[CR]SR vs NR Cage Plates and Cog Clearances.

(Example: Framebuilding:Norris Lockley)

From: "Andrew Gillis" <apgmaa@earthlink.net>
To: <classicrendezvous@bikelist.org>
References: <43.1ceb48bd.2bf9591e@aol.com>
Date: Sun, 18 May 2003 20:37:13 -0700
Subject: [CR]SR vs NR Cage Plates and Cog Clearances.

CRs:

Bob Freeman's previous comments even more extensively articulate my more limited experience with Campy NR derailleurs and the criticality of chain length.

I've been using a 42x52, 13-23 on my Mercian and I needed to shorten the chain to 53" in order to make my 1984 NR rear derailleur shift smoothly into the 42x23. (Rear wheel to BB is 41.3 cm).

I then installed a set of Super Record cage plates onto the NR rear derailleur and I was subsequently able to easily shift into the 42x23 when using my preferred 54" chain. The SR cage plates have the same spacing between the pulleys as NR, but the pivot is 1 cm higher so the pulleys sit 1 cm lower (futher away) from the freewheel. This change gave me enough clearance to apparently run up to a 26 rear cog (I haven't tried it yet, but there is a lot of room on my 23).

The longer 54" chain gives relaxed chain tension, easier shifting, and less fatigue on the pulley tension spring.

I was told long ago that the 'ideal' setup was to have the pulley wheels perpendicular when in top gear (52x13) and also in lowest gear (42x23). This pulley wheel alignment is more easily achieved when using the SR cage plates and a 54" chain, compared to the NR plates and a 53" chain.

I recall that a statement that the Campy SR derailleur was optimized for improved shifting into a 12-tooth cog. However, my observations on the SR cage plates imply that maybe the re-optimization of the derailleur was for the more prevalent 42-53 chainring combinations. Given that the Japanese slant-pantograph design maintains a tight clearance on the cogs to improve shifting, the Campy SR derailleur's increase of clearance between the pulleys and the freewheel contradicts the rumored improvement on shifting into smaller cogs. The increase in the length of the top lip on the outer plate helps make up for the lower placement of the cogs, but there is a lot going on in just a change in the cage plate design.

Incidentally, I couldn't find original SR pulley bolts and nuts, so I found some very serviceable metric hardware from my local the Ace True Value hardware store. (No commercial interest here, just that it worked!)

Andrew Gillis (4.5 hours today on the Mercian today, with a clean chain and new tires!)