I never mentioned a "drive gear" and have no idea where that came from. The driver sitting on the left is irrelevant. Playing with it on a bench has no relation to actual road use but what you're describing is the function of the spider gears and that is relative to driving around corners. It's a matter of physics and how the turning of the gears applies torque to the differential and rear suspension. As the clutch is engaged torque is applied to the driveshaft which in turn applies it to the pinion shaft. When the pinion gear transfers that torque to the ring gear, however, there is a secondary effect which is torque twist. The differential and suspension are twisted. It's similar in effect to what happens on the front axle of an MB when the brakes are applied hard which twists the axle which causes a steering problem. That's why the front torque spring was added. As the rear differential suspension is twisted there is an increase in downward pressure on the left rear tire and a decrease in downward pressure on the right rear tire. The power being transferred from the motor to the driving surface will follow the path of least resistance which is to the right rear tire. That is why, on open differential (no powr-lok etc.), leaf sprung, rear wheel drive vehicles the right rear tire spins first.JAB wrote: I have to disagree. There is no "drive gear" on conventional differentials. I think this is one of the most often perpetuated myths concerning differentials. The reason the right rear usually spins (looses traction) is related more to the fact that the driver is seated on the left side (more weight) or due to road surface conditions or that the vehicle is turning a tight right (right side "lifts" allowing traction to break) more than any other internal friction factor. Look at how the carrier assembly is built, (not including posi, etc. units) & you'll see that no side is favored over the other. They each have their own side-gear which meshes with other gears that are nestled in the carrier. When the carrier rotates the side gears rotate with it but at varying speeds comensurate with the load. If you play with it on the bench it will look like one side is going backwards; while it's possible to do that, in-use it's usually that both sides are actually rotating forward but that one side is rotating at a much slower rate than the carrier itself, which gives the appearance that it's going backwards. Which wheel slips on an "open" rear-end is determined by the load/traction condition, not which side it's on. It is a length thing as all other factors are essentially equal from the mechanical standpoint.
The reason the axle breaks is because each time the tire spins loose, even a little, the torque to the splines is increased significantly at the moment the axle regains traction. After a while the splines get twisted, cracks occur, and eventually it breaks, and there is no predicting when or under what load that will happen. That does not mean that a left rear axle can't break but it's far less likely. There are situations, however, when a left rear axle can become stressed. If you are stuck in snow and you rock the car back and forth by shifting from forward to reverse and back again you can alternately stress one side then the other. If you stop at a red light on a hill and go to start again the vehicle may roll backward just a bit before you engage the clutch. That changing from one direction to another can stress the left rear axle as the afore-mentioned differential twist plants the left rear tire. I've also seen axles break because there was a defect in the metal.
I've built and raced a lot of race cars in my day and I know something about how torque affects traction, how to control it and where the weak points lay. The way to control torque twist on an open differential, spring leaf rear suspension is with a pair of traction bars. With traction bars it's possible to break either axle or, as a fellow racer of mine did, blow the ring gear right through the back of the housing (1962 Chevy 409, 4 speed, 4.56 posi.).
Also, remember that a spring leaf suspension is entirely different than coil spring, trailing arm or any type of independent suspension and reacts entirely different to applied torque.
And comparing torsion bars to axles is like comparing a steering wheel to a tire. They look similar but are entirely different things. How about grease fittings and brake bleeders? I actually watched a rookie apprentice try to shoot grease in a bleeder once but that didn't mean they were the same. Don't compare apples to oranges.