What is a differential?
Simply put a differential is a system that transmits an engine’s torque to the wheel. The differential takes the power from the engine and splits it allowing the wheels to spin at different speeds during cornering but at the same time, it gives equal torque to each wheel when both wheels have the same traction. A system of gears in the differential arrange in such a way that it connects the propeller shaft to the rear axle.
The differential is found on all modern cars and trucks and also on many all-wheel-drive full-time vehicles. In 1927 the first conventional automobile differential work was invented by a French watchmaker Sephora pecker and it was used in steam-powered vehicles. In 1932 Porsche used the first limited slip differential.
Why differential is needed?
When the vehicle is turning the differential allows the outer wheel to rotate faster than the inner wheel. This is essential when the vehicle turns causing the wheel to move outside the turning curve to roll further and faster than the other.
The differential allows the wheels to rotate at different speeds so the car can corner without putting excessive wear on the tires. If the axle does not allow the wheels to turn independently of each other. The tire of one wheel will be pulled across the ground.
Parts of differential:
- Differential side gears
- Sun gears
- Planted gears
- Spider gears
- Ring gear
- Crown wheel
- Bevel pinion
- Axle or half shafts
Working of their parts:
Ring gear will transmit the power to the planet’s pinions, sun gears, and finally to the half shafts so that power will be used to drive the wheels. Two planet pinions or sometimes four planet pinions are placed opposite to each other and are meshed with the sun gears to transmit the power to the rear wheels by half shafts.
The power from the gearbox is transmitted to the propeller shaft or drive shaft and thereby it will send to the bevel pinion. The function of the bevel pinion is to transmit power from the propeller shaft to drive the ring gear. There is not a single shaft which is connecting two rear wheels but two half shafts are used to connect two rear wheels in the automobile and the crown wheel is free to rotate on the half shaft. These half shafts are splined to each of the sun gears. Two sun gears are employed for the working of differential which is opposite to each other and these sun gears are connected to the half shafts to drive the power.
How does differential work?
The power from the engine is transferred to the ring gear through a pinion gear. The ring gear is connected to a spider gear which is at the heart of the differential work system. The spider gear is free to rotate in two different ways one along with the ring gear and two on its axis. The spider gear meshed with two side gears so power from the engine flows from the pinion to the left and right wheels.
You can also read this: types of suspension system work
Let’s consider some cases;
Vehicle moves straight:
When the vehicle is moving in a straight direction then the speed of the rear wheels is the same in the sense both will rotate at the same speed.
In this case, the sun gears, spider gears, and ring gears will act as a single unit and the two half shafts will revolve at the same speed. In this situation when all the parts of the differential act as a single unit there is no relative motion among them and that’s the reason all the rear wheels move at the same speed.
The vehicle taking a turn:
The vehicle is taking a turn spider gear plays an important role here. Along with the rotation of the ring gear it rotates on its axis. So the spider gear is having a combined rotation.
When the vehicle is taking the right turn then the distance traveled by the left wheel is greater than the distance traveled by the right wheel and the speed of the left wheel is greater than the speed of the right wheel. The turning of the sun gear will cause the other sun gear to rotate in the opposite direction which means if the left sun gear rotates in a particular period. Then the right sun gear will also rotate in the same period but the opposite direction. The torque from the final drive is divided between two half shafts and the spider gears are connected using a cross pin which is free to rotate about it and does not change the torque among the wheels.
Therefore, the spider gear acts as a balance and divides the torque equally between the two wheels on the axle even when their speeds are different. In this way, the torque is transmitted where the vehicle is taking the turns.
Types of differential:
The different types of differential are;
- Open differential
- Limited slip differential
- Locking differential
- Torque vectoring differential
- Torsen differential
- Welded differential
These types of differentials are the most essential and only allow individual wheel speed or slip to be changed. In good road conditions, this allows the outer wheel to spin at a faster speed than the inner wheel.
The problem occurs when road conditions are not good such as on wet snow ice or sand. Using an open differential in your car engine is transferred even though the wheel has zero traction so that the sliding tire can spin freely. This type of differential is often found in family sedans and economy cars. If one tire loses traction the other will lose power to maintain traction.
Advantages of open different:
- Open differential allows for different deal speeds on the same axle, so there will be no wheel slip.
- When going around a corner as the outer tire will travel further from an efficiency point of view there will be less through open differential compared to other types.
Disadvantages of open differential:
- Limits the amount of power produced by the vehicle.
When traction is reduced in one wheel it substantially the amount of power produced by the vehicle.
- If one wheel cannot dissipate as much power the other wheel receives an equally small amount of torque.
Limited slip differential.
A limited-slip differential works similarly to an open differential. They transfer torque to each wheel independently under good road conditions. An open differential can cause the tire to slip under hard cornering or heavy acceleration. But a limited-slip differential prevents the amount of torque from passing into the sliding tire the one with the least resistance. This is done by the use of clutches and plates within the differential. This allows the vehicle to power through corners whereas a vehicle with an operational differential cannot. Race cars, off-road vehicles, and other performance vehicles use a limited-slip differential.
Advantages of limited slip differential:
- Limited slip differential allows for different wheel speeds on one axle thus reducing tire wear compared to a locked differential.
- It also allows the wheel to send torque with more traction.
- It allows a very smooth operation and doesn’t have low-speed clunkiness. Typically associated with other limited slip differential types that navigate a tight area.
Disadvantages of open differential:
- It cannot lock completely because the system requires a speed difference between the two sides to transfer torque when it is being used effectively. The effective limited-slip differential will be reduced.
These types of differential use a clutch and springs to actuate the lock which sends the same amount of power to each wheel no matter the traction condition. It essentially forms a solid axle. A locking differential can achieve a greater amount of traction because the full amount of torque is always available to one wheel and it is not limited by the lower traction of a single wheel. At higher speeds this is the drawback but when off-roading or rock climbing it is a huge advantage. These types of differentials are found on many off-road vehicles and some performance cars.
Advantages of locking differential:
- Provide torque with more traction
In various designs, this will allow most of the torque to reach the ground at any given surface condition.
- It is simple and very effective. It also provides a solution for drifting situations where it is desirable to keep the wheel speed constant on the axle.
Disadvantages of locking differential:
- One drawback of the locking differential is that it would not allow for the difference in wheel speed between the right and left wheels which means additional tire wheels as well as tying up within the drivetrain as a result.
Torque vectoring differential:
This type of differential uses a set of sensors and electronics to receive data from various matters road surface, throttle position, steering system, etc. to activate an electronically actuated clutch and a controller. It is also known as an active differential. They operate efficiently, resulting in a truly dynamic high-performance driving experience.
A torque vectoring difference is found in high-performance rear-wheel drive and all-wheel drive vehicles. Full-time four-wheel drive and all-wheel drive have three differentials. One in the front axle second in the rear axle and the third differential is in the middle of the drive train which allows the wheels to turn at the differential speeds.
Advantages of torque vectoring:
- This differential allows the outer wheel to send more torque as it gets closer to the turn.
- It enables complete control by the designer the system can choose under what conditions the vehicle will send more torque to any one wheel rather than being reactive.
- It can send up to one hundred percent of other potential torque to a wheel.
Disadvantages of torque vectoring:
- Though it does not have any drawbacks it hits two things which are cost and complexity.
Torsion means torque sensing. These are types of limited slip differential that use some accelerated gearing to produce an impact without using a clutch or fluid resistance. This can be achieved by adding a collection of worm gears to a conventional gear setup of the open differential. These worm gears working on each axle provide the necessary resistance to enable torque transfer. This is achieved by having the worm gears in a continuous mesh with each other through connected spur gears. The continuous mesh between the two sides of the differential has the advantage of delivering quick torque making it sensitive to changing road and driving conditions.
Advantages of Torsen differential:
- As soon as there is a speed difference between them. It starts sending more torque to the slower wheel. Also, it reacts far more quickly than a limited-slip differential.
- These systems do not require regular maintenance because differential action is dependent on friction throughout the gear.
Disadvantages of Torsen differential:
- When a wheel is in the air a Torsen differential works similar to an open differential and very little torque is transferred to the drive axle.
- This is perfectly acceptable for road use but it can be an issue for more purpose-built vehicles on the track.
This is a type of locked differential and is known as a spool differential. it is welded permanently to a fixed axis by an open gap. This is usually done in specific situations where the features of lock differential make it easier for both wheels to pin simultaneously.
This is generally not recommended because the heat from welding can hazard the component strength and increase the risk of part failure.