Originally Posted by
repechage
the question though was for an 8 speed set of cogs in back.
There are 3 types of constraint in play here.
One is the lateral travel that the paralellogram structure is capable of. Do the pivots have some blockage? Are the limit screws opened up to handle the width of an 8 (same as the width of a 9, 10, or 11)? The rear mech control (right-hand DT shifter) plays a role in terms of whether the lever can be pulled over far enough to move the rear mech sideways far enough to align all the rear sprockets. Available lateral travel is also a point in consideration of front derailleurs.
Second is the ability for the jockey wheel to clear the big rear sprocket. Except for gadgets like a Wolftooth or somehow finagling a longer rear derailleur tang, this is based on the physical geometry of the frame-as-built dropout, the derailleur parallelogram design, and the length, offset, and range of rotation of the chain cage, with consideration of the rotated position of the rear cage. The diameter of the jockey and guide wheels may have an influence as well. A few past designs have allowed the cage to rise and fall to accommodate a larger range of rear sprocket sizes
Third is the chain wrap range, traditionally called "capacity." This is how much chain length can be compensated as the gear is shifted from the small-small up through the big-big position. Many derailleurs have such a spec published. For some manufacturers and derailleur models it seems to be a hard limit and for others it seems you can exceed it significantly. As with the second constraint, the performance can be affected by the available motion of the top pivot of the derailleur, such as if it is sprung.