When a vehicle corners, a difference arises between the rotational speeds of the outer and inner tires.
It is the differential that absorbs this difference in rotational speed. However, conventional differentials also have a drawback. When one tire completely loses traction, such as on a snowy or icy road, most of the engine torque is delivered to the slipping wheel.
As a result, the vehicle may struggle to move. Moreover, during hard cornering, pressing the accelerator pedal transfers engine torque to the inside tire, which has reduced traction. This causes the inside wheel to spin and prevents strong acceleration.
A limited slip differential (LSD) is designed to overcome this drawback.
This article explains:
- The structure and operation of a conventional open differential
- The working principle of mechanical limited slip differentials
- The characteristics of worm gear LSD (Torsen type)
- The structure of helical gear limited slip differentials
How a Conventional (Open) Differential Works?
A conventional differential without a limited slip function is also called an open differential.
In the case of FR vehicles, the engine output is transmitted to the drive pinion through the transmission and the propeller shaft.
The rotational speed is reduced by the ring gear, and the axis of rotation is turned 90 degrees.
The differential carrier is bolted to the ring gear and rotates together with it. The pinions are inserted into the differential carrier and rotate with it, but the pinions themselves can also rotate independently.
The side gears, connected to the left and right wheels, mesh with the pinions.
Open Differential Behavior When Driving Straight
Let’s consider the vehicle’s behavior when starting straight ahead.
The left and right tires are subject to the same amount of rolling resistance. In other words, the same braking force is applied to the left and right side gears.
To make the explanation clearer, we will consider the case with only one pinion.
Since the same braking force is applied to both side gears, the pinion cannot rotate either to the right or to the left.
Therefore, the differential carrier, pinion, and left and right side gears rotate together as a unit.
As a result, the left and right side gears rotate at the same speed, allowing the vehicle to move forward smoothly.
How the Differential Works During Cornering?
Next, let’s look at how the differential works when turning left.
The right tire on the outside of the corner rotates faster than the left tire.
This means that the left side gear rotates more slowly than the ring gear, while the right side gear rotates faster than the ring gear.
For easier explanation, consider the case where the ring gear is held stationary.
The left side gear, which rotates more slowly than the ring gear, can be regarded as rotating in reverse when the ring gear’s rotational speed is zero. Since the right side gear rotates faster than the ring gear, it rotates in the forward direction.
The pinion is driven by the left side gear rotating in reverse and the right side gear rotating forward.
In other words, the difference in rotation between the left and right side gears is absorbed by the rotation of the pinion.
This action allows the vehicle to corner smoothly.
Limitation of an Open Differential
Next, let’s consider the vehicle’s behavior when starting off with the right tire on ice, having completely lost traction.
A large braking force is applied to the left side gear, while no braking force is applied to the right side gear, allowing it to rotate freely.
When the differential carrier rotates, the pinion rotates along the stationary left side gear. This pinion rotation drives the right side gear.
As a result, all of the engine torque is transmitted to the slipping tire, and the vehicle is unable to start moving.
The same phenomenon can also occur when accelerating during hard cornering.
When the vehicle rolls heavily, the traction of the inner tire is reduced. Even if engine torque increases, the inner tire slips and strong acceleration cannot be produced.
This limitation is exactly why limited slip differentials were developed.
Mechanical Limited Slip Differential (Clutch Type LSD)
A mechanical limited slip differential is a mechanism that uses multi-plate clutches to restrict differential action.
It consists of:
- ring gear
- differential carrier
- left and right side gears
- multi-plate clutches
- pinions
- pressure rings
First, let’s take a look at the relationship between the differential carrier, pressure rings, and pinions.
The inner surface of the differential carrier is grooved, and teeth on the outer circumference of a pressure ring engage with those grooves.
The pressure ring can slide in the axial direction of the differential carrier.
While rotating together with the differential carrier, the pinions are located between the left and right pressure rings, and the pressure rings grip the pinions with spring force.
Engine torque is transmitted to the pinions through the differential carrier and the pressure rings.
Clutch Engagement During Acceleration
Next, let’s look at the relationship between the differential carrier and the side gears.
The clutch plate with teeth on its inner circumference engages with the grooves of the side gear.
This plate can slide in the axial direction of the side gear and rotates together with it.
Another clutch plate with teeth on its outer circumference engages with grooves on the inside of the differential carrier.
This plate can slide in the axial direction of the carrier and rotates together with it.
When no engine torque is applied, this system absorbs the speed difference between the left and right tires in the same way as an open differential.
When the clutch is not engaged, if the right side gear rotates while the differential carrier is held stationary, the left side gear rotates in the opposite direction.
Next, consider vehicle behavior during hard cornering acceleration.
When the accelerator pedal is pressed, engine torque is applied in a direction that accelerates the pressure ring.
Rolling resistance from the tires is transmitted to the pinions through the side gears, producing a force opposite to the pressure ring.
As a result, the pressure rings move ahead of the pinion shafts.
Because of the shape of the holes in the pressure ring and the pinion shafts, the pressure ring is forced outward.
The expanded pressure rings press against the clutch plates, engaging the multi-plate clutches.
Through this operation, part of the engine torque is transmitted from the differential carrier to the side gears through the clutch pack.
This generates driving force at the tire that still has traction.
Types of Mechanical LSD
Mechanical LSDs are classified into three types depending on the design of the pressure rings.
Two-Way LSD
Restricts wheel slip during both acceleration and deceleration.
One-Way LSD
Restricts slip only during acceleration, allowing freer movement during deceleration.
1.5-Way LSD
Restricts slip during both acceleration and deceleration, but the locking effect during deceleration is weaker.
Initial Torque in Mechanical LSD
Disc springs are installed outside the left and right multi-plate clutches, lightly pressing the clutch plates at all times.
This creates a constant differential limiting force called initial torque.
When the initial torque is set higher, the differential locking effect builds up more quickly.
However, during parking or low-speed turning, excessive differential restriction can make the vehicle harder to handle in city driving.
The characteristics of the differential limiting function can be adjusted by:
- spring force of disc springs
- number of clutch plates
- shape of the pressure ring holes
Worm Gear Limited Slip Differential (Torsen Type)
A worm gear limited slip differential consists of:
- drive pinion
- ring gear
- differential carrier
- worm wheels
- side shafts with worm gears
For simplicity, we will focus on the differential limiting mechanism.
A worm wheel with spur gears at both ends meshes with the worm gears of the left and right side shafts.
The key point of this system is the relationship between the worm gear and worm wheel.
The worm gear on the side shaft can drive the worm wheel on the differential carrier.
However, the worm wheel cannot easily drive the worm gear due to the friction characteristics of worm gear engagement.
Operation During Cornering
Consider the operation during a left turn with the differential carrier held stationary.
The right side gear rotates forward.
The right worm wheel rotates counterclockwise.
The left side gear rotates in the opposite direction.
The left worm wheel rotates clockwise.
These opposite rotations allow the worm wheels to absorb the speed difference between the left and right wheels.
This enables smooth cornering.
Operation When One Wheel Loses Traction
Now consider the case where the right tire completely loses traction.
A strong braking force is applied to the left side gear, while the right side gear rotates freely.
When engine torque is applied, the worm wheel cannot effectively drive the side gear due to the worm gear characteristics.
As a result, the differential carrier, worm wheels, and worm gears tend to rotate together.
This mechanism allows the system to transmit driving force to the tire that still has traction.
Helical Gear Limited Slip Differential
A helical gear limited slip differential consists of:
- side gears
- long pinions that mesh with the left side gear
- short pinions that mesh with both the long pinions and the right side gear
- differential carrier
Each pinion does not have a fixed central axis and is positioned by the differential carrier.
The inner surface of the carrier pushes the pinions and transmits rotational force.
Differential Limiting Mechanisms in Helical LSD
There are two methods used to generate differential limiting force.
Method 1 – Side Gear Friction
During acceleration, thrust force between the side gears and the pinions pushes the side gears toward each other.
The friction generated between the friction plates and the side gears restricts the rotational difference.
During deceleration, the side gears move apart and friction is generated between the side gears and the differential carrier, again restricting rotational difference.
Method 2 – Pinion Friction
During both acceleration and deceleration, the shapes of the gears cause the pinions to press against the differential carrier.
This generates friction between the pinions and the carrier.
The friction restricts pinion rotation and reduces the speed difference between the two wheels.
Conclusion
Differentials play an essential role in allowing a vehicle’s wheels to rotate at different speeds, especially when cornering. A conventional open differential performs this task effectively under normal driving conditions. However, when one wheel loses traction, most of the engine torque is directed to the slipping wheel, which can prevent the vehicle from moving.
Limited slip differentials (LSDs) were developed to address this limitation. By restricting excessive speed differences between the drive wheels, an LSD helps maintain traction and improves the vehicle’s ability to accelerate and move in low-grip conditions.
Different types of LSDs achieve this effect through different mechanisms:
- Mechanical (clutch type) LSD, which uses multi-plate clutches to limit differential action
- Worm gear LSD, which uses the characteristics of worm gear engagement to distribute torque
- Helical gear LSD, which relies on gear friction and thrust forces to control wheel speed differences
Each design offers its own characteristics in terms of response, smoothness, and torque distribution.
Understanding how these systems work helps explain why limited slip differentials are widely used in performance vehicles, off-road vehicles, and many modern drivetrain systems to improve traction and driving stability.
