Chain drops between rings when downshifting
 

This is my wife and my Burley Zydeco tandem. It came with a 3x7 drivetrain. I replaced the rear hub, cassette, chain, rear shifter, and rear derailleur to 11-speed. I kept the crankset, front derailleur, and shifter. Since then, the chain has a tendency to drop between the big and middle chainrings when shifting down to the middle. The spacing does seem wide.

I could replace the crankset, but I would rather not, as it would be expensive.

One idea is to machine down the surface that the big chainring is on, bringing it closer to the middle ring.

Another is to build ramps on the big chainring somehow, to guide the chain down. I'm not sure how, but I'm picturing machine screws.

Having the chain hang up and hold the drivetrain still on a tandem is a lot worse than on a single bike. I really need to solve it.

Tips and insights will be appreciated.

https://lh3.googleusercontent.com/iw...w2028-h1520-no

I got into a big fight about this on the tandem sub awhile ago. Basically I was told I was wrong & that I was a dumb idiot because it should work, so I "must be doing it wrong."

The only solution I found at the time was to sub in a Sugino 10 speed triple. It's been great ever since. On another unrelated note: That frame is retired now. Cracked from long life & many miles.

Sorry.

I'm surprised an 11 speed chain would even fit on 7 speed chainrings. Given that the 11 speed chain does fit on a 7 speed chainring, maybe something like a Shimano Deore 10 speed triple crankset (48-36-26) would work. The aesthetics would be off a fit, but it wouldn't be very expensive.

You could get a more aesthetically fitting retro looking 3x10 crankset for more money.

Thanks, @base2. I'm pretty good at finagling front derailleurs, but I haven't solved this yet. I change the angle. I bend the cage in various ways. But the spacing between rings looks too great.

I like using my bench grinder, but I'm not sure I can get sufficient accuracy for this job.

I just substituted the big ring which was 48t with a ring with 54t. The problem is no better or worse, so I'll keep trying with the 54t. We need a higher top gear.

made worse because you could not see it being under the stoker with the captain shifting the lever?

perhaps this is where 11 speed cranks benefit closed up the gap between chainrings..

• machine the spider thinner between the chainrings ?

• drill and put some anti jam pins just under the teeth of the middle chainring?/ so it can be shifted out?
• bolt the 2 chainrings together there and so pulling them together tighter..

..

-----

prior to thinning the lands i would check the lateral tooth edge to tooth edge distance between the chainwheels of the chainset and compare this with others you own

i know it sounds wacky but there have been chainsets produced with intentionally overthick lands/spacers so that a rider could run the "X" combination on a two plateau drivetrain without the chain rubbing on the inner face of the outer chainwheel

one such was the Sakae Ringyo Silstar of the late '60's and early '70's era even using a full width 3/32" chain one could experience wedging of the chain between chainwheels

it would be much cheaper to test fit a different drive chain than to replace the chainset...

-----

This type of problem surfaced when 9-speed, with its significantly narrower chain, came out and riders wanted to keep their 8-speed or fewer speed cranks. As an expedient Shimano recommended shifting while sitting down which, of course, reduced the tension on the chain.

You've have a real extreme since you are using the narrowest chain available with a really wide spaced crank so I'm not at all surprised the chain wants to wedge into the chainring gap.

There are really two fixes; 1) a newer crank or 2) thin down the spider arms to reduce the chainring spacing. A 10-speed crank should be fine and a new one doesn't have to be that expensive.

Your crank is spaced for a 7-speed chain width so the 11-speed chain is narrow enough to fall between the rings. You may need to replace the crank with a newer model with narrower spacing between the rings, or carefully file the ring mounting tabs on the crank so they're thin enough to catch the narrow 11-speed chain without letting it jam between the rings. Or you could revert to a 7-speed cluster on the rear wheel and run 7-speed chain on the current crank.

separate the 3 rings & slide in a spacer using the next size smaller/larger tooth ring to the drive ring being used, just with the teeth ground off.

IE:
If yours is this: 26-36-48

It would end up like this:
26---32grounddown---36---42grounddown---48

You're kidding right?
Where do you source a 5 speed shifter?
Getting the FDER to span 5 rings may be a little problematic too.

That big chainring looks like it's from the 6-speed era. Going back in time certainly isn't going to help.

New and used 10-speed triple cranks can be had fairly cheap nowadays. You know it's the right solution to this problem... ;)

Didnt read the other answers, but if the chain drops between the rings, its too narrow for the crank. Imo, you should bite the bullet and get an other crank. Im sure you can find something used that is not too pricey. Like a Tiagra or Deore tripple, depending what gearing you like.

the spacers would replace the step or at least most of the step that bridges each ring currently.

The rings are too far apart as is.
ADDING additional spacing is going to exacerbate the problem unless "something" is machined away.
A proper width crank is the SIMPLE answer.

OK thanks. I see the crankset is really the best solution.

The bike has MTB 26" wheels. It's a tandem, which means we need a very low bottom gear and a very high top gear. Well, I'll admit that "need" is more like "want" when referring to a top gear, but it's extremely easy to spin out in a 113" gear (48/11x26) on the flats. So looking at cranksets, I see the big chainring is very small in the current trends. Many have a 42T large chainring. That makes sense on single bikes but not tandems.

Also, cranks are getting long. My wife is about 5'4" (162cm) tall, and I would not subject her to cranks longer than 170mm. Even I can't stand cranks longer than that, and I'm 5'9" (175cm) tall.

Furthermore, most newer cranks don't use square taper spindles, so I'd need a new BB.

So it take some searching.

I'm not going to undo the upgrade I did in the rear. Not only do I really want the wider range and the closer spacing, but the rear derailleur and shifter are absolute champs. And I had to buy a new hub and rebuild the wheel to get it all together.

I might try another front derailleur, which is not likely to help, but ya nevah know.

One thing I like is that the front shifter is friction based with no click stops. I think indexed front derailleurs are a misguided idea.

Not being able to see it is a small part of the problem. The jamming is sudden, and first, one of us pushes the other person's legs. Then I have to balance the bike while not being able to propel it, and I may not even be able to turn the cranks into the stepping-off position. That position is usually with the left pedal down and my right foot disengaged and reaching for the ground. So not being able to move forward and not being able to stop leaves me with nothing to do. The last time this happened, I managed to stick my right foot out, but the road was heavily sloped downward to the right, and putting my foot down tipped the bike over to the right. I don't remember, but I might have had to jump off and put two feet down. And remember, I'm balancing for both of us. My wife isn't heavy, but I don't have strong arms.

For the time being, my wife has asked me not to risk that shift, which limits our gear range.

I just got a wacky idea. How about putting the big ring in the middle position and then just having two rings? They would be 28 and 54. The front derailleur might or might not be able to make such big shifts.

I’ve had similar problems except not where the chain jammed. I have always been able to spin the crank and redo the shift. On the tandem it would happen on, I think the middle ring was a bit warped. Are your rings true? That crank was not mismatched. I’ve run 9 and 10 speed chains on “5” speed cranks with only very rare failure of teeth to engage (again, just spinning w/o jamming) but these were on singles and much easier to tolerate than on a tandem. Just wondering if switching to 9 speed cassette and chain may work better? But before trying that I would try a different front derailleur with a flat outer plate. It looks like yours has a bump out at the lower end of the cage and I’m wondering if that is limiting the push on the chain inwards as the chain is falling? I guess that wouldn’t eliminate the risk of jamming but with care may make it avoidable.

Great stuff, @L134. Thank you! You've reinforced my idea of trying another FD. I have several in my bins, and it's not hard, much easier than changing chain and cassette. Well, then again, I have some spare 9-speed cassettes.

It's easy enough to experiment with the lands on the existing crank but going by the chain thickness guide a above you may have to remove too much.
Have the land thicknesses changed over time? Are they also narrower on modern cranks which would be a strange place to save material on a critical part.
I don't know but perhaps some modern 11 speed or 10 speed chainrings may work better if perhaps the lands are the same thickness but the rings shaped differently.
Now that's just me thinking I have no idea if that's true or not.
If somebody happens to have an old and a modern crank to compare they may be able to confirm.

I don't think any other front derailleur will solve the problem. The gap between the chainrings is too wide for the chain and that isn't going to change.

You might look for a 10-speed road triple crank. Shimano's HTII 10-speed 105-level FC-5703 crank came OEM as 50/39/30 but had a separate 74 mm BCD for the granny chainring so you could exchange the 30T for as small as 24T. I have these cranks on a couple of bikes set up as 50/39/26 and they work extremely well. They are out of production but should be available NOS with some searching.

First Tiagra 10-speed cranks triple cranks, FC-4603, also came 50/39/30 but the granny was bolted to the middle chainring with a 92 mm BCD so 30T is as small as you can go. These are also out of production and available only NOS.

Thanks, @HillRider. That makes sense. But I can't use a two-piece crank, as the stoker's left crank needs a chainring.

Maybe grinding down the spider makes sense. I might ruin it, but since I'm moving towards the replacement of the crank anyway, that's not a big loss.

We had a similar problem with our tandem, a 10spd triple. I was able to machine down the width
of the mounting arm on the crank and that helped but it was a dicey thing gripping the crank in a
sufficiently rigid fashion and deciding how much to remove. I have a milling machine.
I have found that 11spd cassettes work fine with 10spd chains so you might consider that option.
IF your cassette has no single tooth steps between cogs, then a 9spd chain would probably work
as well. It might be a bit noisier. If you are up for several hours of fiddling an aluminum rated
file with the narrow side teeth removed (ground off) and very good technique would allow thinning
of the crank arms. Filing flat requires some practice, it is easy to rock a bit.

I considered fabing some ramps to mount on the big ring to guide the chain to the middle ring but
my seat on tandem was taken over by the pilots GF and I stopped riding it.

Yeah, I forgot that point. It's too bad there isn't a NDS crank arm available for those cranks that includes a synchronizing chainring.

If the assumption is that the current spacing isn't removed mostly, that'd likely be the outcome.
The rings added would retain as close to original spacing yet put in place a chain guide jam prevention device.