All bevel gears,and in particularhypoid bevel gears, generate veryhigh levels of force that want to push the ring & pinion apart. In our differentials the force applied to the pinion is wellresisted by the pinion bearings and case, however the equal and opposite force applied to the ring gear is arguably less well resisted as it is bourne by the differential carrier, the carrier bearings, and utimately the bearing caps. [/align][/align]These are allpretty beefy components, and until you start to push well over 350lb.ft. of torqueit's not likely to be an issue, however at some gear ratio/torque pointa girdle could keep this from breaking byproviding additional support for the carrier bearing caps.There is also the argument that they increase fluid capacity (1/2 pt. or so) and provide better cooling (the sliding contact of the gear teeth, and controlling of the high loads generates a lot of heat; 150° to 200°+)--the best are finned.[/align][/align]Also, they (diff-girdles) look cool (of courseone wouldhave to lying be ontheir back behind the car to see it), and most offer a drain plug which makes changing the rerar-end oil much more convenient. A drawback is that they weigh 5-6 lbs. more than the OEM cover so if you 'are weight reduction fanatic think about whether you really need one...[/align]