LSD are three letters that incur strong emotions. For many people they represent a utopian ideal that should be shared with everyone. Others view them with suspicion and mistrust, wondering how anyone could possibly benefit from them. For me, LSD represents pleasure beyond description and I believe that it should be shared with all freely.
LSD, in this case, does not stand for "Lucy in the Sky with Diamonds" nor is short for "Acid," "Window Pane" or any other drug reference but rather stands for Limited Slip Differential. A Limited Slip Differential (LSD) is device for getting power from the engine to the ground and if you are serious about going fast in your car you will want one.
First the boring back story- Front or rear wheel drive, a car needs to transfer the turning of the engine into the turning of the wheels. When the wheels are moving in a straight line, they both rotate at the same rate of speed. But when you make a turn, the outside wheel has to turn faster than the inside wheel. If the wheels are not connected to each other, this differential in wheel rotation rate is no big deal. But the set of wheels that drive your car (front or rear) are connected together and need a device to allow one wheel to turn more that the other. This device is called a Differential. Most commonly, manufacturers send cars out into the world with cheap, easy to maintain Open Differentials. An open differential transmits power from the engine to the wheel that is has the least amount of resistance. Unfortunately, the wheel with the least amount of resistance is probably slipping and thus the power is not getting to the ground. To go fast, the engine's power has to get to the ground and so a Limited Slip Differential is used to put more power (but not all of it) to the wheel that is not slipping. This characteristic is also helpful for cars and trucks that will be operated in low traction conditions like ice, snow or frequent rain so this feature is sometimes included in vehicles that will see this kind of service.
Some budget racers and racers that only go in a straight line (drag racers) will weld their differential so that the two drive wheels are locked together and 100% of the power goes to the ground. But when those cars try to turn, the wheels will not turn free enough to prevent the wheels from chirping and chattering, which is not something you want in a car that you take your date out in. A Limited Slip Differential puts enough power to the ground but allows enough slippage so that the wheels can turn smoothly.
The down side to a Limited Slip Differential is that they are more expensive, require frequent lubrication changes (something that most drivers are not used to paying for) and they contain parts that are subject to increased wear. Most Limited Slip Differentials are probably not going to last more than 100,000 miles without a major rebuild. And if the average driver, driving in an average manner, is not going to notice the difference on the average day, the manufacturers will not include a Limited Slip Differential to reduce costs.
But for the spirited driver or the low traction conditions driver, a Limited Slip Differential is a great addition to his car.