I was looking at my bikes the other night and noticed that they all had plastic/nylon idlers to guide the chainline. Thinking back, I had replaced the original idler on my Greenspeed with one that was essentially a large jockey wheel(actual cog shaped, with teeth, etc). The original was just a channel gouged into a plastic wheel.
The idler on my shockproof is the same, plastic with a channel in it. I got to thinking and wondered why they werent being made of some metal product, like the rear cassette is.
Im not trying to replace them, nor am I unhappy wit hthe performance of the nylon/plastic ones, just curious if anyone has experimented with different materials and what the conclusions were.