Why is my chain suddenly jamming

[Tim]

I changed the FD on my 650b rando bike (my only non-Riv). It's set up with Ultegra brifters with a Rene Herse double crank. The old CX-70 FD wasn't working well anymore so I swapped to a shimano 105 FD. The derailer shifts much better but now when I shift from big ring to small ring the chain now falls between the chainrings. It did this immediately upon my first ride so I took it to my LBS thinking I had installed the derailer wrong, although I kinda mostly know what I'm doing. When I went back they told me that my crank was bent. The mechanic said he even asked his boss and he agreed. He went on to explain to me that the Herse crank only had 3 bolts connecting to the crank where most cranks had 5 and he insinuated that the RH was weaker because of this, and that it was one of the bolt "tabs" that had bent. . 

I was very skeptical as the problem had never happened in the 10 years I've had this bike (It's a custom and came to me mostly assembled) and I have never crashed the bike. I brought the bike home to remove the crank and disassemble to check it out as much as i could. I removed the arms from the bike, removed the chainrings, cleaned the crank and reassembled. Th chainrings seemed flat when I laid them on a flat surface. I reinstalled the crank and on the first downshift on the bike stand the chain fell between the chainrings. 

I emailed Rene Herse and their answer was that you can't bend a crank by pedaling it and that the FD is the introduced variable that has caused the problem. I must say that I completely agree that the only change made is the one that caused this problem but I don't at all understand how or why the derailer would immediately cause this problem. So therefore I don't know what to do to fix it. 

Any thoughts?

Peace,

Tim

[Garth]

Are the rings worn and while the CX70 allowed you to get by "normally", the 105 does not ? Different cage dimensions and all that. 

[Bill Gibson]

I wonder if the shape of the new derailleur cage is different in a way that lets the chain move in ways it could not with the old one. I'd still experiment with the position of the FD if possible, both in height and rotation, if it's a clamp-on.

Bill Gibson
Silver City, New Mexico, USA

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[Tim]

I think you just nailed it Garth. I realize now that the chain that came off was noticeably "stretched" when laid side by side with the new one. I just checked them again and the old one gains 1.5 links. I noticed it at the time but didn't make the connection of how badly the chain rings might/must be correspondingly worn. 

Tim

[Tim]

Hi Bill. I did work with the height and rotation. The FD seems well matched to the arc of the chainring and seems parallel with the proper height. 

[Bill Gibson]

So the chainring teeth are hooked? That's how I knew I'd been riding a lot, before the waxed chain revolution.

Bill Gibson
Silver City, New Mexico, USA

My Photographs : https://billbgibson.myportfolio.com/ and on Behance: https://www.behance.net/BillGibson

If you like my photographs, buy me a coffee!

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[Philip Williamson]

Flip the rings over and start with the fresh faces. I don’t know if the RH rings are countersunk to an extent that it wouldn’t work, but Sheldon Brown told me to do that when my rings were worn. It seems doubly smart with rings like yours that aren’t commodity items. 

Philip in SANTA Rosa and SF (and Grass Valley sometimes) who is proud just to be able post from his phone on occasion

[Tim]

This is the first time I've heard about flipping the rings over. I would never have thought about that and, honestly, it doesn't exactly register why only one side of the ring would have worn BUT you invoked Sheldon Brown, which is a trump card even better than, dare I say Grant, so you'll get no argument from me.  I'll check the countersunk holes and see if that will work. I can buy replacement RH rings too if need be.

I think you buried the lede though. The real news, which IMO, deserves its own thread, is that you can sometimes post from your phone!

One other question, because while training for PBP in 2015 I found out I had worn chainrings on my Hilsen when I got out of the saddle to push hard and the chain slipped from the chainring, I awkwardly fell forward, caught myself and tore my rotator cuff. How can you know your rings are worn before it's too late and causes problems? The shoulder injuring chainring was very obviously shark toothed, so visual inspections would have tipped me off but the current chainring doesn't seem extreme like the other one. When looking at the current chainring I'm not really all that sure I would say it was worn out with a visual inspection but with the old chain so stretched I can't see how it couldn't be worn out.

Peace,

Tim  

[Ted Durant]

On Tuesday, May 26, 2026 at 5:24:12 PM UTC-5 Tim wrote:

I think you just nailed it Garth. I realize now that the chain that came off was noticeably "stretched" when laid side by side with the new one. I just checked them again and the old one gains 1.5 links. I noticed it at the time but didn't make the connection of how badly the chain rings might/must be correspondingly worn.

So, I'll ask the obvious question ... did you replace the chain with a narrower one? 

If you wore the chain that much, then you also wore the chain rings. Bite the bullet and buy new chainrings, but I doubt that would solve the chain jamming problem, because narrowing of the teeth from wear wouldn't  also narrow the sections of the rings where the chain would jam. 

RH chainrings are countersunk on one side, so you may have difficulty mounting them reversed. But I wouldn't expect that to solve the problem, anyway. RH sells two different types of chainrings, one of them for narrower chains, and you definitely can't reverse them because of the pins and ramps. Even the standard chainrings wouldn't really work, as they have a pin on the outside to prevent the chain jamming in the crank arm, and their teeth are shaped (though your rings might be the earlier ones, without shaping, and with the amount of wear you might have, probably any shaping gone at this point).

I haven't had any trouble with jamming chains up to 10sp on RH cranks with their original chainrings.

You should be able to see significant chainring wear in the shape of the teeth, especially if you have a newer chainring to use for comparison. Also, with the chain on the big ring, pull on the chain at the forward (3 o'clock) position on the chainring and see how far you can pull the chain from where it is seated on the ring. A fresh chain on a fresh ring won't allow much movement. 

Ted Durant

Milwaukee, WI USA

[Tim]

Hi Ted. The first time it jammed it was the already stretched out chain I put back on without checking it. I didn't think that chain was old enough to worry but I guess it was. Then I replaced it with another 10 sp chain, a SRAM from the LBS. So I don't think it was a narrower chain, unless there are variances in 10 sp chains. I didn't know that RH had different types of chainrings until I was looking at whether to replace these. I see now they have the cranks and chainrings for 9-12 speed, and cranks for 5-10 speed. I'm fairly certain that when the builder got my crank, in 2015, that the 9-12 speed stuff wasn't available yet. I see also that all of their outer chainrings now have their "downshift thechnology."

And also, when looking at the current chainring, I wouldn't look at it and immediately say it was worn out. And in fact, when comparing that chainring with the ones on any of my other three Rivs, I would say it is in the best shape. Yikes! 

So with Ultegra brifters, a 105 RD and the new 105 FD, is there some other reason that this problem could have occurred? I'm fine replacing the chainrings but now worried the problem could persist. 

Tim Kirch

[Ted Durant]

On Thursday, May 28, 2026 at 2:22:21 PM UTC-5 Tim wrote:

Hi Ted. The first time it jammed it was the already stretched out chain I put back on without checking it.

Well, so much for that hypothesis!

 

So with Ultegra brifters, a 105 RD and the new 105 FD, is there some other reason that this problem could have occurred? I'm fine replacing the chainrings but now worried the problem could persist.

Did you also add the brifters when you added the front derailer? I have limited experience with Shimano brifters, but my memory is they do kind of a 2-step, where the front derailer does an initial small movement as you start pushing the lever, then the full movement. Wild speculation on my part, but that bit of hesitation could be letting the chain get between the rings. 

Also, another basic question - did you try easing the FD inner limit screw a bit, so the cage is closer to the frame on the small ring?

There's another possibility that occurred to me, that could fit with the LBS's rude dismissal of 3-bolt chainrings. If somehow the chain got into the gap and was forced down hard enough, it could theoretically bend the outer ring away from the inner ring. I expect that would be immediately visible as runout, i.e. side-to-side wobbling of the teeth as you spin the crank. Does the jamming always occur in the same place on the chainrings? 

Barring something like that, I would be surprised if replacing the chainrings solved the problem, unless somehow you've managed to wear the inner surface of the outer ring a bunch. I've ridden RH chainrings a pretty large amount of miles and haven't seen any wear like that.

[Guy Jett]

It seems to me that the new FD was not the only change made.  I suggest putting your old CX-70 back on and use the new chain.  The new chain may be narrower which now allows it to fall between the chainrings.

GAJett