there is a second, higher tension slot to put the spring in. I believe XO is held together with a machine screw stop? If you think the derailleur can take it, you can even remove the stop and try winding the whole thing forward another revolution. It is all a moot point, though, if there's chain droop in the smallest sprocket combinations. If you didn't want to change the largest chainring to enable a shorter chain, you would need a tensioner.
> 
> It's difficult to see from the fish-eye distortion, but it also looks like the derailleur hanger is twisted, which would also cause what you're describing.


Possibly, and I may be wrong, but the twisted appearance is on account of the derailed chain resting on the RD body and not the pulley.

Checking the hanger alignment is always a good idea.

PK


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## ki5ka (Dec 17, 2006)

Paul, I've been considering that. 
WS, I'll give that a look. I want to pull the derailleur for a cleanup anyway.


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## ki5ka (Dec 17, 2006)

So.... Do y'all think this is a "tuning" issue or a "tandem" issue?? I have never had this happen on my single.
.


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## Okayfine (Sep 7, 2010)

It's not a tandem issue.


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## Okayfine (Sep 7, 2010)

ki5ka said:


> It's a very steep climb followed by a switchback. Can't make it in middle ring, but good thought.


I don't know your ring sizes. Given the standard 22/32/42 and 11-34, you're going to be in a harder gear in 1/5 than if you were in 2/2. Sheldon has a gearing calculator here:
Sheldon Brown's Bicycle Gear Calculator


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## PMK (Oct 12, 2004)

An honest guess, the cable housings are doing something funny based on it happening in a "G"out of down then up. As the suspension linkage moves during the "G"out, the housing can not cope with the movement.

If it is a long chain, the chain slap can do all sorts of not repeatable in the stand issues.

Obviously though, it is the back of the bike, so, sounds like a stoker problem, just like a rear flat tire.

PK


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## ki5ka (Dec 17, 2006)

PMK said:


> Obviously though, it is the back of the bike, so, sounds like a stoker problem, just like a rear flat tire.
> 
> PK


LMAO Paul, you a funny guy  Brave too! Can't figure out how you have survived up front there, with your backside all vulnerable like it is, talking trash like that about stokers.


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## PMK (Oct 12, 2004)

ki5ka said:


> LMAO Paul, you a funny guy  Brave too! Can't figure out how you have survived up front there, with your backside all vulnerable like it is, talking trash like that about stokers.


Every flat rear tire I blame on her. Last one was a few weeks ago. Sharp root punctured the almost new Ramapge tire, slime tube and hit the rim strip.

It exploded and she asked if we got a flat...Told her that was not my end of the bike and it was her fault.

PK


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## ki5ka (Dec 17, 2006)

lol


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## reamer41 (Mar 26, 2007)

I agree with Paul. 
Shift cable and routing? Maybe try a longer run of housing past the rear suspension linkage, or even full length housing. Oh, and new shift housing if you haven't changed it in a while. 

Also I have found the Ventana dérailleur hangers to be on the weak side, both regarding failing and flexing.. Consider a different hanger. I got mine on line. Dérailleurhangers.com, or something. Stiffer I believe. Stoker should like that. 

As to rear der set up: I'll assume it set up per SRAM literature with regard to the spacing of the jockey wheel to cassette cogs? I don't have the instructions with me, but I recall SRAM having a specific spacing and I recalling it being easiest to set before putting cable or chain on. 
BUT it's been a long while since I've set up a SRAM dérailleur. 

Main rear suspension pivot it tight right? No slop in the rear end?

(The positive effects of a stiffer rear dérailleur would not be noticed if there is too much slop in the rear end!)


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## ki5ka (Dec 17, 2006)

Thanks for the reply reamer. I've been through 2 cable/housing sets over several months of riding but I still suspect part of the issue is too much friction in the system, the cables/housing being #1 suspect. I've been wondering if something is causing damage somehow, and so each new set develops the same issue. This WAS an issue when I was carrying the bike on a carrier where the bike was hanging and the cable would get pinched if it wasn't hanging just right. I changed to a car-top carrier, so that problem went away, but maybe there's something else doing damage. 

I am having another shifting issue that I suspect is related and is consistent with excess friction in the cables, that is: Hesitancy in shifting when releasing cable length (upshift), then after I make an adjustment at the barrel (dial in), the bike overshifts (too much cable).

Thanks for all the replies! I STILL haven't gotten to it, but I have some good ideas to follow up on when I can get it on the stand.
To do list:
Check for friction in the cable run
Check for "self-shifting" related to the suspension by pulling the shock and cycling the rear suspension and watching for cable tension release (which would lead to an up shift)
Shorten Chain
Check jockey wheel (another good candidate because I run two different wheelsets with different gearing) The more I think of it, the more I'm wondering if this is not the key issue. Never thought it was that touchy an adjustment... Might have to rethink that assumption.


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## ki5ka (Dec 17, 2006)

Spec from SRAM for jockey wheel is 6 mm. FOund 7 mm for the mountain wheel and 11 for the city wheel. Changed to 6 for the mountain. More problems with the mountain so don't think this is it. 

Next on the list...


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## ki5ka (Dec 17, 2006)

Not on my to do list... but mentioned by one of you, looks like the hanger is bent!


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## ki5ka (Dec 17, 2006)

AND the mounting screws are loose!


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## ki5ka (Dec 17, 2006)

That's better!


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## ki5ka (Dec 17, 2006)

A ride with the "city" wheelset with the hanger straightened, the jockey moved up (thanks Reamer!) and 2 links taken out of the chain (thanks Paul) and we had flawless shifting. We'll have to wait till next weekend to see about the mountain wheel.


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## reamer41 (Mar 26, 2007)

ki5ka said:


> A ride with the "city" wheelset with the hanger straightened, the jockey moved up (thanks Reamer!) and 2 links taken out of the chain (thanks Paul) and we had flawless shifting. We'll have to wait till next weekend to see about the mountain wheel.


Glad you got it worked out!
I suspect the bent hanger was the biggest culprit. I know there others that feel the opposite, but I think the Ventana hangers are too soft. I've bent a few, twice resulting in the wheel eating the derailleur. Not Good! The replacements I got on line have been better. Maybe its all in my head?

Also, I use Chris King hubs with 'fun bolts.' I think the clamping force of the bolt-on wheel stiffens the rear end over all, and specifically the hanger-dropout-wheel relationship.


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## reamer41 (Mar 26, 2007)

ventana Derailleur Hangers

I bought two. After going thru several hangers from Ventana I've yet to have to use my spare from these folks.


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## ki5ka (Dec 17, 2006)

I have fun bolts on Chris Kings as well. 

Fell into a cactus last weekend and landed on the derailleur. Clearly damaged the cable and suspect the hanger got another good jolt as well. Just ordered the hanger you suggested, thanks reamer.


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