Jax Rhapsody

Jockey pulleys. It may sound like some trivial nonsense, but when it comes to them; does the size of them actually matter? Like roadbikes usually have small ones and mtbs have large diameter ones. Anything else important about them, other than their neccessity to allow a derailure to function?

bruce19

One opinion:

JohnDThompson 

The larger pulleys allow more chain to be wrapped for a given cage length. Off-road bikes tend to have wider gear range than road bikes, and the derailleur needs to wrap more chain to cover that wider range. Increasing the cage length creates ground clearance problems. Earlier derailleurs like the SunTour "Lepree" used a third pulley to accomplish this, but two larger diameter pulleys can do the same with lower friction.

subgrade

I don't think it's that clear cut that MTB RD's have larger pulleys than road RD's. I haven't really researched it, but seeing that recently even high end Shimano MTB 11speed RD's like XTR M9000 and XT M8000 (which were designed for 2x drivetrains) had 11 teeth pulleys, while the newest 12speed incarnations (which are designed for 1x drivetrains) have 13 teeth pulleys makes me think that it has more to do with 1x vs. 2x (3x) drivetrains and the need to cope with wide range casettes for 1x. A quick glance at the SRAM site seems to confirm this with the Eagle versions (for wide range casettes) having larger pulleys than the non-Eagle ones.

edit: The use of larger pulleys on some low end RD's like Altus might be explained by the fact that larger diameter pulleys spin slower than smaller ones at a any given cadence and thus don't necessarily require as good bearings.