Is there any set amount of time the rubber motor mounts need to be replaced. My '09 Sportster 1200C mounts seem to be soft as the engine/trans seem to be sagging in the mounts. There is a bit more vibration at cruising speeds (55-65) than I think there should be. Above 65 it smooths out to almost no vibration. I know it's a Harley and there will always be vibration. Any info would be greatly appreciated. Thanks Dave G.