Don't back pedal... it doesn't help you go forward.

 

I'm sure that terrible non-answer is helpful for OP.

Hello folks,
Yes, I'm aware of this problem from my 1X days.
But now it happens even from the 2nd largest rear cog (using the big chainring) on my newly set up bike.
Cassette = Shimano M5100 11-42T (new)
Crankset = Shimano GRX 2x 30, 46T (new)
Chain = SRAM PC-X1 used, with 1/32" of elongation
Bike = 2008 Anthem 26'er (yes it has short chainstays)

The chainline in the 46T chainring is 47mm (GRX default) + 4mm (distance from center to big chainring) + 1.5mm extra (I had to space it out a little so the 46T won't hit the chainstay) = 52.5 mm. OK, that's a pretty bad number, but a few days ago, I was running an old Sunrace 11-46T, and it only dropped from the largest cog. I thought that Shimano was "better" about this, and also this new cassette is a little bit smaller too.

What's the chances that a new and different chain can fix this? Like, maybe it has better internal chamfers or something. Should I be using a non 1X chain instead?

You've done yourself no favors. You're using a GRX crankset (already a wider chainline than typical road cranks) and you've shifted the chainline wider. And you're using it on a mtb frame. GRX is a gravel crankset that's meant for frames with ROAD bb shell width (86mm). I have one of these cranksets on my Salsa Vaya frame. Why in the hell did you put this crankset on a FS mtb frame?

 

I have an Epic with the 1x12 10-52 rear. If the bike is on the work stand, I can peddle it backwards and the chain will stay on the 52 tooth gear. If I am riding and pedal backwards, multiple revolutions, it can jump. I attribute this to the vibration or impacts. However, if I only rotate the pedal 180 degrees, to get the crank in the right position for an obstacle, it is fine. I understand that there are times moving the crank counter clockwise has a benefit, but peddling backwards?

 

Back pedalling 1/2 revolution or more? I can't recall any of my bikes tolerating more than 1 revolution and can't see a need for anymore on the trails. Tight chains & cassette ramp design can come into play w/ back pedalling issues.

 

We are talking about drop in a big-big situation. How likely it is that you have shifted to a large chainring in front and a large sprocket in the rear and then you need to backpedal? If you have a chain in the small chainring do you have issues when backpedaling?

 

Silly me. I forgot that with double front big-big is right in the middle of gear range and often used. Inevitable cross-chaining is why I didn't like the front double.

 

Don't know if this will help but, recently I put a 12 speed SRAM chain on my 11 speed drivetrain (sram cassette, shimano crank, garbaruk chainring) and I can backpedal as much as I like and the chain does not drop down.
It used to drop down with 11 speed sram chain that I used before.
The 12 speed chain even shifts better.

 

Yes, I'm aware of this problem from my 1X days.

The bitter red pill here is that the "backpedalling issue" was always a thing. Except in the days past you never run big/big combos, so people rarely encountered it. Your bike is perfectly normal.

 

Hello folks,
Yes, I'm aware of this problem from my 1X days.
But now it happens even from the 2nd largest rear cog (using the big chainring) on my newly set up bike.
Cassette = Shimano M5100 11-42T (new)
Crankset = Shimano GRX 2x 30, 46T (new)
Chain = SRAM PC-X1 used, with 1/32" of elongation
Bike = 2008 Anthem 26'er (yes it has short chainstays)

The chainline... <snip>

Odds are that your chainline is off. Grab a long straight edge of some sort to verify front rings and rear cogs are not at an angle. If that is not the issue, might be your dropouts or chainsuck from derailleur, ...

I backpedal like a circus clown on 11(9-46 ethirteen), 2x10, 3x9 and single speed bikes with no chain drops, ever.

 

BIG (46t!!!) chainring + very short chainstay + poor chainline + worn non-standard chain = Frankendrivetrain. You've answered the question already.

 

1/32" elongation is worn?
Are you suggesting things will get better if I get a new and different chain?

 

OK, well that would explain why the Sunrace 46T cassette worked better. Maybe time to get a smaller Sunrace cassette then.

Reminder that 2008 Giant anthems came with 44T chainrings

 

Reminder that 2008 Giant anthems came with 44T chainrings

on a TRIPLE crankset. That's a huge functional difference than trying to use a road double.

 

what's the functional difference?

 

a triple has a "wider band" of a chainline (inner ring is narrower, big ring is wider). this allows it to fit the chainstay without resorting to wonky spacing like you've done with this one, which is going to be a big reason why you're having trouble.

also when it comes to shifting, smaller gaps between chainrings shift better than bigger gaps. I have this exact GRX crankset on my gravel bike and I wouldn't want it anywhere near a mtb. not relevant to your question, though.

@uzurpator suggested that using a big-big gear combo is a big reason why you're having chain drop. You never answered...are you doing that? If so, don't. That's cross-chaining and is to be avoided on multi-chainring drivetrains. One thing that has happened with added gears on cassettes is that the cassette itself gets wider every time they add a cog. On mtbs, a lot of people don't notice this because instead of making the freehub body wider, the bigger cogs on the cassette just get "dished" over the hub flange. But the chain now needs to deflect across a wider space than it did for, say, 9spd. So the effect of a big-big cross-chain on a multi-ring 11spd drivetrain is going to mean that the chain is deflected even more than it would be on a comparable 9spd drivetrain. This is going to cause problems.

Saying that your current setup is equivalent to the OEM crankset this bike had BITD, and to a current XT 2x crankset is wrong. It's not equivalent. The "chainline" number for the crankset isn't the only factor at play. Every single little detail is important. You're using the GRX crank in an arrangement it's not designed for (spacing the chainline wider than spec). Things aren't going to work as well as they would if you were using it as it was designed. That road crank is not going to work exactly like an XT double. The XT double is going to be working much closer to its designed tolerances. It's not going to work the same as the OEM triple crankset.

The more you piece together parts that weren't designed to work together (esp if you start putting that stuff on older bikes), the more likely you will be to have a degradation in function. Deviating from the "standard" spec chainline has consequences. Trying to fight people who are telling you that this is why you're having trouble isn't going to help you. You need to accept that certain combos aren't going to play nicely.

I also have to point out that I feel like Shimano's original 11spd mtb stuff had a major failing. I used it for awhile, but it was probably the worst drivetrain with the backpedal chain drop issue. Shimano improved the backpedal chain drop thing with 12spd, but it seems to me that SRAM's backpedal chain drop got worse with 12spd so that they're now more roughly equivalent on that. But for 11spd, Shimano was worse by far. This is part of the reason why my gravel bike is still on 10spd.

 

The only time I backpedal is to get my feet in the right position...or if I'm trying to get someone's attention so I can buzz them with my Hydra hubs.

 

@beanbag Harold is right about shimano road cranks not having adjustable chainline. Adding 1.5mm shim driveside solely for the purpose of getting some chainstay clearance is ... somewhat worrying. Did it on the regular for that purpose ( actually - to get clearance to run 36T ring on a bike designed for 32T ) and got myself cured when I lost the preload bolt and limped with w wobbly crank to the nearest LBS. If you really intend to follow with this plan, I'd at least try to get an alloy preload bolt. Your best bet would be to hunt a shop which has a BB facing tool, so they would shave off some BB off the NDS.

Also - triple anxiety is strong with this one...

Shimano MTB cranks have a hidden feature, that on 68mm BB you can run just one shim or on 73mm BB run no shims and you get 47mm chainline on the middle ring and 55 on the big one.

@Harold apart from the "added 1.5mm to the driveside" issue, you are blowing steam. There is nothing special about GRX other than the weird 16T gap between the big ring and the granny. Mixing road/gravel and mtb drivetrain parts is not straightforward, but it can be done.

 

@Harold apart from the "added 1.5mm to the driveside" issue, you are blowing steam. There is nothing special about GRX other than the weird 16T gap between the big ring and the granny. Mixing road/gravel and mtb drivetrain parts is not straightforward, but it can be done.

Yes, it can be done. sometimes, in some combinations, and sometimes if you're willing to tolerate less than ideal performance. I have a mixed mtb/road drivetrain on my gravel bike. I use the same GRX crankset that OP uses (on a frame meant for road cranks) with a 10spd mtb rear end, an older 105 2x front derailleur, and Gevenalle controls (Tektro brake levers mated to Microshift thumbies). I was very selective, and I was uncertain how well that FD was going to work with the GRX crank. I could not find any other examples online of the combination (some with newer 105 FDs, but none with one as old as mine), so I had to experiment to find out. I was fully prepared for it to not work well because the GRX chainline is outside of the chainline spec of the FD, and the chainring sizes are outside spec for the FD, also. That's the risk you have to take when mixing stuff outside of spec. Sometimes it ends up working well. Other times, not so much.

there are a metric shitton of drivetrain combinations that fall outside mfr spec because some aspect of function is reduced below the manufacturer's preferred tolerance, or because the mfr can't test all possible combinations. but there's no hard limit that it "cannot be done". This is one of those examples. OP is running a drivetrain outside of spec. It "works" but does so at a reduced level of quality. I am only pushing back at OP who does not seem to fathom or accept that even subtle changes outside of mfr specifications for a given part might just be a major contribution to the symptoms.

wide range drivetrains tend to be fairly finicky and require precise adjustments. you can't "good enough" these things in the same way you can with 7spd stuff that can work reasonably well with a ton of slop. there's a narrow band where it's is dead on, and outside of that function suffers. the manufacturers are sometimes pushing these sub-par combos, as well. Such as the examples of a 55mm chainline for a regular boost bike (Yeti is a nice example of this), and shimano deciding that a 52mm chainline is "just fine" for non-boost as well as for boost (current 12spd mtb drivetrains). they might "work", but picky ppl will notice a difference in how well.

 

@Harold You are not specific enough, you speculate with very general rules. Which are true, obviously, but omit specifics, which OP has cornered.

For 51.5mm chainline on the big ring ( which OP is forced to use ) and short chainstays, the big-big combo, as well as maybe some biggish-big combos might drop when backpedalling. No way around it. It would do so with road or not road stuff. Once the chain breaks in, it is going to do it less. It being a gravel crank isn't really a factor here.

 

I said in an earlier post how we might obtain said specifics. A layperson like myself will be unable to obtain these without a substantial investment in bikes and parts to be able to run tests on many combinations of equipment. And for what ends? It's dumping a lot of money into something that's not worth it for me to investigate.

There's a great way to avoid a number of problems created by using a gravel crankset offset to the drive side. That would be to use a different crankset. Such as, oh, a different mtb crankset.

Another one would be to use a frame meant to accommodate the sort of crankset. It's going to be a lot easier to use a modern "gravel" frame without diving too deeply into experimental accommodations. Hell, it might even be easier to use an even older mtb frame for the purpose. Modern gravel frames flirt pretty closely with early mtb frames in a lot of respects.

The pursuit of narrow roadie q factors on a mtb frame is a losing battle and limits your options unnecessarily.

 

Haha, let me tell you how smart I am. By a very clever mix of road and mountain BB parts, I was able to move the chainline over by 1.5 mm, yet only lost 0.5mm worth of crank arm engagement on the non drive side. See this post here, which also contains some rationale on why I went with the GRX crankset:

safe engagement for Shimano Hollowtech 2 cranks?

Hello folks, I need to do some shimming shenanigans for chainline fixing. Does anybody know what is the safe amount of engagement when clamping on the non-drive side crank arm onto the spindle? I think in the default configuration, the little safety tab pin goes right into the center of the...

www.mtbr.com

Well anyway, the whole point is to try to get the biggest chainring you want / need the furthest inboard. (And also to have a low Q) That would be something like a GRX crankset with a non-existent 44T ring, or maybe something like a 2012 era SRAM XX Q156 with 28/42T, and add spacers to move both chainrings inboard. I actually have this crankset, and have added the spacers, to use on my prior road bike in order to change the MTB chainline into a road chainline. (This was my solution to get low gearing on a road bike) Too bad I couldn't reuse this crankset here since I have the BB30 version and needed the GXP version.

 

@Harold None of the thing you speak of are relevant to the OP. His only problem is one that is not caused by mix/matching road and mtb gear.

@arnea I can't say anything about shimano BBs, because I don't use them. Aftermarket BBs tend to have a slinding tube inside. Regarding the fit - I just remove one BB spacer on the drive side, and everything just fits. I've done so on all three bikes I use shimano cranksets. Obviously, the DS pedal is 2.5mm closer to the frame.

 

@Harold None of the thing you speak of are relevant to the OP. His only problem is one that is not caused by mix/matching road and mtb gear.

Is it? That hacked road crank install on a mtb frame looks to be a problem to me.