New-Old-Stock Shimano RX100 Rear Derailleur w/Short Cage...Model RD-A551 (Second Generation Indexing)
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Product Description:

This offering is for a new-old-stock Shimano RX100 rear derailleur (RD-A551) in the short cage (SS) model and with a mostly gray enamel finish. With respect to the shifting capacities and other notable specifications, we have included the following...

  • Index shifting compatibility: 6/7/8 speed (with Shimano branded or compatible shifters and freewheel/cassette)

  • Friction shifting compatibility: Several options (most 5/6/7 speed freewheels/cassettes)

  • Rear capacity: Minimum to maximum...12T to 28T

  • Total capacity: 28T (combined front chainring and rear sprocket difference...also known as chain wrap)

  • Weight: 269 grams (catalog specification)

  • Other notes: The short cage (SS) designation relates solely to the pulley cage length, which represents the technical specification having the most direct impact on rear derailleur capacity limits. This particular model has a short cage length...measuring 48 mm from the center of the jockey/guide (top) pulley to the center of the tension (bottom) pulley. The primary benefit to a shorter pulley cage is less weight (although, in all honesty we are only talking about grams here, so probably not a noticeable savings for most of us). The downside, as most already know, to a shorter pulley cage is lower capacity limits, which limits these derailleurs to double crankset road bikes with a relatively narrow gear range on the freewheel/cassette.

    One other note regarding pulleys, Shimano incorporates a Centeron mechanism with it's jockey pulley on this rear derailleur, which allows for some lateral movement to compensate for imprecise index shifting. In other words, the jockey pulley has some built-in play within it's cage to better align under a sprocket when the indexing adjustment is not optimal.

This derailleur is a second generation 6/7/8-speed indexing model that was introduced in 1994 and remained in current production until Shimano discontinued the RX100 model at the end of the decade (so the last production year was 1999). For those that are interested, the RX100 groupset is linked to the more popular 105 groupset in the sense that the components are nearly identical, with the only meaningful difference being the finish. As most already know, the Shimano 105 groupset was finished in a light gray enamel, with the RX100 groupset is a polished silver.   

A brief glance at other features indicates some modest refinements compared to the first generation model and a pulley cage that is made from a hardened steel, which probably explains the slightly heavier weight relative to higher-end rear derailleur models from this generation (some of which were equipped with a lighter alloy pulley cage).  While the heavier weight may be a drawback, I wonder if the strength/durability of a hardened steel pulley cage is worth the added investment in weight (not sure, but something to consider).  There have also been some improvements to the pulley cage that serve as a better spoke barrier and chain retainer (relative to earlier models). It's also worth noting this unit is among Shimano's competition-level derailleurs from this era (albeit, at the lower end of the pecking order).

In terms of condition, this is a new-old-stock offering that has never been mounted on a frame. I acquired a handful of them, and they were all sealed in heavy plastic (bag) packaging that I opened to take pictures. The units show fairly well, but please expect some shopwear (scuffs and scratches) in the silver finish. Listing Update...I can confirm the uploaded pictures reflect the next unit that will sell/ship.

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