Issue with dropping from 46T under load.

[tuxbailey]

This season I started to experience an issue with my mtb's 1x11 group.

While in the largest cog, it will drop to the next cog under load, such as going up a steeper hill on the trail. This didn't happen last season. I had tried to adjust it and the shifting is smooth on the repair stand, on flat road/trail, on not so steep incline.

The drops are jarring and more than once I had fallen because I wasn't expecting the drop.

Any guess on what could be the issue?

[robt57]

Make sure the chainset has zero lateral play, and the RD as well.

Also, what to the RD pulleys look like? I mean wear wise...

[Peter P.]

I question your chainline.

Is there any way to adjust your chainline spacing?

I had a similar problem with an ATB triple and a Shimano SLX crank. Using the correct BB spacers alleviated the problem.

[jemdet]

I've found that some of the more modern shifting systems work "fine" in the stand, but are really just tolerant to a small margin of error.

I had the same issue on my Seven with a 46t 1x under serious load. Fixed it by adjusting the RD slightly inwards.

It could also be a chainline issue, as others have said. Crosschaining off a big cog can get ugly sometimes.

[tuxbailey]

Is there a way measure the chain line? The only way I have read is that I shift to the 4th cog and make sure the chain line is straight with the chain ring.

The whole set up (chain included) is less than 300 miles old.

Another data point is that I did start noticing this after I took the bike to the LBS for disc brake noise and they replaced the pad and rotors earlier in the season. But I can't fathom how that will affect the chain line in a TA set up.

[RobJ]

Originally Posted by tuxbailey

Is there a way measure the chain line? The only way I have read is that I shift to the 4th cog and make sure the chain line is straight with the chain ring.

The whole set up (chain included) is less than 300 miles old.

Another data point is that I did start noticing this after I took the bike to the LBS for disc brake noise and they replaced the pad and rotors earlier in the season. But I can't fathom how that will affect the chain line in a TA set up.

In “fixing” the brake noise and replacing pads did they make an adjustment to the rear wheel? A slight adjustment of how the wheel fits in the dropouts would affect the alignment of the cassette and resulting chain line.


Sent from my iPhone using Tapatalk

[tuxbailey]

Originally Posted by RobJ

In “fixing” the brake noise and replacing pads did they make an adjustment to the rear wheel? A slight adjustment of how the wheel fits in the dropouts would affect the alignment of the cassette and resulting chain line.


Sent from my iPhone using Tapatalk

That is what I am not sure. I should have brought the bike back and had them take a look at it since everything was working fine prior. I just thought the tension of the cable got messed up and I have been fiddling with it. I am just going to take the wheel off, align the RD with cassette, etc. and see if that will fix it.

[benb]

I'm assuming this is a new bike and well built up so you'd hope this doesn't happen but if it's a FS bike it can also be related to suspension sag under load causing the cable to move.

That is highly dependent on the design of the bike & how the cables are routed, but some bikes could/were vulnerable to excessive lengths of housing causing this.

Otherwise if it is always dropping down to a smaller cog when this happens I'd give the adjuster a small turn towards more tension on the cable and/or check that the range adjustment hasn't come out of adjustment on that side of the derailleur travel.

Could be wear too, that is one monster cog, what is the next cog? Cogs that large should actually last longer.

I've definitely seen these issues in the past with my MTB where it seems fine in the stand and under load it's not quite right.

[Jaybee]

If none of the above work, maybe the derailleur hanger is out of alignment? A friend had similar symptoms on a GX Eagle bike where it would easily shift to the 50t and then would drop under load, rest of the gears were perfectly indexed. A new hanger got him sorted.

[zennmotion]

Gratuitous retrougrouch comment: This is one of several reasons why I've never been attracted to 1X setups. Dropping down to a smaller chainwheel is generally easier and more reliable when hitting a sharp uphill than shifting to a larger rear cog. I think modern shifters and drivetrain have allowed for more precise shifting in general, and user-error more forgiving than the past where precise shifting was more of a developed skill. However, 1X downshifts under pedaling load to a larger cog still work against mechanical principles. I agree with above advice, check the chainline and wheel/cassette alignment.

[jc031699]

Maybe should also check the upper limit screw, to make sure it allows the derailleur to move fully inboard on the final click. This is like what is discussed for the 7900 and 9000 rear derailleurs to prevent fraying of the cables in the shifter.

Perhaps they trimmed the rear derailleur some when they did the brake / rotor service.


Sent from my iPhone using Tapatalk

[tuxbailey]

Thank you all for the great ideas. I will be focusing on the RD area since it was working fine last season. The upper limit screw and cable tension will be a good idea.

Yes this is a full suspension bike and it was put together from a NOS frameset last winter (winter 2020.)

[muz]

Originally Posted by zennmotion

Dropping down to a smaller chainwheel is generally easier and more reliable when hitting a sharp uphill than shifting to a larger rear cog. I think modern shifters and drivetrain have allowed for more precise shifting in general, and user-error more forgiving than the past where precise shifting was more of a developed skill. However, 1X downshifts under pedaling load to a larger cog still work against mechanical principles.

I will have to disagree. It's much easier to shift to a larger cog in the back under pedaling load. The rear derailleur moves the lower part of the chain that is relatively slack. The front derailleur has to move the chain under great load, and has only the spring tension to do it (for mechanical derailleurs).

[zennmotion]

Originally Posted by muz

I will have to disagree. It's much easier to shift to a larger cog in the back under pedaling load. The rear derailleur moves the lower part of the chain that is relatively slack. The front derailleur has to move the chain under great load, and has only the spring tension to do it (for mechanical derailleurs).

I won't disagree with your theory, although I'll respond that you're focused on the jockey pulley end of the derailleur and not at the relevant end of the system, at the chain/cog interface. In my experience, imperfectly adjusted rear derailleurs tend to want to "pop" the chain to the next smaller cog under load (---ugh I hate that!) such as a sudden sharp incline, the chain resists climbing to a larger cog (why it's necessary to "back off" the pressure at the pedals under load). A mis-aligned chain as from a derailleur out of adjustment, causes an unexpected "upshift", the chain wants to tension by ghost shifting or skipping to a smaller cog while it makes a lot of ugly chain skipping noise in the process. On the other hand, shifting to a smaller chain ring under load reduces tension, which is where any mechanical system naturally wants to go- I'm not a mechanical engineer but this is a fairly basic concept. Empirically on the road, IRL, front vs rear "downshifting" is much more obvious with old-school friction shifters and double/triple cranks, anybody who rode/rides these has experienced a chain jumping up a cog when putting the power down going up a hill---ugh when it happens. On the other hand, chain drop at the front derailleur is a thing too, when the front derailleur isn't adjusted (sometimes even when it is!) or when you're somewhat cross-chained. Anywho, my story and I'm stickin' to it! BITD when racing with DT shifters, it was good practice to be sure you were in your big ring before a sudden uphill so you could shift to the small ring and adjust the rear derailleur if you had to, under less chain tension. Anyway, 1X drivetrains are now possible with super wide range 11/12 speed cassettes whether out of meeting real needs/improvements or just marketing. They have their advantages, but the downsides outweigh the upsides for my purposes.

[Peter P.]

Here's how to measure chainline, for us home mechanics, courtesy of the late, great, Sheldon Brown.