I'm looking at a Shimano XT derailluer that comes in both long cage (SGS) and med cage (GS). They both have the same largest/smallest (34/11t) rear cassette capacities, and the same 22t difference for the front chainrings. But, overall the SGS has a 45t capacity and the SG is 33t. Can someone explain this? Where are these numbers derrived?
Difference between largest cassette and smallest cassette (eg 11-34 would have a 23t difference)
plus difference between largest CW and smallest CW (eg 22-44 would be 22t). Add it up for 45t.
The RD has to keep some tension the chain in the loosest configuration (11t cassette and 22t CW)
but not have so little chain it is straightened fully when the chain is on the 44T CW and 34t cog.
The longer the cage the more the chain can be tensioned at extremes of smaller teeth.
Anyone know why the Shimano spec quotes a maximum front difference, as well as max/min rear cog and max total difference. What does the rear derrailliur care about the difference at the front, except in so far as it contributes to the overall capacity?
The RD specs have to cover a variety of possible configurations. The most common
limit is how large a cassette can it increase its radius of action to derail a chain off
and on. Many road RD are limited to 28 or at most 30t cog sizes. ATB RD by default
will all have to cover a 32 and usually a 34t cog as these are typical sizes on ATB.
The other factor is how much chain can they wrap up, and this has to do with the
cage length and the range of angles the cage can move through to accomodate
chain wrap. As the amount of chain occupied by the teeth on the cog and CW
decreases there is more loose chain between the end of the RD and the beginning
of the CW. If the RD can't tension this it droops down in a catenary curve that
is aesthetically displeasing. OTOH when the chain is on the large cog and large CW
you don't want the bike looking like a fixie with the RD fully stretched out, so there
are limits on how long and how short the chain can be.