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As previously mentioned, 4wd vehicles have three differentials: front, center and rear to allow speed differences between wheels. Some differentials allow unlimited speed differences between the wheels—these are termed “open” differentials. The problem with open differentials is that they allow one wheel to spin faster than the other. If one wheel is stuck in mud or raised off the ground, the open differential sends all the torque to that wheel, thus causing it to spin uselessly, while the other wheel doesn’t turn at all. In the case of mud, the wheel just digs in deeper and the vehicle gets more stuck. You may have experienced this in a muddy parking lot.
To avoid this situation, most 4wds have limited-slip differentials. The simplest of these differentials is called a viscous coupling: the diff transfers torque via a silicone fluid in the casing. The silicone allows a speed difference between the two wheels, but if the speed difference increases, the fluid gets churned and gradually becomes harder, to limit the slip between the two wheels. Another type is the Torsen (short for torque-sensing) differential. The Torsen is a mechanical device consisting of worm gears and a worm wheel. The Torsen usually splits torque 50:50 but when one wheel starts slipping, it sends torque to the wheel with more grip, or the opposite of what an ordinary diff would do. Being mechanical, Torsen reacts quicker than a viscous coupling and helps to prevent wheel slipping rather than just reacting after wheel slip takes place. The latest systems such as those on the Mercedes M-class and BMW X5 use open differentials. A computer uses the ABS sensors to monitor differences between wheel rotation speeds, and if it senses wheel spin, it orders the brake at the offending wheel to activate. This slows down that wheel and transfers torque to the other wheels that actually have grip. Theoretically, these cars can get moving even if only one wheel has grip. Such systems save on weight and complexity by not bothering with a limited-slip differential. |
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