MPStoops

Hello all, this is my first post here but I'm not new to cycling by any means. I road raced CAT3 back in the '90s, MTB raced Sport Class back then as well, and have even spent some time on track at the Olympic Velodrome at CalState Dominquez Hills. Back in the day I always bought bare frames and built them up myself with the components I wanted - always Campagnolo on my road bikes and, maybe a bit grudgingly, Shimano on my mountain bikes. Heck, I even used to build my own wheels way back then, so I'm decent with a wrench. These days, I ride recreationally, for fun, and have gotten my girlfriend (a former marathon runner) into both road and MTB riding (she's crazy for MTBing!!!). So we picked up a Specialized Rockhopper for her last spring to get her going, and she loves the bike, but hated having the 2x8 Shimano Altus drivetrain. I upgraded her to SRAM Gripshifters at first (she's 4'11" and with small hands had trouble actuating the double push shifters so the Gripshifters have been amazing for her). Later on I did away with the 2x8 Altus and put on a SRAM NX 1x12 setup - new shifter, new DUB BB, new crankset, rear mech, chain, Sunrace MZ90 11-50T cassette (the only non SRAM bit in the mix) - and I've got a problem that I just can't solve. The bike skips every now and then on just the smallest cog. Sometimes under stress, sometimes just putting light power down. I've got a spacer on the drive side of the BB and a 6mm offset chainring. I've been researching this issue and know I'm not the only one dealing with it. Here's what I've tried thus far:

1. Adjust the high limit screw all over the place - from full out to so tight it won't even let me onto the smallest cog
2. Adjust the B limit screw from super tight to super loose
3. Adjust cable tension on the rear mech, from stupid loose (like, nothing shifts) to so tight it's laughable
4. Checked that the cassette lock ring is tight
5. Checked and adjusted the derailleur hanger for proper angle using an alignment tool
6. Pulled out the spacer on the drive side of the BB
7. Put a 3mm offset chainring on, with and without the BB spacer
8. Tried 30T and 32T chainrings, with and without the BB spacer
9. Adjusted the tension screw at the shift lever to the point I've almost worn it out (not really, but you get the idea)

Again, just to be clear, every part except the cassette is SRAM NX Eagle, including the chain. And, yes, it's definitely a 12 speed chain. No matter what I do, I get the exact same random skip in the drivetrain. I've got the same groupset on a Fuji hardtail, including a change to the same Sunrace cassette, and it never misses a beat. I've also got the same groupset on a brand new Specialized Stumpjumper, and it shifts great. But on her Rockhopper........it's driving me nuts. Everything else shifts beautifully, and when I set the high limit screw on the rear mech to keep it off the smallest cog, it doesn't skip ever. The chain line doesn't look odd at all, especially when compared to both my hardtail and Stumpy. I'm stuck.... what am I missing?

Eric S.

Well, mechanically you know what you are doing and have a good list of troubleshooting. Since you say you have the same setup on another bike, does the chain still jump if you swap the rear wheels? That would narrow it down to some abnormality with the cassette cog.

cxwrench

Did you install the quick link upside down?

woodcraft

+1

cjenrick

use to run a 16 as the smallest cog on the mountain bike.

lots of extreme riding in the middle of the night in muddy conditions,

13,14 and 15 cogs all skipped when the freewheel got packed with mud,

16 cog would not skip no matter how packed, power through those mudholes like nobody's bizzness,

do i expect anybody to take this radical approach? no, i do not.

MPStoops

Wait. What???? Those things are directional??? I've never heard of that, but I'll give it a shot. Thanks (he says as he crawls off, slightly embarrassed)

MPStoops

No mud, no mess. The drivetrain is clean as a whistle and still it skips. BTW, smallest cog here is an 11

MPStoops

The bike in question is running 27.5 wheels while my bikes are both 29ers. Still, I could swap cassettes easily enough. I'm going to look into that quick link thing first though as it's the easier check. Thanks though!

JoeTBM 

I guess I learn something new every day. Working on mostly big box store bikes, we rarely see high end components and we certainly do not purchase high end chains. Apparently there is a direction listed for Shimano, just curious if there are other manufacturers that also have this. We buy mostly KMC 571 & 573 master links and no direction is shown, at least these old eyes have never noticed it.

Shimano chain instruction

AeroGut

I assume you’re using the lock ring that came with the cassette, but if not, lockrings from cassettes with larger smallest cogs are often too big for an 11t and interfere with the chain just enough to cause it to skip occasionally.

cxwrench

SRAM and Shimano are both directional. The arrow should point backwards on the lower run of chain when you join the link. On the 12 speed chains the link is curved and it obviously has to be installed correctly or it can bump into the lockring and cause skipping.

sovende

I was going to suggest that there might be an issue with the chain since on the smallest rear cog, the chain links would experience the greatest degree of “bend” on the link pin. If something were to impinge that movement, a skip could occur. Rusty and/or improperly lubed chains can exhibit this issue. I’ve not used either SRAM or Shimano “quick links” so I can’t speak to there use but if I noticed the curved shape and the arrow, I’d prolly have read any installation instructions to learn of their significance (ya, not very manly ). In this particular instance, I wouldn’t be surprised to learn that this (improper installation) was the likely cause of the issue.

awaumans

Have you checked the chain wear?
Also there could be play on the jog wheels to that could give a problem too.
How old is the cassette?

Iride01 

The backstory and defending your prowess sort make it hard to find the details that matter. Doesn't matter how great a mechanic you are, all of us at various times have a brain fart or simply fail to realize something. But this is a 12 speed cassette with 12 speed chain and 12 speed shifter if I follow you.

Assuming this isn't the kind of skip a quick link installed wrong will cause then....

How certain are you that when the chain is on the largest rear sprocket that the shifter is actually in first and not second?

How certain are you that the cassette is properly on the hub to give it the correct chain line? This isn't a case where you took a wheel hub intended for a narrower spacing and made if work with drops needing a wider spacing?

MPStoops

Had to quote myself here because, well, I am slightly embarrassed. Yep, I managed to install the quick link upside, backwards and maybe even a little bit sideways!! Worse, it was painfully obvious when I slowed down and really looked at it that I screwed up. Super quick fix and I thank those of you who pointed this out and confirmed the suspicion. Everything is working beautifully now and I can't force a skip no matter how hard I grind that smallest cog. BTW, do you know how hard it is to test ride a bike sized for a 4'1" woman when I'm 6'1" with a 36" inseam?!?! I must look like a clown riding her bike around the cul-de-sac, but perhaps a bigger clown for making such a simple, rookie mistake. Thanks again to everyone who responded - have a great weekend!!!

Dancing Skeleton

Have you checked the derailleur hanger?
I had the same problem a while ago, and a new hanger fixed the problem.
I replace mine every year or two, it's a $15 part, and can get bent enough to cause problem from normal use, especially when shifting under power while climbing.

cxwrench

[QUOTE=Dancing Skeleton;21903003]Have you checked the derailleur hanger?
I had the same problem a while ago, and a new hanger fixed the problem.
I replace mine every year or two, it's a $15 part, and can get bent enough to cause problem from normal use, especially when shifting under power while climbing.[/QUOTE]
No, it can't. There is zero stress transmitted to the hanger from pedaling. None. It makes no sense to replace a hanger if it's not damaged. The derailleur and the hanger are not affected at all by how hard you're pedaling, all of that torque is on the other side of the cassette. Generally hanger alignment problems aren't an issue in the small cogs, they become much more obvious in the larger cogs.

Dancing Skeleton

[QUOTE=cxwrench;21903181]That makes sense.
But why does shifting sound "harsher" when shifting under load, like when pushing hard on a steep grade?
I, and several of people in my club replace their RD hangers every season or two, and have found shifting to be better afterwards.

base2 

[QUOTE=Dancing Skeleton;21905638]Because in the 11 cog, at best there is about 5 teeth carrying the full load. Where as in the larger cogs there can easily be ~12-16 teeth or more engaged with the chain depending on cog size. So the force per tooth is massively different (and associated friction & wear) at either end of the cassette.

In any case, though the OP has his issue solved I do understand where the hanger comment comes from. I had a similar issue on one bike where the 11 cog would skip at random. It turns out that the derailleur pivots/parallelogram wouldn't relax outboard far enough. (The bike was a special case retrofit with a hand crafted dropout/hanger.) A 0.5mm washer between the derailleur & the derailleur hanger fixed the issue by physically moving the derailleur physically outboard.

It was quite perplexing, you would expect similar symptoms from a bent hanger, but no. It was just an unfortunate combination of components & the 0.5mm spacer was the solution.

cxwrench

[QUOTE=Dancing Skeleton;21905638]Because the chain is getting pulled on harder it hits the next cog harder...simple. The only reason your shifting would be better after replacing the hanger would be either your old hangers were out of alignment or your derailleur needed to be adjusted. There is no other answer. It makes no sense to replace a hanger 'just because it's a year old'. You know the old saying..."If it ain't broke, don't fix it"...or maybe it could be "If it ain't broke, fix it til it is".

JayLukan

Thank you for this post, I made the same mistake. Glad you experienced guys are willing to share your knowledge.