well from my experience "if understand the question correctly"
take the number of gears on the back derailleur and multiply that
number with how many gears are on the front derailleur
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Medium cage.
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I found: http://bicycletutor.com/adjust-rear-derailleur/
It is specific for Derailleur (I was looking for Free Spirit's
TORK DRIVE SYSTEM - 18 SPEED)