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I have new derailleur and I am somewhat surprised that when I try to spin it with hand (no chain is mounted yet) it basically has no momentum, it almost immediately stops. Almost any other spinning part of the bicycle has much more momentum and spins for a while.

Granted, the wheels in derailleur are the lightest ones, but shouldn't they at least make a one full turn?

The screws are not too tightened because out of curiosity I loosened them already. No change.

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    The cage bolts tighten onto the inner jockey wheel bearing, which is slightly wider than the wheel itself, so you cannot bind the wheel up by over-tightening the bolts. – Argenti Apparatus Jan 15 '19 at 20:52
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    This is fairly normal. I don't try spinning them too often, but on a well-oiled wheel if you can get one free turn out of them that's pretty good. The oil in the bearings creates a substantial drag, when compared to the minuscule momentum of the wheel. – Daniel R Hicks Jan 15 '19 at 21:48
  • @DanielRHicks, thank you, my wheels are below that limit (one free turn), more like one hour of arc of free turn (Deore XT RD-M772). – greenoldman Jan 16 '19 at 15:28
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The derailleur wheels are small and light weight, so have very small moment of angular inertia. The bearings they run on need to withstand a significant force due to the action of the chain, not just the weight of the wheel. So the bearings are fairly robust and lubricated. As a result there is a little "greasy sliding resistance friction" (for want of a better word) in the bearing. The resistance is small and its effect on riding resistance virtually imperceptible, but as the wheel's angular momentum is also very small, it spins down rapidly if spun by hand.

If the wheel was engineered like a fidgit spinner then it would spin for ages, but its bearings would fail within minutes.

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  • Thank you very much, I already started worrying that I have to replace them or something :-). – greenoldman Jan 15 '19 at 20:26
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    Re: If the wheel was engineered like a fidgit [sic] spinner then it would spin for ages, but its bearings would fail within minutes.' - CeramicSpeed would probably disagree. It probably is possible to make small low friction bearings that are robust, they would just be very expensive. – Argenti Apparatus Jan 15 '19 at 20:48
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    To add a slight clarification to the answer: it's not that the jockey wheels have too much friction, rather they have too little mass to continue spinning. – Argenti Apparatus Jan 15 '19 at 20:48
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    Many jockey wheels just run on bushings. These will nearly never spin freely like a fidget spinner. OTOH, they work flawlessly for years and years. Bearings can gum up more quickly (even sealed). I've ordered cheap Chinese jockey wheels that came with "ceramic" bearings running in what appear to be nylon races. These spin freely like crazy, but I doubt they'll last long. I use them for low-stress idlers, and they do fine for that. – WPNoviceCoder Jan 16 '19 at 2:32
  • To add to @WPNoviceCoder 's comment, the Shimano jockeys on my road bike run on ceramic bushings. They've been doing a flawless job for a few years but the grime is slowly wearing them slightly loose. The kind of self-lubricating or chain-oil lubricated bushings are super simple and totally functional. Bearings aren't necessarily needed for those components. – Gabriel C. Jan 16 '19 at 19:04
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There are two types of mounts for jockey wheels - higher end ones come with bearings, and lower end ones come equipped with some "shells" that have no bearings.

The bearings roll much smoother, but add more moving parts to the system. Plus they cost a lot more for relatively small gains.

You do need to remember that the jockey wheels are under relatively low tension because they're on the part of the chain that is looser. The top jockey wheel also has to guide the chain onto the cassette, but the lower one only has to turn and maintain some tension on the chain.

So as long as they move freely and don't bind, a jockey wheel will function fine.

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