Plastic or nylon ones are noisier, but they last longer. Polyurethane ones (with a sealed bearing) are what you need. Not sure what material the DRC ones from Dirt Bike Bitz are? This might be a better option as they are polyurethane...

http://www.becktonsmotox.co.uk/RFX-C...r-38mm/345.htm

Is your chain running on the lower roller all the time?

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