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Almost all derailleurs seem to come from the factory with nylon jockey wheels . What is the rationale for the use of nylon (rather than alloy or aluminum)? Is it merely weight savings (which would seem to be trivial given the size) or is there a functional advantage to nylon/plastic?

Jockey wheels, wikimedia commons

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Caveat: I've only seen two or three sets of metal pulleys in my life. There are several reasons:

  • cost: plastic parts are cheaper to make than metal ones
  • weight: it's easier to make a lightweight plastic part than a metal one (this is also related to cost, "easier" usually means "cheaper")
  • durability/resilience: somewhat counter-intuitively, plastic cogs are less likely to fail completely. When the plain bearing locks up the plastic wheel is likely to slip on the metal bearing (and many people don't even notice that this has happened), or the teeth with abrade off to form a smooth surface that the chain will run on (it's very hard not to notice this). A metal pulley does not do this, if they stick for some reason generally the bike isn't rideable, and it's unlikely that the aluminium pulley will slip nicely on the steel bearing.

The latter is somewhat hard to get experience with, since metal pulleys are sufficiently rare and expensive that few people use them. Also, people who pay extra for them tend to maintain them so I haven't seen them fail other than via "full of mud, not going round". In that case, though, the chain will not go through the rear derailleur at all, so the bike needs to be converted to single speed before you can ride it. Which is an ugly thing to have to do when you're out on a bike ride.

Note that recumbent idler pulleys have the same concerns but often they're on the drive side of the chain so the forces are larger. So are the pulleys to reduce chain bend angles and thus losses. In that case metal cogs are usually also lighter and quieter but significantly more expensive (more than double the cost). Derailleur pulleys would also benefit from being larger, but that's relatively rare, I suspect because weight and air resistance also increase with size (and a larger cog would mean extra chain, further increasing weight)

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  • Interesting... are metal cogs really quieter than plastic? – junkyardsparkle Nov 21 '15 at 1:55
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    in my experience, yes. It's the difference between the noise of a chain hitting a chainring, and a chain hitting a block of plastic. I suspect you don't get that effect with derailleurs because the forces are so much smaller. Some recumbents use O rings on the pulleys to reduce the noise, but IME on the drive side those don't last very long. – Móż Nov 21 '15 at 1:58
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    In fact nylon on steel is the plain bearing on some - the metal sleeve is a spacer clamped tight when the screw is done up. – Chris H Nov 21 '15 at 12:15
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I'd always assumed nylon was used because it was a softer material than alu or steel, and therefore reduced wear on the chain. Since jockey wheels simply route the chain and don't need to operate under the same load as chain rings and cassettes they can be made from a less durable material. I would guess that the routing round the jockey wheels is potentially a high wear area for the chain, so making the jockey wheels from something softer reduces the wear.

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Several reasons. Nylon is lightweight, tough, wears only slowly, and in particular the dynamic coefficient of friction with steel is the lowest amongst the sufficiently tough synthetics. Friction and Wear of Polymers

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I had Bullseye red aluminum pulleys on a Super Record derailleur similar to the one that's pictured. I recall they were slightly noisier than the original nylon ones.

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