I'm wondering about the variations in jockey wheels / derailleur pulleys and what differences they make; There's a lot of conflicting and confusing information out there, so I'm after some general opinion on them.
Common sizes appear to be 10T, 11T, 13T. I can see that the larger the pulley, the lower its rotational speed and chain bend and thus the less friction it will cause, at the expense of some weight and space. Some stock pulleys are apparently over 30g, and some light alloy ones are under 10g, which is a reasonable range for the weight weenies to fuss over!
Larger jockey wheels will require a longer chain, which I've seen described as also increasing gear capacity; is it effectively like having a longer cage? If so, is it reasonable to use a 13T bottom pulley to gain capacity?
The upper pulley is clearly going to affect shifting most, as it feeds the chain onto the cassette, so a larger pulley will move the chain closer to the cassette - is this likely to reduce capacity?
I've seen jockey wheels sold in pairs specifically labelled as upper and lower, and often one of them will have some degree of float or play, rather than being rigid on its axle.Is this meant to make it more forgiving of slightly misaligned gears? Is it necessary if you have well-aligned gears?
Alloy pulleys seem to be more highly regarded than plastic. Is it also the case that plastic wheels may have more flex than alloy ones, providing float-like behaviour in the wheel rather than the bearing?
Bearings will usually be smoother and lower friction than bushings, and ceramic bearings are better than steel - but how big is this difference?