Polaris OBark

I have a 2014 11-speed Ultegra Di2 drivetrain. The chain and the cassette are new. The rear derailleur pulleys are new. The crankset is about 5 years old. It is a GRX 46/30T which has worked flawlessly until last week. I was JRA and up-shifted the front and the chain very undramatically came off the 46T ring. I was pedaling quite slowly when I shifted, so I caught it immediately and put it back on. I continued my ride. When I got home, I re-adjusted both the rear and the front derailleur, dutifully setting both the mechanical and electronic limits. All was well on the stand. Despite this, I subsequently dropped the chain in the same manner twice in one 60 mile ride yesterday. The second time yesterday was because I was playing around with it, trying to reproduce the problem. Briefly, it appears as long as I am pedaling briskly, the up-shift will go ok, but if I am slowing down (this was right at the end of the ride) and upshift, I could reproduce the problem.

Any ideas what to try next?

FWIW, I've had this GRX crankset for nearly 5 years with no issue, and a White Industries 46/30T for several years before that, and have never before had this problem.

Kontact

Look for a bent tooth on the chainring, a stiff link in the chain and any sort of bend in the front derailleur cage.

soyabean

It's also showing symptoms of a chain that is about to break.

Doesn't mean it is, but fine comb each link pin roller.

Do it while the chain is on, and off.

You're looking for anything that might be bending, a slightly sheered pin, who knoiws.

mpetry912 

just run the chain across your fingers, you may feel a popped link

almost now way to fix it. are 10 speed cassettes and chains still available at reasonable prices ?

probably need to replace the chain if you have a popped link

/markp

Kontact

Why can't you replace a bad link with a new quick link?

Yes, 10 speed parts are common.

Bill Kapaun

Along those lines-
Crank the chain backwards and observe the RDER pulleys. Often, chain "aberrations" will show up as a noticeable "twitch".
Quick & free to do.

Polaris OBark

11 speed. Two new chains. Happens with both.

Polaris OBark

Thanks everyone for the suggestions. I haven't been able to find any issues with the chains, crankset, front derailleur cage, etc. I'm going to take it apart, clean everything, and carefully reassemble it and re-adjust.

TiHabanero

My guess is a chain ring tooth profile problem as in worn teeth all the way around. Replace ring and see what happens.

Polaris OBark

I'm thinking this is the most likely explanation, and a new GRX crankset is less than $120 on BTD. Meanwhile, cleaning is free...

Polaris OBark

It looks like the problem is quite reproducible if I am in cassette gear #5. I wonder if a damaged or defective cassette tooth could do this? (Ultegra 11sp 11-34T). It is nearly new, but who knows?

I have another one I can put on to test that hypothesis.

Another possibility is that gear #6 is the lowest I can be in before up-shifting in the front.

Kimmo

The cog probably matters via chain tension?

Maybe see if you can get the FD closer to the rings.

Polaris OBark

Right now it is looking like a conspiracy of small things ...

I took everything apart, cleaned it, and re-installed and re-adjusted.

The front derailleur was definitely too far up the seat tube, possibly as much as 3mm. The teeth on the chainrings are definitely worn a bit, which could exacerbate the first problem. The high limit screw was probably set too high -- I likely overdid it, fearing that the GRX crankset might nearly exceed what the Ultegra derailleur could handle -- although it did fine for several years. The lower limit also needed adjusting, but I am not sure that is relevant to this problem. I think the cassette is ok, but up-shifting the front while in a larger (lower) cog probably made it easier for the chain to hop off the large chainring. By design, the front derailleur over-shoots and then corrects. The hard limit on the H screw helps to limit the over-shoot. They got rid of it in the next iteration of Ultergra and GRX derailleurs, fwiw.

It now behaves better on the stand. I'll put it to the test on the next ride.

This is probably my fault because the GRX crankset is technically out of spec with respect to the otherwise Ultegra drivetrain, and I probably got away with it for awhile until the chainring teeth started wearing down.

Koyote

I'm trying to recall: the GRX crankset is supposed to be used only with a GRX FD, correct? b/c the crankset puts the chainrings a few millimeters further outboard than other models, and the GRX FD is designed for that?

At least that's what I recall from when I got a Di2 Ultegra bike with a GRX crankset and FD -- they had to go together. Maybe your bike worked okay with the non-standard pairing, but you were at the limit of the tolerances -- and a bit of wear and tear pushed it beyond that limit. So I wonder if a good cleaning and maybe a new chainring will do it? Hopefully.

Polaris OBark

Exactly.

I was one of the people (via previous user-name before going into the witness protection program) who was saying you could get away with keeping the Ultegra front derailleur -- based on my previous experience. It worked great until it didn't. I think I have a tendency to mount my derailleurs too high, not just with this bike, and I've moved it down about 2 mm now. I suspect I didn't bother at the time, since it worked, and I reasoned it would be easy enough with the band clamp on my steel frame (it was).

What I am not clear about is why it just started happening very recently.

​​​​​​​

Koyote

Okay, thanks for the confirmation. When I got a GRX 11sp crankset (48-31), the shop tried using it with my Ultegra FD -- no dice. Had to add the GRX FD. My understanding is that some bikes will allow the GRX cranks without changing the FD, some will need the dedicated FD. I'm guessing that yours is an edge case, and a little fettling will get it working again.

Polaris OBark

I'm really surprised yours wouldn't work. A 48/31 is a less drastic difference than my 46/30T. (I wanted the 165mm crank-arms.)

It might be because mine has L and H screws it gives a wider range of motion than the ones that have only electronic limits. I didn't realize mine was different from the newer Ultegra and GRX derailleurs until I set up my wife's GRX Di2 drivetrain.

Polaris OBark

This is tempting ...

Steve B.

This was initially what I thought might be part of the problem, I recall that GRX F derailer sits a bit further out from the seat tube, or some such, to allow appropriate chainline on cassettes on 142mm rear spacing, or something along those lines. I cannot recall if you are supposed to use a specific bottom bracket, but was wondering if the system was barely functional for a bit, with some wear and tear now making it not function.

Polaris OBark

It uses the same bottom bracket. The rings and chain-line sits 2.5mm further outboard, which for a 135/142 mm rear hub is actually an improvement.

This does require about 3mm or more adjustability head-room for the front derailleur H-limit. Mine has more than enough. So I think in that respect, it was fine.

What I think I never bothered to do was lower the derailleur to compensate for the change from 50T to 46T. I've only just now lowered it.

It actually functioned perfectly, shifting flawlessly, for many years. It is still far better than "barely functional", but it is no longer "a perfect shift every time." (It still shifts much better than my 2008 mountain bike triple.)

​​​​​​​But I do agree, that with some wear and tear and dirt, it likely got worse.

Polaris OBark

Also, there is some wax build-up on the inside of the front derailleur cage.