Limited Slip Differentials

By SRT-Tom · Mar 30, 2019 ·
  1. SRT-Tom
    A limited-slip differential (LSD), or “anti-spin” differential is a type of differential that allows the rear wheels on a vehicle to turn at different speeds when executing a turn. They are widely used in high performance and four-wheel-drive vehicles because they provide superior traction abilities. Various types of differentials can be classified as "anti-spin." These include limited slip, locking and spool differentials. Each performs differently on and off the road. Generally, only limited slip differentials are installed in factory vehicles. A locking or spool-type differential is used for off-road or racing use.

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    The advantages of an LSD in high-performance automobiles were first demonstrated during the original muscle-car era, from the mid-1960s through the early 1970s. Cars of this era normally were rear wheel drive and did not feature independent suspension for the rear tires (but instead used a live axle). With a live axle, when high torque is applied through the differential, the traction on the right rear tire is lower, as the axle naturally wants to turn with the torsion of the drive shaft (but is held stationary by being mounted to the vehicle frame). This coined the terms "one-wheel peel" or "one-tire fire." Muscle cars with LSD had at a distinct advantage to their wheel-spinning counterparts. Some factory names for LSDs were: Sure Grip (Chrysler), Trac-Lok (Jeep), Posi-traction (Chevrolet), Positive Traction (Buick), Anti-Spin (Oldsmobile) and Safe-T-Track (Pontiac).

    An LSD allows the vehicle's wheels to have a limited amount of slip before the differential attempts to correct the slippage of the wheel. This slip is usually a very limited amount. Your tires don't have to spend five to ten seconds or more skidding before the limited-slip differential attempts to correct it. The LSD just needs to establish that there is a loss of traction in that wheel and many limited slip differentials can notice slippage in under two seconds, sometimes in several milliseconds. So the slipping of the wheels is corrected before you can notice it.

    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 type of mechanism that applies a torque (internal to the differential) that resists the relative motion of the output shafts. In simple terms, this means 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. Types of limited-slip differential typically are named from the type of the resisting mechanism. Examples include viscous and clutch-based LSDs. The amount of limiting torque provided by these mechanisms varies by design. All of these can tolerate slight differences in wheel speed to facilitate the vehicle cornering before the differential sends power to the other wheel. Here is a brief description of each type:

    1. Mechanical (Clutch-Based)- A mechanical LSD has two clutches inside the differential. Whenever a wheel starts to slip it activates the clutch to the shaft leading to that wheel to cut the power to that wheel thereby sending the power to the other wheel.

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    ZF LSD Clutch Stack

    2. Viscous Coupling -
    This means that there is a thick liquid inside the LSD. The liquid spins in the direction of the wheels and, if one wheel starts to spin considerably faster than the other, the viscosity of the spinning liquid forces the other wheel to spin as well. This action is similar to a locking differential, as power is not actually removed from the slipping wheel.


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    Viscous LSD-Nissan 240SX

    3. Electronic -
    An electronic LSD may have two clutches inside it and uses an ECU to determine when one of clutches should restrict the power to a wheel, thereby sending the power to the other wheel. An electronic LSD may operate by itself with its own ECU or may use the systems that operate Traction Control to also operate the LSD.

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    Electronic LSD

    A Traction Control system can be used as an alternative or a supplement to a traditional limited-slip differential. The system harnesses various chassis sensors, such as speed sensors,anti-lock braking system (ABS) sensors,accelerometers, and microcomputers to electronically monitor wheel slip and vehicle motion. When the chassis control system determines a wheel is slipping, the computer applies the brakes to that wheel. In most cases, individual wheel braking is enough to control wheel slip. However, some traction-control systems also reduce engine power to the slipping wheels. On a few of these vehicles, drivers may sense pulsations of the gas pedal when the system is reducing engine power much like a brake pedal pulsates when the anti-lock braking system is working. These features, however- braking and power reduction- are not conducive to drag racing.

    In 2008, the Challenger SRT came equipped with a Traction Control system. For the 2009 SRT model, a mechanical limited-slip differential was added to further help control the 6.1 Hemi’s 420 lb/ft of torque.
    On new models, this is accomplished by a Launch Control button or a Track driving mode. SRT models provided a dash control button to completely disable the Traction Control for drag racing.


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    2008 Challenger SRT

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    Traction Control Disable Button (on right)

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    Launch Control Button

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    Driving Modes (Street, Sport & Track)

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