I did some googling. The purpose of the idler , in this instance is to reduce / eliminate the pogo effect of the rear suspension. When you look at the idler, it doesn't look like it's doing anything, but when you sit on the machine, things change, and you can't see. As the rear settles with your weight on it, or a load, the idler then keeps the chain down close to the pivot point of the rear swinging arm. One chap ( bentrider online) removed the idler due to the noise, and noted that the bike was almost silent. Then another reader, said that he had removed the one thing that stops the pogo. Ah! Said the first, I had noticed it bobbing up and down more, but thought the rear shock was on its way out. The plastic idler should be smooth, but Terracycle do a 23 t idler in alloy, about£70 , but £18 shipping outside Europe from Germany. ( if they will even ship to UK, now ) . I might try and wrap the surface in tape, of 'rings'of MTB inner tube, it really is quite chewed up.