Maximum sprocket size - 1x vs 2x
 

Hi. I'd like to install a larger cassette on my tandem to make it easier to ride a loaded tandem through the Rockies. I have a Shimano Ultegra 2x11 drivetrain. Currently I have a 11-40 cassette. Going to 11-46 cassette would help. 11-50 might be better, but 11-46 might be enough. Shimano specs on their RDs sometimes specify a maximum low sprocket for 1x and a lower maximum low sprocket for 2x. For example, the maximum low sprocket for the Deore 11s RD-M5120-SGS is 46T for 1x10 and 42T for 2x11 and 2x10. Microshift lists the XCD RD-M865M as having a maximum cog of 40-46T without explaining why there is a range.

Is this difference or range strictly a function of total capacity of the RD to take up chain slack?

If that was the case, then I might be able to manage it by avoiding being in the small chainring and one of the smaller cogs to avoid a slack chain. That would be a hassle, but might be worth being able to get a larger cassette. I might also need to minimize big-big to minimize noise/wear/etc., but overall it seems that it would work.

What am I overlooking? Thanks.

"What am I overlooking?"
A triple

Smaller chainrings

The 1x rating is the largest cassette it can clear. It will have plenty of chain wrap as a single. The double rating accounts for as much chain wrap as it can handle with the appropriate front double (36-26)

What are you chainring sizes on the 2x part of your 2 x 11 drivetrain? Getting smaller rings or a crank with smaller rings might be the better way to get smaller ratios.

Though with a tandem there are probably things for their drivetrains that might make some difference I'm not aware of.

Still, for the big 40 tooth cog you say you already have, I can't imagine that is not enough. Is someone not pulling their share?

Look at the "Total Capacity".

The Deore RD-M5120-SGS lists the Total Capacity as 41T.

So, say you have a 11/42 cassette, and a 50/34 crankset.

You get:

42-11 = 31T
50-34 = 16T
========
Sum 47T.

You're greater than the 41T "capacity" or "wrap" of the derailleur.

It may still work, but you may end up with problems when cross-chained. Either slack on the small/small combo, or over-stretched on the big/big combo.

I configured one bike that way. With a "Shadow" style derailleur. I adjusted the chain tight (no slack on small/small, won't work on big/big). On the few instances I mis-shifted, the bike failed to shift to the improper larger sprocket, and I quickly realized my error without any problems.

I'd be concerned about a mis-shifting on a Tandem. You've got a lot more power going into the shifts, may have less feel of the drivetrain, and have to coordinate between two people.

The current chainrings are 50-34. They are 110 BCD, so it isn't possible to go lower than 34T on the small chainring. Absolute Black would allow going to 48-32 chainrings, but that's not much of an improvement. Adding a third granny gear to the current chainrings isn't really possible since we are already near the extreme ends for FD capacity. Most triples are something like 50-40-30, so it might help a little. Swapping out the entire crank set for a smaller BCD would allow smaller chainrings, but I doubt there are crank sets available with a small BCD that also will allows for a tandem setup with a timing chain on the nondrive side. More specifically, a Gates belt drive. I've pretty much determined that going to smaller chainrings would require major surgery. If it works, changing the cassette and RD (with a Wolftooth tanpan or Jtek converter) would be an outpatient procedure.

At 34-40, our lowest gearing is 22.4 gear inches with 650b wheels. For a loaded bike going up more than 6%, that's tough. Getting gear inches below 20 would be nice. With our current low gear, at a cadence of 72 rpm we go 4.8 mph. If we could get to below 20, at 72 rpm speed would drop to 4.2 mph It may not seem like a lot, but might make the difference between biking and pushing.

I would avoid Big/big not being able to work. I know we all say: " it will be ok i will never use that combo" but life happens. and the results are not good if you are at speed

something else to look at might be a gravel crankset at 46/30

Going to smaller chainrings doesn't require major surgery. It just requires buying cranks with smaller rings.

For a tandem rear, they have to have chainrings on both sides. I.E. Either specific for tandems, or compatible with existing parts. So, it can become an expensive hassle.

Another option would be various "triplizer" options if the right parts could be found.

Of course, one still ends up with issues of limited capacity for both the front and rear derailleurs.

totally forgot about this part of tandem setups so maybe OP can look at a FSA tandem triple or ??? https://www.benscycle.com/fsa--full-...24548-cr6135/p

Getting a 30T small chainring (which seems to be as low as 3x systems go) with a 40T cog gets similar gear inches as a 34T chainring with a 46T cog (19.7 or 19.4). Replacing a cassette and RD seems a lot easier than replacing the crankset, chainrings and FD, especially since a new crankset would need to be able to handle the non-driveside Gates belt drive sprocket. Back to my original question, is there any significance to the maximum cog size being different for 1x vs. 2x except for the RD's capacity and inability to take up enough chain slack if the small chainring is used on a 2x drivetrain?

My understanding is that chain slack capacity has no direct relation to the max rear cog capacity. I.e the shimano GRX 812 which is a 1x RD, with 42 max cog is not able to handle 42 because it has a 31 tooth capacity, but because it is designed for 42. and to note the GRX 810 with is a 2x RD, has a 40 tooth capacity, but max cog of 34 and max front chain ring difference of 17 th

For most of my riding, I don't need the super low gears. But there are a few applications when they would come in handy.

I'm surprised there isn't a bigger market for modifying derailleur cages.

Adding a monster lower tension pulley to the derailleur might give a few more links of chain pull.

Suntour made a 3 pulley derailleur. Surely long out of patent by now. I think the idea was increasing the derailleur capacity without the cage hanging lower. I haven't used one, but I assume that the third pulley pulls away from the chain with the big/big combo, then picks it up as it takes up more slack to the small/small combo. I'd be curious if it periodically just loses the chain. :eek:

A 74 BCD crank or tripilizer will fit a 26T sprocket. If someone makes a triplizer with a 110/74 BCD, that mIght be the way to go.