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Shorten chain after dropping big chainring?

6K views 14 replies 8 participants last post by  thomllama 
#1 ·
This is probably another stupid question, but I've removed my big chainring from my triple, and I'm having problems with my chain dropping at the most inopportune times. I've searched and found a couple of posts that suggest shortening the chain after removing the big ring, but my hesitation is that I'm not sure what's the best way to determine what the new length should be. Will largest chainring (now 32t) to largest cassette sprocket (36t) plus 2 work fine? Will my XTR medium cage RD care if the chain is that short? Do I have nothing to worry about?

PS. I'm not suggesting this is an actual fix for dropping my chain, but it only really seems to happen when there's a lot of rear suspension movement, so I'm hoping this will at least help.
 
#3 ·
Yes you need to shorten the chain. In theory it would seem like "whats the difference right? I'm just not using the big ring anymore" Somehow that is not the case. Just use the exact same formula you would for your triple....... Big ring front (now your middle ring) Big ring back while bypassing the derailleur plus two links.
The advantage to the 2x9 system you now have is that you can run a medium cage or sometimes even short cage without a problem. This makes the whole system tighter. Crisper shifts, less chain noise.
 
#5 ·
I'm actually already running a medium cage RD, so from this and everything else I've read about cages makes me think that shortening the chain is almost required in my situation.

It's not clear from your description if the chain is dropping when shifting up front, or just riding along. If it's during shifts then I would suspect the FD needs to be aligned. If it's coming off while just riding along, then it could well be the chain is too long.

Did you lower the height of the FD? In my experience it helps to do this. Simply leaving it where it was for 3-ring operation and dialing in the high limit to restrict it to 2-ring use has resulted in dropped chains during shifts for me in the past. Lowering the FD was the remedy.
So as to not make things too complicated I left off a few details. I actually don't have a granny ring currently installed, so I've been riding 1x10 for the past couple of months, using the FD as a chain guide. I'll be installing the missing granny ring sometime in the next week. I ride a Niner Jet 9 RDO which has a direct mount FD--so no way to lower it. I currently have a Shimano FD-M780-E on there. The chain drops when trying to pedal during or right after a slight downhill stretch, especially if it was particularly bumpy, and it is always falls off to the outside (right into my carbon cranks :madmax:)

Now I'm wondering if I should also move to a 2x10 FD like the M785-E2 in the hopes that it might sit lower. The M785 is listed as accepting 38-40t top ring while the M780 is 42t, so it should sit lower, right?
 
#4 ·
It's not clear from your description if the chain is dropping when shifting up front, or just riding along. If it's during shifts then I would suspect the FD needs to be aligned. If it's coming off while just riding along, then it could well be the chain is too long.

Did you lower the height of the FD? In my experience it helps to do this. Simply leaving it where it was for 3-ring operation and dialing in the high limit to restrict it to 2-ring use has resulted in dropped chains during shifts for me in the past. Lowering the FD was the remedy.
 
#6 ·
Sounds like the chain is flopping around on those descents and starting to derail where it come onto the ring, and then when you start pedaling it rides right off to the outside - so it doesn't sound like the FD is doing anything to guide the chain back on the ring.
- Shorten the chain if possible
- Get 'clutch' style RD if you don't already have that.
Those would be my suggestions. I think your method for sizing should work fine but if you have any doubts I'm sure you could call the guys at Niner.
 
#10 ·
I guess I should clarify that I did use the big-to-big plus 2, and that resulted in taking 6 links out. The derailleur is a bit more stretched now than I'm used to seeing, but after comparing it to pictures in some of the chain length guides I think it's good. The chain has just enough of an s-bend in it, and it shifts fine (ignoring my other RDO shifting problems.)
 
#12 ·
I'm curious, is the standard big-to-big plus 2 links still the standard for 2x or 1x setups? My understanding was that was basically to prevent any breakages or jamming of the drivetrain if accidentally selecting that gear in a traditional 3x drivetrain and still keep the chain short enough for the derailleur to take up the small-small end of cross-chaining, but with a 2x or 1x setup, big-big is actually a gear you'll be legitimately using. I'm wondering if that would be too much stress on the derailleur in the long term, or cause poor shifting since the derailleur is all stretched out? Perhaps balancing derailleur take-up in small-small with enough chain for big-big would be a better alternative?

Just thinking out loud.
 
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