cage length & cassette size

Quoted message said:

Hi,

What is the benchmark/guideline for rear derailleur cage length vs.
cassette size?

Manufacturers all specify a maximum cog size, and a total capacity
(delta front + delta back). The total capacity is designed on the
assumption cross-gears will be used, so can be nudged a bit.
Similarly, max cog size can probably be exceeded by a tooth w/o
much issue.

Dan

Cage length has nothing to do with cassette size; it has to do with
total capacity (the total range of front and rear).

A Dura-Ace short-cage derailleur will work fine on a downhill bike with
a 12-34 cassette - assuming it only has one front chainring. A Dura-Ace
short-cage will NOT work on a 12-21 rear cassette, if you use a triple
front. You need to add up the total capacity and compare that with the
derailleur's spec (available on Shimano's web site).

sunderland said:

Cage length has nothing to do with cassette size; it has to do with
total capacity (the total range of front and rear).

A Dura-Ace short-cage derailleur will work fine on a downhill bike with
a 12-34 cassette - assuming it only has one front chainring. A Dura-Ace
short-cage will NOT work on a 12-21 rear cassette, if you use a triple
front.

But ultegra short cage will work with 12-27 cassette and a 52/39/30
rings.

email hidden said:

Hi,

What is the benchmark/guideline for rear derailleur cage length vs.
cassette size?

The manufacturer's recommendation for maximum tooth count range is a
starting place, and many users have found that it's often safe to
exceed that recommndation by two teeth, sometimes more.

If the range isn't known, then I figure that a short-cage der is
usually good for up to a 24 tooth large sprocket, medium-cage der will
handle up to about 28 teeth, and a long-cage will handle whatever
you're likely to be able to buy. A lot depends on the specific der
and cassette or freewheel combination; some will just barely work at
the ranges described, others won't get there, and some can go beyond.
If the specs are not known, the only way to be sure is to try it,
fiddling with the adjustments to see if it can be made to work.

http://sheldonbrown.com/derailer-adjustment.html contains a bunch of
helpful tips about tweaking the adjustments on the rear der.
--
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No it won't, unless you avoid several gear combos.

Capacity of a RD-6600-SS is 29 teeth.
(from
http://www.shimano.com.au/publish/content/australia/en/cycling/customer_support/service_diagrams/road_2005.MainContentPar.0024.Schematic.0018.File.tmp/RD-6600_SI-5VG0B.pdf)

Your range is (27 - 12) + (52 - 30) = 37 teeth. The capacity of a
RD-6600-GS (mid length cage) is 37 teeth; that will work. The SS cage
is meant for a double ring only.