Sarah sat in the driver’s seat for a moment, thinking. She pulled the hood latch, exited the car and removed the jack and tool pouch from beneath the hood.
She chocked the driver’s front wheel.
She popped off the driver’s rear wheel cover, then inserted the jack into the jacking port and raised the car enough so that the tire was off the concrete.
Next, she put the transmission into third gear, pressed the accelerator once to set the choke and turned the key to the “On” position.
Using the jacking bar and the lug socket from the pouch, Sarah began to turn the rear wheel counterclockwise.
Soon, the engine chugged to life. Sarah put the car into neutral, lowered the jack and stowed the tools. After replacing the wheel cover and removing the chock, she got into Schultz and happily drove to work—a little late and sweaty, but happy.
(The Limited-Slip Differential gearing allows the wheel with the least resistance to spin. The wheel on the ground provides too much resistance and will not turn)
In 1932, Ferdinand Porsche designed a Grand Prix racing car for the Auto Union company. The high power of the design caused one of the rear wheels to experience excessive wheel spin at any speed up to 100 mph (160 km/h). In 1935, Porsche commissioned the engineering firm ZF to design a limited-slip differential that would perform better. The ZF “sliding pins and cams” became available, and one example was the Type B-70 for early VWs.
The main advantage of a limited-slip differential is demonstrated by considering the case of a standard (or “open”) differential in off-roading situations where one wheel has no contact with the ground. In such a case (with a standard differential), the non-contacting wheel will receive 100% of the power, while the contacting wheel will remain stationary. The torque transmitted will be equal at both wheels, and therefore, will not exceed the threshold of torque needed to move the wheel with grip. In this situation, a limited-slip differential prevents 100% of the power from being allocated to one wheel, and thereby keeping both wheels in powered rotation.
Basic principle of operation
Automotive limited-slip differentials all contain a few basic elements. First, all have a gear train that, like an open differential, allows the output shafts to spin at different speeds while holding the sum of their speeds proportional to that of the input shaft.
Second, all have some sort of mechanism that applies a torque (internal to the differential) that resists the relative motion of the output shafts. In simple terms, this means that they have some mechanism which resists a speed difference between the outputs, by creating a resisting torque between either the two outputs, or the outputs and the differential housing. There are many mechanisms used to create this resisting torque. The type of limited-slip differential typically gets its name from the design of this resisting mechanism. Examples include viscous and clutch-based LSDs. The amount of limiting torque provided by these mechanisms varies by design.