PromptCritical 

We just purchased a "new to us" Santana tandem and we had some shifting problems yesterday on our first longer ride.

The crankset is a 48/42/24 and the cassette is a Shimano 9-speed 11-42T.

Since the rear derailleur is 17 years old, I figured a new one might be in order and I scoured the interwebs for a suitable one.

What I found (or didn't find) was one that meets both the maximum cog size (42) and maximum capacity (48-24 = 24 + 42-11 = 31 for a total of 55).

It appears that as bikes have transition to 2x & 1x drivetrains, the maximum capacities have decreased.

Does anyone have any suggestions for an RD that would work?

We really need to keep the bailout gear as we ride hills and are no longer as young as we once were.

TIA

alcjphil

Not provided is the model of your current rear derailleur and the shifter you are using

PromptCritical 

It is a Shimano Deore XT. No other identifying information on it.

dedhed

Number on back like RD-M7XX

zandoval 

I get ya about that bailout gear! I spend more time in it then I would like to admit. I have had lots of problems with derailleurs shifting on my multiple, assembled out of the parts bins, mismatched wheel sets, Franken Bikes. My experience is to first check that long cage derailleur. Then check how far that derailleur can push in towards the wheel.

On one of my Franken bikes I had to extend the derailleur attachment. It was in an odd ball position. By welding on another derailleur achievement I was able to drop the position of the derailleur just 3mm and that solved my problem.

On another Franken bike I was using an old steel wheel set. I had to reset the Dish on the wheel. Just moving the Dish 3mm fixed the problem.

I use such odd ball components some of these fixes might seem strange. But then a Tandem is kinda odd ball in itself... Ha

And of course, you probably know this.

bboy314

Any “shadow” style 9 speed MTB derailleur should work, but I don’t think any of these are made by Shimano anymore for 9 speeds, except for the new CUES stuff which will require replacing more parts. You could swap shifter and derailleur for Microshift Advent and keep the cassette. Or possibly the Sunrace M9 derailleur could be a direct replacement.

But I wouldn’t assume the derailleur needs replacing just because of its age - there are plenty of other factors that could be affecting your shifting.

Andrew R Stewart 

A few comments-

An aprox 15" gear (24x42 on a "27" wheel") is a very low gear and one that being able to balance the bike (especially a tandem), at the 3ish MPH that this low ratio is often run at, would be hard to do for many to actually do. But I am a proponent of low gearing and do spin (as well as have a lot of tandem miles in my past) so if you can get it all to work that's great IMO.

Do know you're mixing different "generational" versions of what is "the right way" to design a gear system. The triple crank is "old school" (all my bikes that I ride much have them) but the cassette range is "new school". The ders that the triple crank was intended to be used with generally didn't have the large rear cog capacity to handle more than a 36 at most (and pretty much all of that era had a 34T max as the listed limit). trying to use a cassette with so huge a large cog was never their intention. As the industry has moved on from front shifting in general and have had to instead go to larger rear cogs for the low gear ratio the chain wrap capacities have reflected this.

An attempt at a work around that can work for some is the der hanger extended adaptor (aka "Wolf Tooth"). This mounts the rear der farther away from the cogs, thus allowing a larger diameter/greater tooth count. One issue with these adaptors is that the smaller cogs are also further from the der's guide pulley and the additional chain links, in between the pulley and cog underside, all have to go through their amount of lateral flex/slop before the "top" link will even try to climb off the current cog it's on before the shift is tried. When all is new and not too worn this can be OK... Add wear or other issues and...

For the rear der to "like" shifting into a cog it wants the guide pulley to be lower down then the cog's underside. Too far under and things get sluggish, too close and the chain might not be able to climb onto the next larger cog well or at all. This balancing act is why slanted parallelograms and sprung B pivots have been a standard (since the 1980s aprox) when a wide range of chain wrap is needed. (A notable exception was the Huret DuoPar). The book "The Dancing Chain" is a great resource for those looking to better understand all this.

One other issue might be that the guide pulley bushing is worn enough to have more lateral slop then when new. If that pulley is rocking about it can't control where it's feeding in the chain WRT where the intended cog's teeth tops are. Pulleys can be replaced if that were the only issue, however with the miss match between old and new this is not the only issue.

On the der's backside of the inner parallelogram link is where Shimano usually places the actual version/model number. RD-M761 is an example of one Deore XT version. These model numbers can be hard to see and often covered with grime. For all the flack Shimano gets for various reasons they have done a far better job at stamping their parts with model numbers then other companies.

Now besides all the mechanical crap affecting shifting there's that elephant on the back of the tandem, so to speak. A tandem with its far longer cables and that added power unit (stoker) which usually lags behind any "commands" from the captain just increases the challenge of shifting.

I suspect this tandem is your first as a team. I also suspect that who ever was servicing this tandem didn't pay attention to der capacity limits or such either. My first tandem was long before my stoker was always the same person. My rides were far less pleasant and more challenging to "work" the bike than after I married. (That first tandem came from a couple who never could work in harmony. My second and third tandems saw over 30K miles with my late wife, you do the math). Besides my stoker learning how to be steadier in their movements I learned how to better communicate and we learned what it took for the bike to work better. This didn't happen on the first ride and we "forgot" our techniques far more often when either distracted (like my wanting to keep up with others) or tired.

When I worked in a tandem specialist shop I got to watch many teams go through this as they tried to see if their hope of a more intimate riding experience was going to turn out well. When we did service work on our customer's bikes the mechanic would test ride after the work alone first then have ne of the other shop people stoke for them. That added power and lessening of control over that power really changed the dynamics and the bike would sometimes need a revisit to the repair stand before the work was done as well as we wanted. Andy

PromptCritical 

Thanks for the suggestions, and I understand that, but am trying to work the problem in parallel, as well as the condition of the bike indicates a replacement would be prudent (lots of stuff is well worn and loose). The Park tool for checking derailleur alignment should arrive tonight, so that will be checked although it "looks" properly aligned.

Amazon and eBay have a lot of NOS, but it is very challenging to find out exactly what is being sold.

We are going on a 6 day tour in the Zion/Bryce canyon area and we really need the granny gear (yes, we are both grandparents - best promotion ever!)

PromptCritical 

Thanks for the great explanation! Yes, my stoker is new to tandems (and doing great, but we aren't quite a well oiled team yet), but I have been riding them since the early '80s, although until last month my tandem was a 1995 3x7. And, you are also correct about the maintenance of the bike, which hasn't been so great. The cables are all new with top quality cables and housings. I did add a Wolf Tooth DR extender when I changed the cassette from a 36 to 40 granny gear.

I'll look for the RD serial number tonight.

Do you think changing from an STI shifter to a friction shifter would help? Yesterday, the chain jumped into the spokes twice, which as you know is quite the disaster on a slow speed uphill with traffic. The limit screws were properly adjusted and worked great in the garage, so I'm guessing as you suggested the length of the cables and power of two riders (and likely a very worn and loose DR) caused this.

soyabean

The Deore XT is a premium all-metal shifter and I have never seen one of these go bad unless they got smashed.

If you're looking for a possible "better than Shimano", you may want to go all SRAM with 1:1 actuation ratio on the cable.

zandoval 

Thanks Andy, this explains allot. Most of the components I have used on my Franken bikes are quite worn. And I have noticed that on the bikes that I extended the derailleur position on I was not able to shift quickly. They all shifted but not fast. Dork Disks were a rule... Ha

FBinNY 

The combination of a super large cassette with a wide range triple means much greater than normal take up will be needed. Offhand I don't know of any specific RD that will fill the bill, but that doesn't mean one doesn't exist.

However, you might consider a solution that will serve your needs by deciding that you will not use the granny with smaller cassette sprockets. If you go that route, be sure to measure the chain to safely loop big/big with an inch or so to spare. You should be able to use middle and outer chainrings with any cassette sprocket, but the granny will probaby be limited to the larger half of the cassette. Should you accidentally shift to a too small combination, the lower loop will sag, but nothing serious will happen.

FWIW - my touring bike was set up this way for years allowing me full use of my normal riding combinations, with the bailout granny only used when needed.

Andrew R Stewart 

"Shifting into the spokes" is a classic indication of the chain's movement not being controlled properly. Once limit screws are set (or at least the low gear one) they rarely spin in their threads as to move and thus change their limiting. A very common way this can seem to happen is when the der is bent inward, from an impact/fall down, but this would have to happen after the limit screws were set. But then the indexing "coordination" WRT cog locations will also be upset across the cassette and the shifting into the high gear likely problematic. I doubt this is the case. A worn out guide pulley is the other way this overshifting into spokes can happen. As one sets the limit screws while the bike is on a stand (meaning with no power being transmitted through the chain) the pulley can act nice and stay fairly stable. But with power and/or a more forceful lever pulling that pulley can rock and want to move the chain over that extra little bit.

Of course the best solution is a fully matched system that also provides the gear ratios wanted. But we don't always get what we want, or what we need, (sorry Mick J). Andy

PromptCritical 

Andy,

Amazon just delivered the Park DR alignment tool, so that will be checked tonight.

I would also imagine that worn bushings in the DR wouldn’t help. I replaced the idler wheels.

What I would give for NOS matching components…..


Thanks again for your insight.

PromptCritical 

I would LOVE a current SRAM setup, but that isn’t going to happen before our ride in two weeks.

One of my thoughts is an AXS rear DR with a POD controller. I have an email into SRAM asking about compatibility.

PromptCritical 

So, the Park DR tool came today and I spent a couple hours on the bike. The rear DR was waaaaay out. Like 15+mm. It is now less than 1mm out. I readjusted the B screw and the two limit screws (why are those things that get so gunked up always phillips head screws?).

BTW - the RD# is a DR-M750.

soyabean

Oh it is indeed a fantastic tool.

I resell bikes and it's required to correct every bike I get, 19/20 of them.

But even the home user with one bicycle, will benefit from the tool because it will save them SO MUCH time by elminating doubt.

Before I got the tool, I did my best to eyeball it, then I had to road test the bike. This process could take as much as an hour.

Now with the tool, it's done once in minutes, I don't have to road test the bike after a RD repair.

The tool also shows me RD's that are smashed. Dropout is aligned but the RD itself is bent.

shelbyfv

FWIW, they resemble Phillips but aren't. Best to use a flat head unless you can source the proper driver. They are easy to bugger with a Phillips.

veganbikes

Shimano screws are JIS. Modern screwdrivers are going towards just a unified crosstip so eliminating both Phillips and JIS while being compatible with both without camming out.

In terms of derailleur replacement the first post mentions nothing about needing a derailleur to solve the problem. Those old XT derailleurs are pretty much cockroach like in that they can survive just about anything and will probably still be around long after we are gone. I have one that was throughly used and abused before I got it and did the same and it shifts like a dream every time running 9 speed (it is RD-M737) and had the previous iteration which was 7 speed running 9 speed just fine but I sold the bike it was on. I hope the alignment and adjustment have helped it to work better. If you are still having issues I would make sure chain and cassette are replaced and maybe chainrings and make sure the cable is moving smoothly and has the right tension and all of that.

The derailleur could be an issue but those derailleurs are hard to kill and would be something that I wish Shimano would re-release, just an exact replica maybe even an 11 speed version and make a lot of people happy.

shelbyfv

Good to know! Would have saved many limit screws if that had been done years ago. Now mfgs are moving to Allens....

PromptCritical 

Yep. Around here the wait to get the LBS is 10+ days, so one more reason to do all my own work.

PromptCritical 

I thought they were JIS; thanks for confirming that. Even with my JIS screwdriver, the B screw was challenging and is no an Allen. The rest will be soon.

So, everything except the rear DR is now new. Fingers crossed!

shelbyfv

Good luck! As has been mentioned above, the large cog and gear combo as a whole are well beyond any design specs so you may have to temper your expectations. Kudos to you and stoker for being able to keep a tandem upright in 24/42!

PromptCritical 

Yeah, it was bit "sporty" when the chain jumped into the spokes on a busy road while in 24/42.

grumpus

As already stated, these are JIS - they work a lot better with the correct driver. I use bits of inner cable as tiny wire brushes to clean out crusty fastener heads.