Why do current, entry-level shimano rear derailers have large-diameter pulleys? I have seen (and recently bought) entry-level rear derailers from Shimano (Tourney for MTB, 2300 for road bike), and I keep wondering why they are making these parts with such large pulleys.

I don't think this make the parts cheaper to produce, since large size means more material, and I can't see why customers would find this more attractive or adequate than the smaller-sized ones (unless customers would prefer more expensive models because they DON'T HAVE large pulleys...).

Also, I imagine there might be advantages to larger pulleys (less chain link rotation, less wear, slower pulley rotation), but the reason why entry-levels are big, while more expensive models continue to be the same old size, is very unclear to me, not to say arbitrary.
There are 2 versions of derailleurs which come with extra large pulleys.

The first, which is what is being discussed here, are the low end mega-range compatible derailleurs.  A mega-range freewheel has one distinct characteristic which requires compatible derailleurs to have very large cogs.

As seen in the image below, there is a large tooth count difference between the second to largest cog and the largest cog.



This large difference in size requires a large derailleur pulley, to make sure that the chain has cleared the teeth on the last cog, in order to shift into this gear.

If you use a smaller pulley on the derailleur, you may be able to get it to shift, but it will be clunky and loud at best.  At worst, it will trap the chain against the side of the largest cog, and refuse to shift into that gear at all.

The second type of large pulley derailleur is decidedly not low end.


The Berner Carbon Fiber derailleur upgrades are designed to reduce resistance on the drive-train by decreasing the severe chain bends associated with smaller cogs. While expensive, my experience has been an average 10 watt reduction in energy used by the cyclist, with no apparent loss of efficiency in shifting, or in anything else other than the cyclist's wallet.