Bearings are what allow the gears to rotate smoothly. They are typically made out of bronze or brass, and they in shape into races (or grooves) within the gearbox housing. Synchro rings have teeth on them that mesh with the enamel to the gears, and so they also have friction https://filter-gearbox80233.blog2freedom.com/34096479/facts-about-filter-gearbox-revealed