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Limited slip differentials (LSD) are essential for traction on low friction surfaces and for cornering stability and safe towing, but they take many forms. The LSD is used on many road vehicles, especially those with a performance bias, but is it appropriate for electric vehicles with their more varied drive train configurations?
Answering this means explaining the role and function of the differential, exploring the problems solved by the LSD, and only then discussing the relevance of the LSD to electric vehicles (EV).
The open differential is a mechanical device that allows one wheel on an axle to turn faster than the other while ensuring an appropriate distribution of torque between them.
This is achieved by splitting the axle and putting a set of gears between the two halves. This allows one side to turn faster or slower than the other, which is key to getting around a corner smoothly.
All wheel-drive vehicles often have a center differential in the driveshaft between the front and rear axles. Its purpose is to permit different wheel speeds and torque split between the front and rear axles.
However, the open differential has one major weakness. It cannot transfer more torque than the lowest friction wheel can apply, leading to a loss of traction on slippery surfaces because the torque goes to the shaft with the least resistance. This becomes a problem when one wheel is on a low friction surface and the other has good grip. The wheel without grip spins while the one with grip doesn’t move.
A limited slip differential (LSD) limits the difference in speeds between the two shafts. This reduces the difference in torque going to the left and right sides of the vehicle, which improves handling and towing stability by ensuring there’s always some torque going to the wheel with grip.
The most extensively used types of limited slip differentials include:
Mechanical plate style LSDs use a clutch pack to provide frictional resistance to differential speeds. These work without driver intervention, relying instead on the forces generated during acceleration, braking and cornering.
Viscous couplings use a special fluid to create drag inside the differential assembly, with the drag increasing with the difference in wheel speeds. These relatively simple devices work smoothly but tend to be expensive, cannot lock the differential completely and can overheat.
Helical gear differentials use a complex arrangement of helical-shaped gears that mesh with increasing force until wheel spin is slowed or completely stopped. Some types work in conjunction with a clutch pack. They are often used as a center differential because of their ability to provide an unequal front-rear torque split.
The eLSD is a form of mechanical LSD with electronic control over clutch pack operation. The Eaton IntelliTrac™ eLSD uses an electronically controlled hydraulic pump and piston to operate the clutch. With this approach LSD operation is managed to maximize traction in response to steering, acceleration, braking and wheel speed inputs.
An EV has the same cornering challenges as an internal combustion (IC) vehicle. However, vehicle electrification is enabling engineers to re-imagine drivelines for optimal performance, efficiency, and safety. So this complicates the answer.
The simplest EVs replace the IC engine with an electric motor while retaining a conventional front or rear wheel drive layout. Other configurations include dual motor designs where each axle has its own motor, three motor designs—independent motors for the wheels on one axle while a single motor powers the other axle—and four motors or motor-in-wheel designs. In addition, some hybrid vehicles feature one IC-powered axle and one that is electrically driven.
In instances where a motor drives a single wheel, there is no need for a differential because motor speed can be managed electronically for optimal traction and cornering. However, when one motor drives wheels on both ends of an axle a differential is essential. But does this have to be an LSD?
The differential in an EV is subject to a different duty cycle to that seen in an IC vehicle. This is because there are torque loads while the vehicle is regen braking. In addition, an EV can produce near-instantaneous torque, which creates unique challenges for driveline engineers. Vehicle integration experts can partner with engineers to effectively manage these systems.
As such, it’s important to use a differential designed specifically for an EV. An LSD is strongly preferred for its ability to ensure stability under high torque conditions. Mechanical designs of LSD can only respond to conditions that are already occurring. An eLSD can, using advanced control algorithms, act in preparation for loads or conditions that will be experienced imminently.
The bottom line is that yes, an EV should have an LSD when wheels on an axle are powered by a single motor.
