Originally Posted by
mcgreivey
His mountech might already be long-cage. But if not, a long cage mountech (or a Suntour VXGT or something) can be had on the "bay" for $10-30.
Careful. "Long cage" does not equate to being able to handle larger cogs. It has to do
only with the amount of chain slack the RD can handle. Note that Al1943 said, "A 14-34 freewheel would require a long cage
mountain bike rear derailleur." Key is the bolded phrase. The parallelogram geometry of MTB RDs is such that the shifting arc is steeper than for road RDs. This is what makes them able to handle the big 34T cogs.
Larger cogsets do tend to produce more slack, so may want a longer cage. But a "long cage" RD may not be able to handle the big cogs. The equation is not commutative.