Match the tow ball to the trailer coupling first: 50mm is the standard for most Australian trailers and caravans, while 70mm couplings appear on heavy commercial trailers. Every component in the towing chain has a rating - towbar, ball mount, tow ball and coupling - and the lowest rating sets the limit, so a 4.5 tonne ball does not upgrade a 3.5 tonne bar. Check the shank diameter and length suit your ball mount, and torque the nut to specification. Zinc plated and chrome finishes both meet AS4177.2, so choose by preference and exposure. The CURT RockerBall suits drivers wanting reduced jolting on rough roads through its cushioned isolator design.