Short cage rear mech with much wrap?

[LittleWheelsandBig]

HK’s recumbent has a 355 rear wheel, a short cage Ultegra 9sp mech and Shimano 9sp bar end levers. It is already overmatched running big-big to small-small. She wants to swap from a double crank to a triple, so will need more chain wrap. The small wheel means a long cage mech would be right down in the weeds and the short cage mech is already bad enough.

What is a nice 9sp-compatible rear mech with a short cage but lots of wrap? The nearest I have found is the RD-M310 Altus or possibly RD-M360 Acera, though that looks to have a longer cage.

[BFC]

Any mech with larger/oversize jockey wheel(s) will increase the total gear range - only way to find out if it's enough is to try it.

Ultegra 9s were ceramic (floating) bush top pulley, sealed bearing bottom pulley when made, the mech (pivots and general precision) is a very good unit - as durable as DA but not as many weight reductions (bolts aren't Ti, merely SS, big bits are hot forged rather than cold forged on DA). I would prefer a carefully bodged Ultegra with with bigger pulleys to a low series off the peg unit (cast big bits, sloppy pivots).

May need an extension to the rear mech hanger or major B screw adjustment to work with bigger pulleys (problem is more likely to be with the big sprockets). Should need a longer chain to take the scenic route around bigger jockey wheels.

I passed on most of my 9s bits, only things left are a DA octalink triple crank (7703, 172.5mm, 30/39/53) with worn out rings and it's threaded BB (BSA, english...), and the rocking horse shit part - a braze on 6503 ultegra front mech (that works with 6500 or equivalent double shift levers).

[DaveReading]

What is a nice 9sp-compatible rear mech with a short cage but lots of wrap? The nearest I have found is the RD-M310 Altus or possibly RD-M360 Acera, though that looks to have a longer cage.

If it helps, Shimano quote the capacity of the M360 as 43 teeth.

[LittleWheelsandBig]

Thanks for suggesting modifying the existing mech, I hadn’t thought about that option before. The cassette will probably wear out before next year, so a new chain will be necessary anyway.

The Capreo cassette fitted to this trike (required to get reasonable high gears with 355 tyres) is 9-26t and I’ve never seen a Capreo-compatible cassette with more than 32t. Having capacity for larger sprockets is probably irrelevant. I am not sure what are the largest pulleys that could fit on an Ultegra mech but I am pretty sure that it wouldn’t be able to absorb 43t of wrap.

The current chainrings are 52/ 34 110 BCD. I am thinking of going up to 56t or so, down to 30t or 28t and 42t or a bit larger in the middle. Drilling and tapping out the forging (granny bosses exist but aren't drilled/ tapped) for a granny ring is pretty easy. The 160mm cranks are necessary for HK’s heels to clear the cross-member as the boom doesn’t need to stick out very far.

I see the main issue as ground clearance. The shorter the cage, the better. Replacing a mech every couple of years because of wear isn’t an issue. You would get five or more cheap mechs for the cost of one expensive derailleur.

[cycleman]

Ice trikes run a shorter   chain which pulls the mech forward and up increasing ground  clearance. 

[LittleWheelsandBig]

Yes, the chain is currently at the minimum possible length for big-big. A link shorter would rip the derailleur off.

Late last night, I risked a whole £20 to see if a M310 would do the job. It is supposed to have 43t of wrap. Something to fit before next weekend.

[Kim]

Possibly worth asking how much wrap you actually need in the real world.  The ICE chainline is pretty tolerant of the chain going slack, and that's only likely to happen in small:smallish gear combinations which you're unlikely to end up in by accident.  Barakta's chain has a couple of extra links so the boom can be extended to fit me without chain jibbling, and it's fine in the granny ring for the lower half of the cassette.  Granny ring plus higher sprockets only gets used as a deliberate tactic to raise the derailleur out of the grass on campsites, and if you overdo it the chain tube literally rubs on the ground.

[BFC]

With through the night riding for uprights and general riding for recumbents it is easy to lose track of where the rear shifter has ended up, generally not an issue on shorter rides but losing track of what gear your in on longer rides is the time when a "bad" gear shift can destroy the ride.

If the transmission can be made intrinsically safe (won't self destruct, even if it runs like shit and makes a lot of noise) is a worthwhile target, sweeping the road/grass with a chain seams a lot better than ripping the rear mech off.

[Kim]

With through the night riding for uprights and general riding for recumbents it is easy to lose track of where the rear shifter has ended up

Depends entirely on your shifter.  Bar-ends and trigger shifters with indicators neatly avoid this problem.  Twist-grips OTOH are a work of Stan in this respect, as if they weren't already for other reasons.  Roadie shifters which are similarly afflicted are an upwrongular compromise that have no place on a self-respecting recumbent 

If the transmission can be made intrinsically safe (won't self destruct, even if it runs like shit and makes a lot of noise) is a worthwhile target, sweeping the road/grass with a chain seams a lot better than ripping the rear mech off.

Absolutely.  Outside of limited and specific circumstances (I'm thinking record attempts or something) it seems daft to have a drivetrain that will self-destruct, get stuck in a particular gear or otherwise ruin your day if something gets knocked or you're not paying attention.

[LittleWheelsandBig]

Altus fitted and indexing ok on 9 speed with enough ground clearance on the 355 wheel. Lots of available chain wrap left for when a triple chainset is fitted. HK notes that the shifting is not as crisp as with the previous Ultegra, so at some point I will have to check hanger alignment and cable tension.

[LittleWheelsandBig]

The Altus is apparently shifting very well, now that the hanger alignment was tweaked, the worn Shimano indexed bar end lever was replaced by a Sunrace 9sp equivalent and the cable tension adjusted.

It easily handles the 9-28t Sunrace cassette and new triple crankset with capacity to spare.

The Altus still has more pivot play from brand new than I would like but it is functioning perfectly over a 9sp Capreo cassette, once the angle adjustment screw was backed out all the way. It has a mile of ground clearance compared any long cage alternative and roughly as much ground clearance as the previous short cage Ultegra.

[LittleWheelsandBig]

HK is currently borrowing (with a view to purchase) a Birdy Blue with 355 wheels and a 3x7 SRAM Spectro hub/ derailleur combination. It weights too much, has too many gears for her liking and the twist grip shifters don't suit.

The 3sp hub only adds 2 gears at both the top end and bottom end of the middle range and the resulting top end is too high for touring anyway. An 11sp 11-42t cassette and trigger shifter might do the job nicely and drop a lot of weight. The Birdy drags long arm derailleurs in the dirt and the rear mech needs to absorb a lot of chain when the swingarm is folded. What is the shortest arm 11sp rear mech that wraps a lot of chain?