You start with an 'O'-ring which is slightly smaller than the hub diameter of the pulley wheel. The hub is the bottom of the groove which you cut out of the skate wheel. The O-ring should be about twice the thickness of the gap between a chain's sideplates and it's rollers, when looking at it from the end of a link. Don't confuse this with the width of the rollers, which corresponds to the thickness of the chainrings and cassette sprockets. I don't recall exactly what this dimension ((sideplate dia. minus roller dia.) divided by two) is on our chains but I use a ring which is 3/32" thick. Cut a groove around the middle thickness of the pulley hub where the center of the chain runs. The groove depth should be half the thickness (half 3/32") of the 'O'-ring. It will retain the O-ring in the center of the pulley hub. The idea is that the 'O'-ring will ride on the center of the chain's rollers, which reduces the contact pressure of the chain's sideplates on the pulley hub. You've probably noticed that your pulley's hub has two rows of dimples worn into it by the chains's sideplates. The O-ring reduces this wear considerably, especially if you change the ring before it wears down to the surface of the pulley hub.
Thursday, 29 January 2009