Hi All,
An update for those who have contacted me by email to ask...perhaps it will help others in need of a new rear derailleur for their Thorn...
I found a great deal on a new Deore Shadow SGS 9-sp rear derailleur and took the plunge. I installed it last evening, and am very pleased so far. In fact, it appears to have addressed all the drawbacks I've experienced with the original-style, double-sprung slant-parallelogram derailleur mechs and I'm happy at this point.
The Deore Shadow has no upper tension spring, and the derailleur is actuated via an extension of the back-side parallelogram instead of collapsing the plates diagonally on conventional designs. As such, it snuggles in very closely to the frame's derailleur hanger, and hides within the outline of the hub's QR nut in all but the top 2 or 3 cogs, making it much less vulnerable to damage in a fall-over or in packing and transport (it would be ideal for air travel; just shift onto cogs 1-7 and you wouldn't have to remove it when boxing the bike). Because there is no upper tension spring, the derailleur does not bounce on rutted roads or off-road and so remains silent. Wheel changes are nearly as quick and easy as on the front, and in the event of a heavy bashing, it appears the aluminim upper hanger would distort before the frame's derailleur tab, which is nice. The parallelogram spring is exposed and incredibly easy to clean, unlike on a conventional derailleur, where the parallelogram forms a box for dirt to enter with no room for a cleaning brush. There is no "return loop" of cable housing to catch on roadside plants and brush, and the stop is fixed so the housing stays in the same place no matter the gear. There is a straight shot for the cable, so it should last longer. The only omission I can see is the absence of a tension adjuster on the derailleur body. Instead, one must use the adjuster at the gear lever or downtube stop (as in the case of bar-cons, on my Sherpa).
Setup was easy, but as with all single-spring derailleurs (i.e. the old SunTour line), the "B" or angle-adjustment screw must be adjusted so the upper (jockey) pulley is the minimum workable distance from the teeth on the largest cassette cog. Get that and the cable tension right, and you're set. Once installed and adjusted properly, shifting is as good as the double-sprung Deore. The Shadow is supposed to be able to handle a 12-36 cassette as well, and the SGS requires a minimum 32t cog. Works fine on my 11-34 cassette and provides a full 45t of firm, no-sag wrapup using a 22/32/44t chainset.
It is kind of neat to watch the derailleur shift in the workstand. It sort of collapses on itself and then blooms again, Transformer-like, as it runs through the gears. I was concerned it might have less chain wrap around the smaller cogs because the body is offset to the rear. No worries; where the conventional design uses an offset jockey pulley, the Shadow's is concentric with the spring pivot and so has the same relationship and chain-cog engagement as the original.
I will post an update after I put some heavy miles on it. I've attached some composite photos comparing the Shadow to the conventional Deore and also showing the reach, range, and extension of the Shadow at high-low extremes mounted on my Sherpa.
One last note: The newer, 10-sp Deore DynaSys series of derailleurs and shifters require a different amount of cable pull to actuate and so are incompatible with 9-sp equipment. The Shadow-design rear mechs are also not interchangeable between DynaSys 10-sp and existing 9-sp.
Best,
Dan.
