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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?

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    The one that has play is the upper one (the jockey pulley, guide pulley) in order to allow some tolerance to indexing adjustment when guiding the chain from one sprocket to another (this play also makes the derailleur a bit quieter as a side effect). The lower pulley (the tension pulley) is there simply to control tension when switching sprocket sizes. You should not interchange them, and they are generally marked as being different. There are systems with very little/no play in the upper pulley. They also often ship with different types of bearings in the two pulleys. – Batman Sep 14 '16 at 21:10
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    Correction, my comment was wrong. – mattnz Sep 14 '16 at 21:50
  • Cross-compatibility means that in many cases you can't separate pulley performance form derailleur performance. Obviously you can swap plastic to alloy if the dimensions mtach, but you can't just still a 13T in where a 10T was; it would hit the cage.My experience is that plastic pulleys on bushings don't inherently have play -- some may be designed in but the wheels themselves are stiff – Chris H Sep 15 '16 at 12:57
  • Cheap jockey wheels are $5 or $6. If your current ones are worn, replacement is a totally valid choice. – Criggie Oct 3 '16 at 1:24
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Main gain is less friction when using ceramic ones and less friction on the transmission when using up to 15T pullies. This translates into couple of watts after a big number of km's.

If you are into pro racing or you are a really weight weenie or you just have money to burn, then you can go and change both pullies if you want to. From several studies, at very high performance you inded gain some power from using larger pullies, but that is really almost irrisory and only really valuable at really high performance levels, say pro riders at Tour de France, Vuelta, Giro, etc. Other than that, ceramic ones have less friction so is supposed to save you some watts on long rides, which translates on some miliseconds on the clock after couple hundred km's.

If you check prices, good ones are up to 400 usd...so, what you will do, change pullies or fully upgrade your transmission? I'll go for the 2nd option ;)

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