We want one lower gear. Anyone using 22T low chainring?
 

Our drive train: 24/39/50 x 12-36 9-speed cassette with 700X35C tires, for about 18 gear inches on our Co-Mo Speedster. Shifting requires a little care, but it got us fully-loaded across America this summer without ever having to walk uphill.
But!
We kept wishing we had just one lower gear, especially through Appalachian climbs we barely made.
Our FSA Gossamer MegaExo cranks have a 130/74mm bcd spider, which won't take lower than a 24T.
Is anyone using a 22T granny gear on your tandem, and how?
-Any tandem crank with a 104/64mm or 110/58bcd spider and the same FSA external-bearing bottom bracket?
-Anyone switch to square-taper BB to get smaller spiders? (I see DaVinci and Middleburn have options with interchangeable spiders, but expensive.)
-Any other solutions you've found to get one lower gear combo? (Bigger big cassette cog, perhaps?)

Doing the math, you are already at 50 teeth of derailleur capacity. That's a lot. Off hand the most capable derailleur I can think of is Shimano XTR M-9000-SGS Shadow Plus. & it's capacity is 47. I'm sure that capacity can be found in lower tier models by now.

My point is findind a lower climbing gear is going to require a smaller crankset all the way around. Meaning a smaller big ring too. You will be trading the high end for more low.

Irrespective of mechanical considerations, at some point it becomes very difficult to get enough purchase with each pedal stroke to stay upright. On a lone mountain bike, my own personal limit is about 15 gear inches. At that point either the front wheel comes off the ground, the rear is limited by traction, or the bike is so slow balance becomes an issue...& you can rule out starting out in a gear that low. I can't imagine a loaded tandem would be any easier.

I think you are at about the limit of practicality with out dumping serious cash on a derailleured Rohloff type situation.

With 26" wheels my Rohloff gives me a 17" low gear. Rohloff doesn't recommend a gear ratio between front and rear of 2.5 to 1 for GVW above 100 kilos. I have a 50 tooth up front and a 20 tooth in the rear. A Schlumpf Mountain Drive might solve your problem. They are less expensive than a Rohloff or Pinion. You could have 727 percent range. Would a gear inch range of 15 to 110 work for you. The crank is an internal 2 speed. If this interests the OP than look into it. Cycle Monkey is a US importer for the Schlumpf cranks. Recumbent bicycle shops also may have them.

I do believe you are correct. The Schlumpf's come in a standard 5x110 bcd & a type "08" version. It is possible to run the 5x110bcd as a double & run the timing chain to the unused chain ring. They are rated at 250nm of torque, so using one on a tandem should be ok.

As far as Cycle Monkey is concerned, I have not had luck in any form of contacting them this past year. After many fruitless attempts both email & phone, eventually their voice mailbox said it was full & could not accept more messeges. My only guess is the covids caught up to them & they are no longer in business. Or, so far behind it doesn't even matter anymore.

An email to Haberstock Mobility directly got a prompt response & a new Speed-drive with crank arms Cycle Monkey doesn't even stock to my door within a week via DHL. (Silver, 172.5mm)

I wouldn't have any problem bypassing Cycle Monkey again on the future.

Thanks for the response. All good points!
Sorry, I should have made clear: I know our current setup is totally illegal, but it works just fine for us. It has worked for 5,000 miles, including 3,800 miles across all the mountains between SF and Yorktown, Va.
Yep. My thought exactly. Hence my questions about smaller spiders and switching to square taper. I just want to make sure I'm considering all the options other people have tried before I buy a whole new crankset to get to, say, 22/32/44.
You're absolutely right, and by noting we rode it across country, I meant to indicate that we can handle our current low gear and think we can still handle one more. We were climbing at 3-4mph at 18.1 gear inches and balancing is more challenging, but we're never close to losing control.

15-110 would be beyond my dreams! I was thinking of the 16.6-99.6 gear inches I would get with a 22/32/44 X 12-36 (vs. my current 18.1-113.2). I shall look into it.

I have more than an average interest in The Schlumpf Mountain drive. I intend to put one in a long tail cargo bike. I thoroughly look into products before purchasing. I followed the Rohloff hub through online sources for over 12 years before purchasing my Co-Motion Pangea Rohloff. The same has been with the Schlumpf mountain drive. Kinetics bicycle shop has a gear calculator that is some type of executable file for windows and it is the only gear calculator that I have found with the Schlumpf in it. Type in (K_gear.exe) to locate it with your browser. If you look for info on the Schlumpf they do say that they have Tandem versions of their setup. This being said it is more complicated than that. You will need a bicycle shop that is familiar with what you want. Utah Trikes sells allot of Schlumpf drives. After you or a knowledgeable shop has figured out fitment and compatibility. Not being the common setup it will probably be a special order. There are Two ways to mount a Schlumpf into a standard BB. The most common way is with the use of a hand mill. You cut a taper into the bottom bracket. They also have them with a torque arm. The setup has a light seal so they use a light molly grease that needs refreshing every few thousand miles. you can also use lighter lubrication that they sell or heavy gear oil to reduce friction but you will need to add it more frequently and wipe seeping oil off with a rag. I believe the unit has a grease port for this. By the way: you do no how to shift it don't you. Remember the Wizard of OZ. Dorthey click your heels.

You can achieve the same result of 22 in front by going with a 40 in back. For example with a SunRace CS-M980 9 Speed 11-40t or similar.

With the 11 in back you will also have more top end. In fact the 39-11 combo matches your current 50-14 combo.

So if you are willing to lose your topmost gear of 50-12, then you can just run the system as 2x9 and remove the 50 ring altogether.

Which is likely going to be required anyway since if you run 11-40 with 3 chainrings you will have exceeded all of the capacity ratings of the derailleur and it may shift horribly.

1. There's always a triplizer middle ring if one would survive on a tandem. Might even work with the current cranks set. Wouldn't be hard to get one made if not. I know a guy who could if you can't find one that survives on a tandem, assuming there's no other clearance issue.
2. We cheated by having a da Vinci tandem, so I just changed the middle ratio. Instead of a 34->17 in the middle it's 34->22. Add a (I think) 34 on the back with the 12 on the front and 26" wheels and we're at about 13 gear inches on the low end, and some days it feels like not enough with some of the hills around here. I wouldn't normally let a random pair of strangers ride our bike, but seeing as you've ridden a tandem more on one trip than I do in a year you're welcome to try it out if you want to see how that ratio feels. If your location is correct you also live within about 3 miles of me as the crow flies, so it wouldn't be hard to arrange.

TA makes craksets with inter-changeable spiders. Not sure if they make a tandem crankset but Peter White lists a 58mm granny diameter.

TACARSPI94x58B 94mm - 58mm Triple, Black, Smallest ring, 20 teeth

Talk to Peter (or maybe a British or French bike shop) and he should be able to tell youif such a beast exists.

I don't worry about the high gear much on a touring tandem. By the time you can go fast enough to use it, you might as well be coasting. I also don't like 11T cog because I can feel each tooth engaging. 12T is OK but I have yet to find 11S 12-42T Shimano cogset.

Another option: https://boxcomponents.com/products/b...-bike-cassette

I know it is likely out of bounds for this conversation. (ie, new bike) But, a DaVinci drivetrain has the ability to have a larger gear inch range. The intermediate shaft is usually geared 2:1 from the crank (though that can be modified) so your front cogs run 2x the speed and are smaller. (ie 13-20-29 on the cogs is the standard crank/chainring equivalent of 26/40/58) While there are some gearing tricks inherent in that setup, the range bonus becomes more apparent when talking about chain wrap. With the front cogs being half the size you use up less chain wrap allowing for a larger rear range. For example, a 24-50 standard crank is a 26 tooth chain wrap leaving you with only 21 teeth in the rear (ie 11-32) for the 'legal' 47 tooth chain wrap. But if you are running a 2:1 on the DaVinci the 24-50 becomes actual gears of 12-25, dropping your teeth differential to 13, allowing you 34 teeth in the rear. So you could have a 11-45 cog set.
Again, this is more hypothetical and really only talks to the gear inch RANGE that can be achieved. If you are stuck with a standard Tandem drivetrain then I would just take some teeth off the top end and move them to the low end if you need the low gear inches.
simon

We went down to a 24T granny and then put on an 11-40 cassette off Amazon. That's an easy fix for you. Had to use a Wolf Tooth (has to be the real Wolf Tooth) with an MTB RD as mentioned above, although ours is the old Shimano XTR RD-M952 9 Speed Long Cage Rear Derailleur which came on our Speedster. Ebay if you don't have one. This setup works very well. Though the gaps are larger than we'd like, it was an easy fix.

[Edit]: I hadn't seen the above comments about chain wrap when I wrote this. No, you won't be able to use any cog which any chainring, but (hilariously) you don't need to use every cog with every ring. We set ours up so that the 53/40 won't go, though only one link shy of that, so no damage if you screw up. The small cogs with the 24T are useless, and yep, they sure are. So you just shift a ring without doing the math in your head. :) We do use the 53 X 11. Nothing wrong with pedaling downhill, keeps the blood in the legs flowing.

Due to the amount of riding we do and hence chainring wear, and hence scarcity of original 9-speed rings, we changed the left brifter and FD to 10-speed and use the 10-speed 39T triple middle ring and the 10-speed 53T double outer ring. That solved our shifting problems. Only downside is that the 10-speed brifters use very small drums, which results in early cable failure on tandems, so we replace that cable every year before it breaks..

What I find most inspiring about this thread is the enthusiasm of working this problem. I really have no experience with Tandums but what fun tackling this need. Looks like you have plenty of chain, room, and available parts for a successful outcome. Fun, Fun, Fun...

Thanks. I'll have to try this. Our Speedster has that very RD, and I already put a real Wolf Tooth on when I want to the 24T granny.