Limited Slip Differentials

A limited slip differential reduces individual wheel spin for maximum traction.

What is a limited slip differential?

Mechanical limited slip differentials are arguably the most common type of differential due to their wide range of applications. They work by shifting a portion of the torque to the wheel with the most traction while limiting the slip on the wheel with the least traction. Because of this, limited slip differentials are often referred to as “torque sensing”.

Like an open differential, the wheels can rotate at different speeds. However, with a limited slip differential, torque is not always balanced between the wheels. This allows the wheel with traction to receive more torque in order to continue to move the vehicle. In other words, the differential is said to “bias” more torque to the higher traction tire. The amount of torque variation the differential can shift between wheels is referred to as the bias of the differential.

Eaton makes two types of limited slip differentials: the Eaton Posi (positraction) and the Detroit Truetrac.

Limited Slip Differential (noun):

A differential which utilizes some means to partially reduce, but not totally eliminate, individual wheel spin (e.g., clutches, springs, biasing pinion gears, etc.).

How do limited slip differentials work?

Limited slip differentials use various methods to transfer torque to the wheel with traction.

Eaton Posi (clutch pack style):

The Posi prevents wheel slip before it can get started. Race-bred carbon disc clutch packs, preloaded by a central spring assembly, are located behind each differential side gear. When torque input changes (one wheel slips), the clamping load on the clutch packs increases, causing them to smoothly transfer power to the other wheel as needed.

Detroit Truetrac (helical gear style):

The Detroit Truetrac operates as a standard (open) differential under normal driving conditions, allowing for unobtrusive operation. When a wheel encounters a loss of traction or the terrain changes, the helical-shaped gears mesh with increasing force, transferring torque to the high traction wheel until wheel spin is slowed or completely stopped. When the vehicle exits the low traction situation, the differential seamlessly resumes normal operation.

Are there any disadvantages to a limited slip differential?

The key disadvantage when compared to a locking differential is that the amount of torque that can be transmitted to the wheel with traction is limited (thus the name, limited slip). Since they cannot provide 100% lock up, during extreme situations (i.e., complete loss of traction at one wheel), there may not be enough torque to the wheel with traction to move the vehicle.

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