hyhuu

Hi,
My mountain bike 1x 10 (11-42) won't shift from cog 42 down to 36 but otherwise shifts perfectly in any other gears up or down. Any suggestion on how to diagnose the problem?

Thanks,

ridelikeaturtle

Does it shift fine from 36 up to 42?

hyhuu

Yes it shifts fine from 36 up to 42.

JohnDThompson 

Not much to go on here. Chain too short, perhaps?

ridelikeaturtle

I'm thinking rear derailleur alignment. Check the cable and all housing is in good condition, and the shifter is functioning as you expect.

Try shifting it up to 42, then loosening the cable via adjustment until it will drop easily to 36. Once you get it shifting between these two, then see how it affects the shifting across the rest. Repeat until you've dialled it in.

dsbrantjr

First off, is your derailleur rated for the 42-tooth cog?

Andrew R Stewart 

If the upper pulley (the guide one) is too close to the cog's underside the chain acts as a trap and keeps the chain from riding up (or actually down as this in under the cog) and over the cog's teeth when shifting off. dsbrantjr likely is heading ion this direction with his question. Andy

ridelikeaturtle

"b-screw" adjustment, usually one of the last things anyone ever checks.

hyhuu

To answer the questions: The cable and housing are fairly new, the derailleur is rated for 42T, b-screw is not the problem as there are plenty of clearance. If I adjust the cable so that it shifts from 42T to 36T then everything else will be off.

Shimagnolo

A source of problems I've had in shifting from a large cog to a smaller one, is a frayed cable inside the shifter.
But usually, that causes problems with shifting from any larger cog to a smaller one,

Andrew R Stewart 

Which is why some don't understand it's use as well as they might. Like it's an after thought.

I'll make this next point in another thread in a few minutes- When we use a system at it's maximum capacity is it no wonder that there are niggling issues and less performance then when the system is less stressed? I think of cars many times when I explain bike stuff. Take your car engine's maximum RPM limit (the red line on the tach.) By the way many people who ride and work on bikes would seem to have it it's perfectly OK, and without ant added stress/wear/performance loss, to operate the engine to that max all the time. So one is to shift the gears, on their car, when the engine reaches that red line each and every time. (This is satire for those who can't read between the lines). Andy

ridelikeaturtle

Then you've talked yourself into the only solution: do nothing, and accept it working everywhere but from 42T to 36T. Enjoy!

Kovkov

dsaul

Any time I've had that issue with those huge 1x cassettes, its always B-tension or chain length.

GrainBrain

When did this start happening? Has it worked fine before? What model of derailleur and shifter?

You can't go wrong with wiping down and relubing the cable.

Bill Kapaun

Maybe too much?
Think more of "enough" than "plenty".

As to chain length-
This is how I do a quick check in the big:big combo, or in your case, put the chain on the largest cog and make sure there is some slack.

hyhuu

The bike has the Shimano XT shifter paired with the SLX7000 derailleur, which has the 46T max capacity. Even with the b-screw all the way out, there is still too much clearance for my liking but I guess that's how it's designed. When I had the Deore derailleur on it previously, I needed to have the b-screw almost all the way in just to get enough clearance. The shifting problem only happens recently. I've checked the cable and there doesn't seem to be any binding/friction issue but I'm going to put a new one on to see if it makes any difference. For chain length, I used the big-big plus 2 but I'll check again with the method above. I ride on plenty of rocky terrain so it's possible the derailleur or the hanger is bent but I can't see it with my eyes. I've had bent derailleur before and it still shifted through all the gears even though crappy.

Retro Grouch 

2 things to try:

1. Check your derailleur hanger alignment. Whenever I encounter a rear shifting issue that doesn't respond to normal tuning that's the first thing that I check. MOST times, that's it.
2. Try backing off your low limit screw about 1/8 turn. If the screw is tight against the stop, it can sometimes bind and delay the shift to the next sprocket.

MobiBike

At least the method I've adopted which has proven to save time and energy for me... Start with hanger alignment and security and material inspection (cracked or soft or wrong replaceable) check that with the derailleur still in place before you remove it to check for alignment of the hanger: by pulling back and forth and trying to go and test to see if it has extra movement at the hanger bolt or p-knuckle while you're there. Some hanger issues are hidden when the wheel is in place and sometimes caused by improper installation and even insecure quick releases or nuts. This frustrating area on some bikes seems to plague in Neverending wiggle Fest back there working sometimes and others it doesn't. At this point I'm making this a lot more detailed and complicated than your situation is probably going to warrant.
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so the simplified version is he a check that hanger first then inspect those cables and I found damaged ends under cable caps if you can pull caps off of cables or if they're loose enough sometimes the longitude wires drift and may need to be trimmed back to make the cable non-sticky as they pass through. That's just the next area where my inspection goes.
my brain wants to say it's something like this because you said it was working and then it quit working so the first suspect is going to be a bent hanger or damaged wire slash cable or cable housing. I have found bent tensioning bolts/barrel adjusters at the derailleur or even some shift levers can cause issues, and kinks in The Wire that are underneath some of that housing are also culprits. This happens on certain bikes depending on how they're loaded on Carrier racks bending and pinching cable housing and cables themselves.

I forgot to mention that you really must use a good alignment tool for that derailleur hanger here's one that's more money and precision than most would need but it's an awesome awesome tool. https://www.abbeybiketools.com/products/hag