Originally Posted by
nomadmax
The larger pulleys do need more chain length for the same gearing set up as the smaller pulleys, BUT they do not increase chain wrap/total capacity. Only a greater pulley distance center to center (ie a longer cage) will increase chain wrap/total capacity. The additional chain needed for the larger pulleys is used up going around the larger pulleys, ie the chain is functionally the same length as the smaller pulley set up. In fact, a larger pulley may decrease total cog capacity because the larger top pulley will come in contact with the cogs sooner. I would think that a Road Link could mitigate that but it won't increase capacity either, it just places the top pulley further from the cogs for interference sake.
My guess is, the selling point is that the chain doesn't travel as torturous a route as it does with smaller pulleys, ie doesn't bend as much going around them so it provides less friction loss.
Not quite true. Think about the limit at the Big/Big combo, a gear we try not to use much, but it should shift into without breaking anything. When you're straining the wrap capacity of the mech, in big-big the chain is almost straight, not wrapping around the pulleys much if any. So that's why big pulleys increase total chain wrap, because you're comparing the wrapped state in small-small to an almost straight chain.
Another reason the big pulleys reduce friction loss is that they turn more slowly. Whatever superduper bearings you put in 'em, you can use those same bearings in a small or large pulley, so it's not a question of bearing quality. The bigger radius does make each link go through a smaller angle change though, as you said, so you pick up a tiny friction decrease there too. The total friction change from both sources is small but it adds up on a long ride or a performance-critical one like time trial.
Downside is pretty much just weight, and maybe looking funny, but otherwise it's all upside, unless I'm missing something.
Mark B in Seattle